According to its chemical structure, DNA ( Deoxyribonucleic acid) is biopolymer, whose monomers are nucleotides. That is, DNA is polynucleotide. Moreover, a DNA molecule usually consists of two chains twisted relative to each other along a helical line (often called “helically twisted”) and connected to each other by hydrogen bonds.
The chains can be twisted both to the left and to the right (most often) side.
Some viruses have single strand DNA.
Each DNA nucleotide consists of 1) a nitrogenous base, 2) deoxyribose, 3) a phosphoric acid residue.
Double right-handed DNA helix
The composition of DNA includes the following: adenine, guanine, thymine And cytosine. Adenine and guanine are purins, and thymine and cytosine - to pyrimidines. Sometimes DNA contains uracil, which is usually characteristic of RNA, where it replaces thymine.
The nitrogenous bases of one chain of a DNA molecule are connected to the nitrogenous bases of another strictly according to the principle of complementarity: adenine only with thymine (form two hydrogen bonds with each other), and guanine only with cytosine (three bonds).
The nitrogenous base in the nucleotide itself is connected to the first carbon atom of the cyclic form deoxyribose, which is a pentose (a carbohydrate with five carbon atoms). The bond is covalent, glycosidic (C-N). Unlike ribose, deoxyribose lacks one of its hydroxyl groups. The deoxyribose ring is formed by four carbon atoms and one oxygen atom. The fifth carbon atom is outside the ring and is connected through an oxygen atom to a phosphoric acid residue. Also, through the oxygen atom at the third carbon atom, the phosphoric acid residue of the neighboring nucleotide is attached.
Thus, in one strand of DNA, adjacent nucleotides are linked to each other by covalent bonds between deoxyribose and phosphoric acid (phosphodiester bond). A phosphate-deoxyribose backbone is formed. Directed perpendicular to it, towards the other DNA chain, are nitrogenous bases, which are connected to the bases of the second chain by hydrogen bonds.
The structure of DNA is such that the backbones of the chains connected by hydrogen bonds are directed in different directions (they say “multidirectional”, “antiparallel”). On the side where one ends with phosphoric acid connected to the fifth carbon atom of deoxyribose, the other ends with a “free” third carbon atom. That is, the skeleton of one chain is turned upside down relative to the other. Thus, in the structure of DNA chains, 5" ends and 3" ends are distinguished.
During DNA replication (doubling), the synthesis of new chains always proceeds from their 5th end to the third, since new nucleotides can only be added to the free third end.
Ultimately (indirectly through RNA), every three consecutive nucleotides in the DNA chain code for one protein amino acid.
The discovery of the structure of the DNA molecule occurred in 1953 thanks to the work of F. Crick and D. Watson (which was also facilitated by the early work of other scientists). Although DNA was known as a chemical substance back in the 19th century. In the 40s of the 20th century, it became clear that DNA is the carrier of genetic information.
The double helix is considered the secondary structure of the DNA molecule. In eukaryotic cells, the overwhelming amount of DNA is located in chromosomes, where it is associated with proteins and other substances, and is also more densely packaged.
In prokaryotic cells, deoxyribonucleic acid is located in a cytoplasmic colloidal (“glue”) matrix along with other components. The ground substance contains this type of nucleic acid, represented by a double-stranded helix, in chromosomes. Otherwise it is called covalently closed circles DNA (abbreviated as cccDNA).
Bacterial chromosomes are less condensed. They float freely in the cytoplasmic matrix within a small nuclear region - the nucleoid. Moreover, they are curled into supercoiled “balls”. If you stretch one of the chains in length, it will be 1000 times larger than the size of the cell itself! It can be wrapped around a protein.
Bacterial macromolecules as cytoplasmic inclusions are covered with histone-like proteins: H-NS, HU, JHF, FIS. But the density of this “shell” is very small. Only some archaea from the euarchaea group have nucleosomes.
The size of the bacterial genetic macromolecule ranges from 600 thousand (for mycoplasma - Mycoplasma) to 10 million (for myxococci) nucleotide pairs. Prokaryotes are haploid. Their single chromosomes have a circular or linear (in three species: Borrelia, Streptomyces, Rhodococcus) shape.
The genetic material in prenuclear cells consists of many loops emanating from a single center. Due to the absence of an envelope in the nucleoid, these domains penetrate even into the peripheral cytoplasm. This feature significantly affects the transcription process.
Prokaryotic chromosomes are attached to the cell membrane. They have quite a lot of attachment points:
- oriC - “origin of the chromosome” - point of origin of replication;
- terC - “terminus of the chromosome” - the point of completion;
- replication fork.
