Molecular basis of heredity
The structure of the gene, its function. Characteristics of the human genome. DNA is a molecule of heredity. Chemical and structural features of
It is already known from the school biology program that a gene is a segment of a DNA molecule. Only this macromolecule from a rather extensive spectrum of macromolecules existing in every cell of every living organism is capable of self-reproducing, and hence transmitting in the generations of cells or organisms the information contained in it. The ability of DNA for self-reproduction is due to the peculiarities of its chemical structure. The DNA molecule is made up of three components: sugar, represented by deoxyribose, phosphate groups and 4 types of nitrogenous bases - cytosine( C), thymine( T), which are also called purines, adenine( A) and guanine( D).These are pyrimidines.
In 1953, Watson and Crick published their historical article on the physical structure of DNA.According to the Watson and Crick model, the DNA molecule is a double helix. Each spiral wraps around another spiral along a common axis. The chains of this helix form deoxyribose and phosphate groups. At regular intervals, a nitrogenous base, facing the inside of the spiral, is attached to each chain. The two bases of each chain, located on the same level, are connected together.
The most remarkable thing about the DNA molecule is that each nitrogenous base can only connect to another strictly defined and complementary( suitable only for it) base, namely adenine with thymine, and guanine with cytosine.
This property of nucleotides complementarily pairing provides the basis for the exact reproduction of the nucleotide sequence of each DNA strand. The nucleotide chains of DNA are polar. The polarity is determined by how the sugars( deoxyribose) are joined together. The phosphate group attached to C5( 5-carbon) of one sugar is connected to the hydroxyl group at the C position,( 3-carbon) of the next sugar by the phosphodiester bond. As a result, the terminal nucleotide at one end of the chain has a free 5-, and on the other - a free 3-group. The sequence of nucleotide bases is usually written in the direction from the 5- to the 3-end. The two strands of DNA are antiparallel to each other, since they go in opposite directions and the 5-end of one chain corresponds to the 3-end of the other chain and vice versa.
The DNA model of Watson and Crick explained by then the well-known rule of English biochemist Chargaff, according to which in any DNA molecule the amount of purines strictly corresponds to the amount of pyrimidines.
In the double helix, purine DNA-( adenine, guanine) is always combined with pyrimidines( thymine and cytosine).Between the cytosine and guanine, three hydrogen bonds are formed, and between thymine and adenine - two, so another way to connect simply can not.
An elementary unit of DNA is a nucleotide that contains one deoxyribose, one phosphate group and one nitrogenous base.
DNA replication
Because DNA is a heredity molecule, to implement this quality, it must accurately replicate itself and thus retain all the information available in the original DNA molecule in the form of a certain sequence of nucleotides. This is provided by a special process preceding the division of any cell in the body, which is called DNA replication. The essence of this process is that a special enzyme breaks weak hydrogen bonds, which connect the nucleotides of two chains together. As a result, DNA strands are disconnected and free nitrogenous bases( the so-called replication fork) "stick out" from each chain. A special enzyme DNA polymerase begins to move along the free chain of DNA from the 5- to 3-end of the molecule, helping to join the free nucleotides continuously synthesized in the cell to the 3-end of the newly synthesized DNA chain.
As a result of replication, two new, absolutely identical DNA molecules are formed that are identical to the original DNA molecule before the beginning of its reduplication.
It can be said, somewhat simplifying that the phenomenon of exact doubling of the DNA molecule, which is based on the complementarity of the bases of this molecule, is the molecular basis of heredity.
The rate of DNA replication in humans is relatively low, and in order to ensure the replication of the DNA of any human chromosome, it would take weeks if replication started from a single point. In fact, in the DNA molecule of any chromosome, and each human chromosome contains only one DNA molecule, there are many places of initiation of replication( replicons).From each replicon, replication goes in both directions until the neighboring replicons merge. Therefore, DNA replication in each chromosome proceeds relatively quickly.