Places of attachment are divided into permanent and sliding. Prokaryotic genes are grouped into operons. The unifying features are the similarity of functions and the unity of promoters. The latter are sets of gene nucleotides, upon exposure to which the transcription process is launched. Structural genes take up much more space than regulatory genes.
Some segments of “hereditary” molecules are capable of moving within a prokaryotic cell between genetic loci - these are transposons. There are two types of such moving elements:
- IS elements are the simplest modules of transposase genes;
- Tn elements are actually transposons.
The former move randomly and are extremely mobile. The longer the transposon, the more passive it is. The genetic elements of prokaryotes are not only chromosomes, transposons, but also plasmids. They are completely autonomous extrachromosomal molecules. Transposons should not be confused with plasmids, because the former cannot exist independently of chromosomes.
Thus, the peculiarities of the localization of hereditary information in prokaryotes are associated with the absence of a membrane in the nucleoid, as well as in some organelles. Segments with hereditary information are localized near the nuclear region, and are also “stretched” throughout the peripheral cytoplasm.
Localization of DNA in eukaryotic cells
The localization of deoxyribonucleic acid molecules near the cellular “center” was first established by Feulgen using the Schiff reaction closer to the middle of the twentieth century. Spatially, DNA molecules are localized by proteins - histones. Such complexes are called nucleosomes.
Eukaryotic chromosomes are localized mainly in the nucleolus of the nucleus, although it does not have its own membrane. The molecules are associated with chromatin. If we compare it with prenuclear organisms, here genetic macromolecules are not represented by transposons moving freely in the cytoplasm, as well as plasmids. But eukaryotes have hereditary molecules in organelles: mitochondria, plastids.
Mitochondrial DNA (abbreviated as mtDNA) no longer constitutes the nuclear genome, but a cytoplasmic plasmon. Most eukaryotes have mitochondria: plants, fungi, animals. In the cytoplasm they move to where energy demand increases.
Types of mitochondria:
- young – protomitochondria;
- mature;
- old - postmitochondria.
Carriers of hereditary characteristics are located in a matrix bounded by a second, internal membrane. Otherwise it is called the pink substance. mtDNA has a linear and/or closed circular shape. It is much smaller than nuclear. Maxi- and minicircles of mitochondrial DNA can combine to form catenanes. The coding sequences of the mitochondrial genome are called codons.
If there are several mitochondria, then they have identical and unique types of macromolecules. mt-DNA is most often inherited through the maternal line. There are eukaryotes with mitochondria that do not contain genetic macromolecules - mitosomes.
Mitochondria are not the only organelles of eukaryotes that have their own genetic apparatus. The plastid genome is called the plastome or pDNA. In these semi-autonomous organelles, operons are created by analogy with the cellular formations of eukaryotes. Genetic carriers are located in the plastid matrix - the stroma.
Usually, when talking about the plastid genome, they mean chloroplasts and their cDNA. But there are many more types of plastids:
- propplastids;
- leukoplasts;
- amyloplasts;
- elaioplasts;
- proteinoplasts;
- etioplasts - dark plastids;
- chloroplasts;
- chromoplasts.
In a simplified manner, the features of DNA localization in “prenuclear” and eukaryotic organisms can be represented using the table:
Genetic elements are found in non-cellular forms - viruses. Their localization and quantity in varieties of prenuclear/nuclear smallest units of life are very diverse. The similarity of prokaryotic and eukaryotic cells indicates that these are elementary structural and functional units of living matter, as well as the unity of the origin of life on Earth. The existing differences in the localization of macromolecules confirm the evolutionary theory.
Mitochondria and plastids have their own circular DNA and small ribosomes, through which they make part of their own proteins (semi-autonomous organelles).
Mitochondria take part in (the oxidation of organic substances) - they supply ATP (energy) for the life of the cell, and are the “energy stations of the cell.”
Non-membrane organelles
Ribosomes- these are organelles that deal with... They consist of two subunits, chemically consisting of ribosomal RNA and proteins. The subunits are synthesized in the nucleolus. Some of the ribosomes are attached to the EPS; this EPS is called rough (granular).
Cell center consists of two centrioles that form the spindle during cell division - mitosis and meiosis.
Cilia, flagella serve for movement.
Choose one, the most correct option. The cell cytoplasm contains
1) protein threads
2) cilia and flagella
3) mitochondria
4) cell center and lysosomes
Answer
Establish a correspondence between the functions and organelles of cells: 1) ribosomes, 2) chloroplasts. Write numbers 1 and 2 in the correct order.
A) located on the granular ER
B) protein synthesis
B) photosynthesis
D) consist of two subunits
D) consist of grana with thylakoids
E) form a polysome
Answer
Establish a correspondence between the structure of the cell organelle and the organelle: 1) Golgi apparatus, 2) chloroplast. Write numbers 1 and 2 in the order corresponding to the letters.
A) double membrane organelle
B) has its own DNA
B) has a secretory apparatus
D) consists of a membrane, bubbles, tanks
D) consists of thylakoids grana and stroma
E) single-membrane organelle
Answer
Establish a correspondence between the characteristics and organelles of the cell: 1) chloroplast, 2) endoplasmic reticulum. Write numbers 1 and 2 in the order corresponding to the letters.
A) a system of tubules formed by a membrane
B) the organelle is formed by two membranes
B) transports substances
D) synthesizes primary organic matter
D) includes thylakoids
Answer
1. Choose one, the most correct option. Single-membrane cell components -
1) chloroplasts
2) vacuoles
3) cell center
4) ribosomes
Answer
2. Select three options. Which cell organelles are separated from the cytoplasm by a single membrane?
1) Golgi complex
2) mitochondria
3) lysosome
4) endoplasmic reticulum
5) chloroplast
6) ribosome
Answer
All of the following features, except two, can be used to describe the structural features and functioning of ribosomes. Identify two characteristics that “drop out” from the general list and write down the numbers under which they are indicated.
1) consist of triplets of microtubules
2) participate in the process of protein biosynthesis
3) form the spindle
4) formed by protein and RNA
5) consist of two subunits
Answer
Choose two correct answers out of five and write down the numbers under which they are indicated in the table. Select double membrane organelles:
1) lysosome
2) ribosome
3) mitochondria
4) Golgi apparatus
5) chloroplast
Answer
Choose three correct answers out of six and write down the numbers under which they are indicated. Plant cell organelles are double-membrane.
1) chromoplasts
2) centrioles
3) leucoplasts
4) ribosomes
5) mitochondria
6) vacuoles
Answer
NUCLEUS1-MITOCHONDRIA1-RIBOSOME1
Analyze the table. For each lettered cell, select the appropriate term from the list provided:
1) core
2) ribosome
3) protein biosynthesis
4) cytoplasm
5) oxidative phosphorylation
6) transcription
7) lysosome
Answer
MITOCHONDRIA2-CHROMOSOME1-RIBOSOME2 
Analyze the table “Structures of a eukaryotic cell.” For each cell indicated by a letter, select the corresponding term from the list provided.
1) glycolysis
2) chloroplasts
3) broadcast
4) mitochondria
5) transcription
6) core
7) cytoplasm
8) cell center
Answer
LYSOSOME1-RIBOSOME3-CHLOROPLAST1 
1) Golgi complex
2) synthesis of carbohydrates
3) single membrane
4) starch hydrolysis
5) lysosome
6) non-membrane
Answer
LYSOSOME2-CHLOROPLAST2-RIBOSOME4 
Analyze the table. For each lettered cell, select the appropriate term from the list provided.
1) double membrane
2) endoplasmic reticulum
3) protein biosynthesis
4) cell center
5) non-membrane
6) biosynthesis of carbohydrates
7) single membrane
8) lysosome
Answer
LYSOSOME3-AG1-CHLOROPLAST3 
Analyze the table “Cell Structures”. For each cell indicated by a letter, select the corresponding term from the list provided.
1) glycolysis
2) lysosome
3) protein biosynthesis
4) mitochondria
5) photosynthesis
6) core
7) cytoplasm
8) cell center
Answer
CHLOROPLAST4-AG2-RIBOSOME5 
Analyze the table “Cell Structures”. For each cell indicated by a letter, select the corresponding term from the list provided.
1) glucose oxidation
2) ribosome
3) splitting of polymers
4) chloroplast
5) protein synthesis
6) core
7) cytoplasm
8) spindle formation
Answer
AG3-MITOCHONDRIA3-LYSOSOME4 
Analyze the table “Cell Organelles”. For each cell indicated by a letter, select the corresponding term from the list provided.
1) chloroplast
2) endoplasmic reticulum
3) cytoplasm
4) karyoplasm
5) Golgi apparatus
6) biological oxidation
7) transport of substances in the cell
8) glucose synthesis
Answer
1. Select two correct answers out of five and write down the numbers under which they are indicated in the table. Cytoplasm performs a number of functions in a cell:
1) communicates between the nucleus and organelles
2) acts as a matrix for the synthesis of carbohydrates
3) serves as the location of the nucleus and organelles
4) transmits hereditary information
5) serves as the location of chromosomes in eukaryotic cells
Answer
2. Identify two true statements from the general list, and write down the numbers under which they are indicated. Occurs in the cytoplasm
1) synthesis of ribosomal proteins
2) glucose biosynthesis
3) insulin synthesis
4) oxidation of organic substances to inorganic ones
5) synthesis of ATP molecules
Answer
Choose two correct answers out of five and write down the numbers under which they are indicated. Select non-membrane organelles:
1) mitochondria
2) ribosome
3) core
4) microtubule
5) Golgi apparatus
Answer
The following features, except two, are used to describe the functions of the cell organelle depicted. Identify two characteristics that “fall out” from the general list and write down the numbers under which they are indicated.