The concept of "genetic code"
For a molecule of heredity, which is DNA, not only is it itself capable of self-reproducing, it is only a part of heredity. DNA must somehow encode all the signs of the body. Most signs of any organism, unicellular or multicellular, are determined by proteins: enzymes, structural proteins, carrier proteins, protein channels, receptor proteins. Therefore, DNA must somehow encode the structure of proteins and the order of amino acids in them.
Amino acids are joined together by a peptide bond, which is formed by condensation of an amino group( NH2) of one amino acid with a carboxyl group( COOH) of another amino acid. The amino acid sequence in the polypeptide chain is recorded from an amino acid with a free NH2 group to an amino acid with a free COOH group.
Scientists have determined that the code is triplet, which means that each amino acid is encoded by a triple of nucleotides. Indeed, since 20 different amino acids are used to construct proteins, the code can not be a single nucleotide, since there are only 4 nucleotides. The code can not also be dinucleotide, since only 16 combinations of 2 nucleotides are possible. At 3 nucleotides, the number of combinations increases to 64, and this is quite enough to encode 20 different amino acids. In addition, it also follows from this that the genetic code must be degenerate, namely, one amino acid can be encoded by more than one triple of nucleotides. Another important property of the genetic code is that it is non-overlapping, with each successive new amino acid in the polypeptide chain coding sequentially a new DNA triplet. The genetic code does not contain punctuation marks, and the coding triplets follow one after another. The genetic code is universal and is used identically both by prokaryotes and eukaryotes. Coding triplets of nucleotides are called codons.
The most important are the first two nucleotides of each codon. The third nucleotide is nonspecific. Three codons determine the signal for stopping the synthesis of the polypeptide chain( termination of translation): UAA, UAG and CAA.This means that at the place of the information RNA( mRNA), where any of these codons are located, the synthesis of the polypeptide chain of the protein ceases. The codons indicating the termination of the synthesis of a polypeptide chain are called stop codons.
Information RNA and transcription process
It should be explained why it was so necessary to introduce the concept of information RNA.As is known, DNA is contained in the chromosomes of cells and, consequently, in the nucleus, and the protein is synthesized in the cytoplasm of cells. In order for information about the structure of the protein, written in the DNA language, to enter the cytoplasm of the cell, it is first rewritten( transcribed) into an mRNA molecule.
RNA differs from DNA in that in the RNA chain the sugar residue is represented by ribose( hence its name), thymine is replaced by uracil, which has approximately the same complementarity to adenine as thymine.
In order for the nucleotide sequence of the gene encoding the primary structure of a certain polypeptide chain of the protein to mRNA to be written off, a special enzyme, RNA polymerase, is attached to the DNA chain at a certain distance from the gene, to a special sequence of nucleotides called the promoter.
The starting point of transcription is the DNA base corresponding to the RNA base, which is first included in the transcript. Transcription of mRNA is continued until RNA polymerase II encounters a termination signal of transcription.
Biosynthesis of the polypeptide chain
In the polypeptide chain, the information encoded by the genetic code is decoded and the mRNA template is constructed on a polypeptide chain of a specific protein. In this process, two more types of RNA are involved: ribosomal( rRNA) and transport( tRNA).For both types of RNA, there are numerous genes in the genome, on the matrix of which these RNAs are synthesized.
The formation of a polypeptide chain from sequentially delivered tRNA mRNAs with the corresponding amino acids occurs on ribosomes.
The structure of the gene in higher organisms is quite complicated. It includes a promoter containing a transcription initiation site, exons and introns. Exons contain the coding sequences of the gene, the function of the introns remains unknown. On the border of exons and introns is a consensus sequence, which is recognized by splicing enzymes, i.e., by enzymes for excision of introns from the primary mRNA transcript. At the 3-end of the gene, there is already in the non-coding portion a site providing the addition of 100-200 adenine residues to mRNA to ensure its stability. The gene is characterized by the so-called open reading frame, that is, the presence of a sufficiently long sequence of triplets encoding amino acids not interrupted by stop codons or meaningless triplets.