1) serves as an energy station
2) breaks down biopolymers into monomers
3) provides packaging of substances from the cell
4) synthesizes and accumulates ATP molecules
5) participates in biological oxidation
Answer
Establish a correspondence between the structure of the organelle and its type: 1) cell center, 2) ribosome
A) consists of two perpendicularly located cylinders
B) consists of two subunits
B) formed by microtubules
D) contains proteins that ensure the movement of chromosomes
D) contains proteins and nucleic acid
Answer
Establish the sequence of structures in a eukaryotic plant cell (starting from the outside)
1) plasma membrane
2) cell wall
3) core
4) cytoplasm
5) chromosomes
Answer
Choose three options. How are mitochondria different from lysosomes?
1) have outer and inner membranes
2) have numerous outgrowths - cristae
3) participate in the processes of energy release
4) in them, pyruvic acid is oxidized to carbon dioxide and water
5) in them biopolymers are broken down into monomers
6) participate in metabolism
Answer
1. Establish a correspondence between the characteristics of a cell organelle and its type: 1) mitochondria, 2) lysosome. Write numbers 1 and 2 in the correct order.
A) single-membrane organelle
B) internal contents - matrix
D) the presence of cristae
D) semi-autonomous organelle
Answer
2. Establish a correspondence between the characteristics and organelles of the cell: 1) mitochondria, 2) lysosome. Write numbers 1 and 2 in the order corresponding to the letters.
A) hydrolytic cleavage of biopolymers
B) oxidative phosphorylation
B) single-membrane organelle
D) the presence of cristae
D) formation of a digestive vacuole in animals
Answer
3. Establish a correspondence between the feature and the cell organelle for which it is characteristic: 1) lysosome, 2) mitochondria. Write numbers 1 and 2 in the order corresponding to the letters.
A) the presence of two membranes
B) accumulation of energy in ATP
B) the presence of hydrolytic enzymes
D) digestion of cell organelles
D) formation of digestive vacuoles in protozoa
E) breakdown of organic substances to carbon dioxide and water
Answer
Establish a correspondence between the cell organelle: 1) cell center, 2) contractile vacuole, 3) mitochondria. Write numbers 1-3 in the correct order.
A) participates in cell division
B) ATP synthesis
B) release of excess fluid
D) “cellular respiration”
D) maintaining a constant cell volume
E) participates in the development of flagella and cilia
Answer
1. Establish a correspondence between the name of organelles and the presence or absence of a cell membrane: 1) membranous, 2) non-membrane. Write numbers 1 and 2 in the correct order.
A) vacuoles
B) lysosomes
B) cell center
D) ribosomes
D) plastids
E) Golgi apparatus
Answer
2. Establish a correspondence between cell organelles and their groups: 1) membrane, 2) non-membrane. Write numbers 1 and 2 in the order corresponding to the letters.
A) mitochondria
B) ribosomes
B) centrioles
D) Golgi apparatus
D) endoplasmic reticulum
E) microtubules
Answer
3. Which three of the listed organelles are membranous?
1) lysosomes
2) centrioles
3) ribosomes
4) microtubules
5) vacuoles
6) leucoplasts
Answer
1. All but two of the cell structures listed below do not contain DNA. Identify two cell structures that “drop out” from the general list and write down the numbers under which they are indicated.
1) ribosomes
2) Golgi complex
3) cell center
4) mitochondria
5) plastids
Answer
2. Select three cell organelles containing hereditary information.
1) core
2) lysosomes
3) Golgi apparatus
4) ribosomes
5) mitochondria
6) chloroplasts
Answer
3. Choose two correct answers out of five. In what structures of eukaryotic cells are DNA molecules localized?
1) cytoplasm
2) core
3) mitochondria
4) ribosomes
5) lysosomes
Answer
Choose one, the most correct option. Where in the cell are there ribosomes, except for the ER?
1) in the centrioles of the cell center
2) in the Golgi apparatus
3) in mitochondria
4) in lysosomes
Answer
What are the features of the structure and functions of ribosomes? Choose the three correct options.
1) have one membrane
2) consist of DNA molecules
3) break down organic substances
4) consist of large and small particles
5) participate in the process of protein biosynthesis
6) consist of RNA and protein
Answer
Choose three correct answers out of six and write down the numbers under which they are indicated. The structure of the nucleus of a eukaryotic cell includes
1) chromatin
2) cell center
3) Golgi apparatus
4) nucleolus
5) cytoplasm
6) karyoplasm
Answer
Choose three correct answers out of six and write down the numbers under which they are indicated. What processes occur in the cell nucleus?
1) formation of the spindle
2) formation of lysosomes
3) doubling of DNA molecules
4) synthesis of mRNA molecules
5) formation of mitochondria
6) formation of ribosomal subunits
Answer
Establish a correspondence between the cell organelle and the type of structure to which it is classified: 1) single-membrane, 2) double-membrane. Write numbers 1 and 2 in the order corresponding to the letters.
A) lysosome
B) chloroplast
B) mitochondria
D) EPS
D) Golgi apparatus
Answer
Establish a correspondence between the characteristics and organelles: 1) chloroplast, 2) mitochondria. Write numbers 1 and 2 in the order corresponding to the letters.
A) the presence of stacks of grains
B) synthesis of carbohydrates
B) dissimilation reactions
D) transport of electrons excited by photons
D) synthesis of organic substances from inorganic ones
E) the presence of numerous cristae
Answer
All of the characteristics listed below, except two, can be used to describe the cell organelle shown in the figure. Identify two characteristics that “drop out” from the general list and write down the numbers under which they are indicated.
1) single-membrane organelle
2) contains fragments of ribosomes
3) the shell is riddled with pores
4) contains DNA molecules
5) contains mitochondria
Answer
The terms listed below, except two, are used to characterize the cell organelle, indicated in the figure by a question mark. Identify two terms that “drop out” from the general list and write down the numbers under which they are indicated.
1) membrane organelle
2) replication
3) chromosome divergence
4) centrioles
5) spindle
Answer
Establish a correspondence between the characteristics of a cell organelle and its type: 1) cell center, 2) endoplasmic reticulum. Write numbers 1 and 2 in the order corresponding to the letters.
A) transports organic substances
B) forms a spindle
B) consists of two centrioles
D) single-membrane organelle
D) contains ribosomes
E) non-membrane organelle
Answer
1. Establish a correspondence between the characteristics and organelles of the cell: 1) nucleus, 2) mitochondria. Write the numbers 1 and 2 in the order they correspond to the numbers.
A) closed DNA molecule
B) oxidative enzymes on cristae
B) internal contents - karyoplasm
D) linear chromosomes
D) the presence of chromatin in interphase
E) folded inner membrane
Answer
2. Establish a correspondence between the characteristics and organelles of cells: 1) nucleus, 2) mitochondria. Write numbers 1 and 2 in the order corresponding to the letters.
A) is the site of ATP synthesis
B) is responsible for storing the genetic information of the cell
B) contains circular DNA
D) has cristae
D) has one or more nucleoli
Answer
Establish a correspondence between the characteristics and organelles of the cell: 1) lysosome, 2) ribosome. Write numbers 1 and 2 in the order corresponding to the letters.
A) consists of two subunits
B) is a single-membrane structure
B) participates in the synthesis of the polypeptide chain
D) contains hydrolytic enzymes
D) located on the membrane of the endoplasmic reticulum
E) converts polymers into monomers
Answer
Establish a correspondence between the characteristics and cellular organelles: 1) mitochondria, 2) ribosome. Write numbers 1 and 2 in the order corresponding to the letters.
A) non-membrane organelle
B) presence of own DNA
B) function - protein biosynthesis
D) consists of large and small subunits
D) the presence of cristae
E) semi-autonomous organelle
Answer
All of the features listed below, except two, are used to describe the cell structure shown in the figure. Identify two characteristics that “drop out” from the general list and write down the numbers under which they are indicated.
1) consists of RNA and proteins
2) consists of three subunits
3) synthesized in hyaloplasm
4) carries out protein synthesis
5) can attach to the EPS membrane
Answer
© D.V. Pozdnyakov, 2009-2019
““Nucleic acids” chemistry” - Structure of chromatin. Spiral pitch. Review DNA analysis data. Practicing and consolidating acquired skills and knowledge. Structure and functions. Formation of a DNA superhelix. Nucleic acid. DNA reduplication diagram. Questions for self-control. Keywords. Nucleotide. Designations of nitrogenous bases. DNA is a double strand.
“Nucleic acid” - Sugar - ribose. The meaning of nucleic acids. Drawing up a comparative table. Triplet. Functions of DNA. Gunin. Purpose of the lesson: The structure and functions of nucleic acids were studied by the American biologist J. Storage, transfer and inheritance of information about the structure of protein molecules. "Nycleus" - core.
“RNA and DNA” - Repetition and consolidation of knowledge: Transfer RNA (t-RNA). Integrated lesson on the topic: “NUCLEIC ACIDS.” Completion task. (In the nucleus, cytoplasm, mitochondria, chloroplasts). (In the nucleus, mitochondria, chloroplasts). (Double helix). Construction of a complementary DNA strand. Nucleic acids.
“Nucleic acids” - 1892 - chemist Lilienfeld isolated thymonucleic acid from the thymus gland in 1953. History of discovery. The principle of complementarity (supplementation). Structure of nucleotides (differences). The length of DNA molecules (American biologist G. Taylor). Laboratory workshop. Biological role of nucleic acids. James Watson and Francis Crick deciphered the structure of DNA.
“DNA and RNA molecules” - Types of RNA. Cell matrix ribosomes and mitochondria. Physicochemical properties of DNA. Subject to hydrolysis. Structure of extranuclear DNA. Problematic question. The RNA molecule is a polymer whose monomers are ribonucleotides. Molecular structure of DNA and types of chemical bonds in the molecule. Types of nucleic acids and their structure.
"DNA and RNA" - Phosphate. James Watson and Francis Crick discovered the truth in 1953. Abbreviated: Nucleic acids. Nucleotides come in five different types. Monomers of nucleic acids are. There are three types of RNA: messenger, ribosomal and transport. Molecular text consists of four letters and might look something like this:
There are 10 presentations in total
9. Nucleic acids. DNA, its composition and structural organization,
localization in the cell. Biological role.
Nucleic acids are natural high-molecular organic compounds that ensure the storage and transmission of hereditary (genetic) information in living organisms.
In nature, there are two types of nucleic acids, differing in composition, structure and functions. One of them contains the carbohydrate component deoxyribose and is called deoxyribonucleic acid (DNA). The other contains ribose and is called ribonucleic acid (RNA).
DNA, its composition
DNA is a double-stranded biological polymer whose monomers are nucleotides containing one of the nitrogenous bases, deoxyribose and a phosphoric acid residue. DNA nucleotides: purine bases adenine (A) and guanine (G) and pyrimidine bases cytosine (C) and thymine (T).
structural organization
The polynucleotide chains of the DNA molecule are antiparallel and connected to each other by hydrogen bonds according to the principle of complementarity to form a double helix.
localization in the cell
DNA is found in the cell nucleus in the form of a complex with nuclear proteins (histones). There is also its own special (circular) DNA in mitochondria (mtDNA) and in chloroplasts in plants (chlDNA). Bacteria do not have a nucleus, therefore DNA floats freely in the cytosol (intracellular fluid, cytoplasmic matrix).
Biological role
DNA has one function - storing genetic information.
10. RNA. Types of RNA, their structure and chemical composition, biological role. RNA splicing (processing), alternative splicing and RNA of structural genes of eukaryotes. The concept of ribozymes.
Unlike DNA molecules, ribonucleic acids are represented by a single polynucleotide chain, which consists of four types of nucleotides containing sugar, ribose, phosphate and one of four nitrogenous bases - adenine, guanine, uracil or cytosine. RNA is synthesized on DNA molecules using RNA polymerase enzymes in compliance with the principle of complementarity and antiparallelism, and uracil is complementary to DNA adenine in RNA. The entire variety of RNAs operating in the cell can be divided into three main types: mRNA, tRNA, rRNA.
Matrix, or information, RNA (mRNA, or mRNA).
Transcription. In order to synthesize proteins with specified properties, “instructions” are sent to the place of their construction about the order of inclusion of amino acids in the peptide chain. This instruction is contained in the nucleotide sequence of matrix, or messenger RNA (mRNA, mRNA), synthesized in the corresponding sections of DNA. The process of mRNA synthesis is called transcription. The synthesis of mRNA begins with the detection by RNA polymerase of a special region in the DNA molecule, which indicates the place where transcription begins - the promoter.
After binding to the promoter, RNA polymerase unwinds the adjacent turn of the DNA helix. Two DNA strands diverge at this point, and on one of them the enzyme synthesizes mRNA. The assembly of ribonucleotides into a chain occurs in compliance with their complementarity to DNA nucleotides, and also antiparallel with respect to the DNA template strand. Due to the fact that RNA polymerase is capable of assembling a polynucleotide only from the 5" end to the 3" end, only one of the two DNA strands, namely the one facing the enzyme with its 3" end, can serve as a template for transcription ( 3" → 5"). Such a chain is called codogenic. The antiparallel connection of two polynucleotide chains in a DNA molecule allows RNA polymerase to correctly select the template for the synthesis of mRNA. Moving along the codogenic DNA chain, RNA polymerase gradually accurately rewrites information until it does not encounter a specific nucleotide sequence - the transcription terminator. In this region, RNA polymerase is separated from both the DNA template and the newly synthesized mRNA. A fragment of the DNA molecule, including a promoter, transcribed sequence and terminator, forms a transcription unit - transcripton. During the synthesis , as RNA polymerase moves along the DNA molecule, the single-stranded DNA sections it has traversed are again combined into a double helix. The mRNA produced during transcription contains an exact copy of the information recorded in the corresponding section of DNA. Triples of adjacent mRNA nucleotides that encode amino acids are called codons. The codon sequence of the mRNA encodes the sequence of amino acids in the peptide chain. mRNA codons correspond to specific amino acids. The template for mRNA transcription is the codogenic DNA strand, facing the enzyme with its 3" end
Transfer RNA (tRNA). Broadcast. Transfer RNA (tRNA) plays an important role in the process of using hereditary information by a cell. By delivering the necessary amino acids to the site of assembly of peptide chains, tRNA acts as a translational intermediary. tRNA molecules are polynucleotide chains synthesized from specific DNA sequences. They consist of a relatively small number of nucleotides -75-95. As a result of the complementary combination of bases that are located in different parts of the tRNA polynucleotide chain, it acquires a structure resembling a clover leaf in shape. It has four main parts that perform different functions. The acceptor “stem” is formed by two complementary connected terminal parts of tRNA. It consists of seven base pairs. The 3" end of this stem is slightly longer and forms a single-stranded region that ends with a CCA sequence with a free OH group. The transported amino acid is attached to this end. The remaining three branches are complementary paired nucleotide sequences that end in unpaired regions that form loops. The middle one of these branches - the anticodon - consists of five pairs of nucleotides and contains an anticodon in the center of its loop.An anticodon is three nucleotides complementary to the codon of the mRNA, which encodes the amino acid transported by this tRNA to the site of peptide synthesis. Between the acceptor and anticodon branches there are two side branches. In their loops they contain modified bases - dihydrouridine (D-loop) and the triplet TψC, where \y is pseudouraine (T^C-loop).Between the aiticodone and T^C-branches there is an additional loop, including from 3-5 to 13 -21 nucleotides In general, different types of tRNA are characterized by a certain constancy of the nucleotide sequence, which most often consists of 76 nucleotides. The variation in their number is mainly due to changes in the number
nucleotides in the extra loop. The complementary regions that support the tRNA structure are usually conserved. The primary structure of the tRNA, determined by the nucleotide sequence, forms the secondary structure of the tRNA, which is shaped like a clover leaf. In turn, the secondary structure determines the three-dimensional tertiary structure, which is characterized by the formation of two perpendicularly located double helices. One of them is formed by the acceptor and TψC branches, the other by the anticodon and D branches. The transported amino acid is located at the end of one of the double helices, and the anticodon is located at the end of the other. These areas are located as far as possible from each other. The stability of the tertiary structure of tRNA is maintained due to the occurrence of additional hydrogen bonds between the bases of the polynucleotide chain, located in different parts of it, but spatially close in the tertiary structure. Different types of tRNA have similar tertiary structures, although with some variations. One of the features of tRNA is the presence of unusual bases in it, which arise as a result of chemical modification after the inclusion of a normal base in the polynucleotide chain. These altered bases determine the great structural diversity of tRNAs in the general plan of their structure. Of greatest interest are modifications of the bases forming the anticodon, which affect the specificity of its interaction with the codon. For example, the atypical base inosine, sometimes found in the 1st position of the tRNA anticodon, is capable of complementarily combining with three different third bases of the mRNA codon - U, C and A. The existence of several types of tRNA that can bind to the same codon has also been established. As a result, in the cytoplasm of cells there are not 61 (by the number of codons), but about 40 different tRNA molecules. This amount is enough to transport 20 different amino acids to the site of protein assembly. Along with the function of accurately recognizing a specific codon in mRNA, the tRNA molecule delivers a strictly defined amino acid, encrypted using a given codon, to the site of synthesis of the peptide chain. The specific connection of tRNA with its “own” amino acid occurs in two stages and leads to the formation of a compound called aminoacyl-tRNA.
Attaching an amino acid to the corresponding tRNA:
I-1st stage, interaction of amino acid and ATP with the release of pyrophosphate;
II-2nd stage, attachment of adenylated amino acid to the 3" end of RNA
At the first stage, the amino acid is activated by interacting its carboxyl group with ATP. As a result, an adepylated amino acid is formed. At the second stage, this compound interacts with the OH group located at the 3" end of the corresponding tRNA, and the amino acid attaches to it with its carboxyl group, releasing AMP. Thus, this process occurs with the expenditure of energy obtained from the hydrolysis of ATP to AMP The specificity of the connection between an amino acid and a tRNA carrying the corresponding anticodon is achieved due to the properties of the enzyme aminoacyl-tRNA synthetase. In the cytoplasm, there is a whole set of such enzymes that are capable of spatial recognition, on the one hand, of their amino acid, and on the other, of the corresponding tRNA anticodon . Hereditary information, “recorded” in DNA molecules and “rewritten” on mRNA, is deciphered during translation due to two processes of specific recognition of molecular surfaces. First, the enzyme aminoacyl-tRNA synthetase ensures the connection of tRNA with the amino acid it transports. Then aminoacyl-tRNA complementarily pairs with mRNA due to anticodon-codon interaction. Using the tRNA system, the language of the nucleotide chain of mRNA. translated into the language of the amino acid sequence of the peptide. Ribosomal RNA (rRNA). Ribosomal cycle of protein synthesis. The process of interaction between mRNA and tRNA, which ensures the translation of information from the language of nucleotides to the language of amino acids, is carried out on ribosomes. The latter are complex complexes of rRNA and various proteins, in which the former form a framework. Ribosomal RNAs are not only a structural component of ribosomes, but also ensure their binding to a specific nucleotide sequence of mRNA. This establishes the start and reading frame for the formation of the peptide chain. In addition, they ensure the interaction between the ribosome and tRNA. Numerous proteins that make up ribosomes, along with rRNA, perform both structural and enzymatic roles.
1. Messenger RNA transfers the genetic code from the nucleus to the cytoplasm, thus determining the synthesis of various proteins.
2. Transfer RNA carries activated amino acids to ribosomes for the synthesis of polypeptide molecules.
3. Ribosomal RNA, in combination with approximately 75 different proteins, forms ribosomes - cellular organelles on which polypeptide molecules are assembled.
4. Small nuclear RNAs (introns) Participates in splicing.
5. Small cytoplasmic RNAs
6. snoRNA. She is a small nucleolus. In the nucleoli of eukaryotic cells.
7. RNA viruses
8. Viroid RNA
After polyadenylation, the mRNA undergoes splicing, during which introns (regions that do not code for proteins) are removed, and exons (regions that code for proteins) are stitched together to form a single molecule. Splicing is catalyzed by a large nucleoprotein complex, the spliceosome, consisting of proteins and small nuclear RNAs. Many pre-mRNAs can be spliced in different ways to produce different mature mRNAs encoding different amino acid sequences (alternative splicing).
Briefly: splicing is when introns that do not code for anything leave and a mature molecule capable of encoding a protein is formed from exons.
Alternative splicing - different proteins can be obtained from one pre-mRNA molecule. That is, we are dealing with variations in intron loss and different exon stitching.
Ribozymes
RNA molecules with enzymatic activity (usually autocatalysis)
Regulation of gene expression by antisense RNAs is characterized by high specificity. This is due to the high accuracy of the RNA-RNA hybridization process, based on the complementary interaction of extended nucleotide sequences with each other.
However, antisense RNAs themselves do not irreversibly inactivate target mRNAs, and high intracellular concentrations of antisense RNAs are required to suppress the expression of the corresponding genes. The effectiveness of antisense RNAs increased sharply after they were supplemented with ribozyme molecules - short RNA sequences with endonuclease activity. Many other enzymatic activities associated with RNA are known. Therefore, ribozymes in the broad sense are called RNA molecules that have any enzymatic activity.
The RNA variant of suppressing HIV infection was tested on model systems. For this purpose, an unusual property of some RNA molecules is used - their ability to destroy other types of RNA. Americans T. Cech and S. Altman received the Nobel Prize in 1989 for this discovery. It was believed that all biochemical reactions in the body occur thanks to highly effective specific catalysts, which are proteins - enzymes. However, it turned out that some types of RNA, like proteins, have highly specific catalytic activity. These RNAs were called ribozymes.
Ribozymes contain antisense regions and sites that carry out enzymatic reactions. Those. They not only attach to the mRNA, but also cut it. The essence of the method of suppressing HIV infection with the help of ribozymes is shown in Fig. 32. By attaching to a complementary target RNA, the ribozyme cleaves this RNA, resulting in the cessation of synthesis of the protein encoded by the target RNA. If such a target for the ribozyme is viral RNA, then the ribozyme will “spoil” it, and the corresponding viral protein will not be formed. As a result, the virus will stop reproducing in the cell. This approach is also applicable to some other human pathologies, for example, for the treatment of cancer.
