In prophase, chromatin material condenses into chromosomes. Chemical analysis of chromatin shows that it is composed primarily of Deoxyribonucleic acid (DNA) and proteins. These are organic molecules.
DNA is the genetic material. It is a long molecule about 2 nm thick. It runs continuously within each chromosome. Genes are short lengths of DNA in chromosomes. DNA is acidic in nature.
Chemical composition of DNA
DNA is a complex macromolecule. Macromolecules are very large molecules. It is composed of repeating sub-units called nucleotides. Nucleotides are joined together in long chains called polynucleotide. The nucleotides of DNA are deoxyribonucleotides. Each deoxyribonucleotides is made up of: (Fig 17.4a)
1. A phosphate group,
1. A deoxyribose sugar, and
1. A nitrogenous base.
There are four different nitrogenous bases: Adenine (A), Guanine (G), Thymine (T) and Cytosine (C).
Adenine and Guanine are larger double ring bases, called Purines. Cytosine and Thymine are smaller single ring bases called Pyrimidines. As each nucleotide is named by the base it contains, there are four different types of nucleitides; Adenine nucleotide, Guanine nucleotide, Thymine nucleotide and Cytosine nucleotide
Many of these four types of nucleotides join together to form polynucleotide chain of DNA molecule (fig 17.4c).
Structure of DNA – Watson and Crick’s model
In 1953, James Watson and Francis Crick gave the model for the structure of DNA. They proposed that DNA molecule is made up of two polynucleotide strands, which are twisted around each other in the form of a double helix, The double helix looks like a spiral staircase of uniform diameter(Fig. 17.5).
The railings of this staircase are composed of deoxyribose sugars and phosphates, while the steps are made of pairs of nitrogenous bases. Each step consists of a purine paired with a pyrimidine. Their Pairing is highly specific. Adenine always pairs with thymine, and guanine always pairs with cytosine. Two hydrogen bonds are formed between A and T, while three hydrogen bonds are formed between G and C.
A = T, G = C
Nitrogen base pairing between adenine and cytosine or between guanine and thymine is not possible because such hydrogen bonds cannot be formed. The two polynucleotide strands are held together in their helical configuration by these hydrogen bonds between bases in opposing strands. The base pairs in DNA are 0.34 nm apart, and each turn of the double helix has 10 base pairs. The sugar phosphate backbones of the two complementary strands are anti – parallel
due to opposite chemical polarity.
Significance of Base Sequence
The two strands of a DNA double helix are complementary to each other. If the sequence of bases in one strand is known, the sequence of bases in the other strand will be automatically known due to specific base-pairing. This property of the two strands of the double helix makes DNA a unique molecule best suited to store, copy and transmit genetic information from generation to generation. Genetic information is nothing else but this base sequence of DNA in a linear order. All sorts of coded biological information is stored in the form of this base sequence. Genes are parts of DNA comprising the base sequences. A change in the base sequence is called point mutation; due to which mutated gene becomes different in behavior form its parental gene.
CTT Point Mutation CAT
Activity No.1: Study of Base Sequence in Structure of DNA
The following is a DNA molecule (Fig 17.6). Carefully looks at its base sequence on one strand and constructs the complete molecule.
Many base-pair sequences in your DNA are identical to those of your parent’s DNA. Despite this striking similarity, much of your DNA is different from theirs due to mutation or genetic recombination. This makes you and your DNA very unique.
“DNA stores genetic information in the sequence of its bases in a linear order”
Replication of DNA
The human genome contains about 6 x 10 9 nucleotide pairs of DNA. All this genetic information must be duplicated immediately before each cell division, so that each daughter cell must have identical information like the parent cell. DNA is the heredity material only because it is capable of self- replication (Fig 17.7).
Watson and crick proposed mechanism of DNA replication based on complementary base – pairing. The DNA molecule begins to unwind at one end. It unzips like a zipper. The hydrogen bonds between base pairs are broken, and the two complementary strands of DNA double helix separate like two halves of an open zipper. A separates from T, and G separates from C. the bases along each parental strand are exposed free. Each strand of the parent DNA now serves as template. Its exposed bases can be paired by fresh proper partners. All four types of deoxyribonucleotides are floating in nucleoplasm. Their bases are attracted by complementary bases of each exposed strand. The incoming nucleotides pair their complementary bases on the template.
Each daughter DNA molecule contains one old parental strand and one new strand. That is why this replication is semiconservative – half new, half old.”
A complete complementary new strand is formed opposite to each old parental strand. The new strand id exactly like the one that had been separated. Thus two identical double helix molecules are formed from parental DNA molecule.
“DNA polymerase enzyme helps in binding the fresh nucleotides to the template strand.”
How do Genes Function? How is a Phenotype Produced?:
Every phenotype is based on specific biochemical molecules, e.g.; proteins. These proteins are made in the cells under the instructions of genes. Genes sit like a ‘Boss’ seated comfortably on chromosomes in their office – the nucleus. They get their job done in cytoplasm through their managers – RNAs.
R N A
RNA is ribonucleic acid (Fig 17.8). It is also a polynucleotide but its basic unit is ribonucleotide. A ribonucleotide consists of a phosphate group, Ribose sugar and a nitrogenous base. Its four bases are adenine (A), guanine (G), cytosine (C) and Uracil (U). DNA controls synthesis of RNA.
“RNA is slightly different from DNA:
1. RNA has ribose instead of deoxyribose
2. RNA has uracil instead of thymine
3. RNA is a single polynucleotide strand, but DNA is made up of two polynucleotide strands.
4. RNA is found throughout the cell but DNA is concentrated in the nucleus.
5. RNA is of three types: messenger RNA (mRNA) transfer RNA (tRNA) and ribosomal RNA (rRNA) but DNA has no such types.”
A gene expresses itself in two steps (Fig 17.9):
1. Transcription: The coded information in DNA, which is there in the form of base sequence, is copied into the base sequence of messenger RNA.
“Copying of DNA – encoded information into RNA – encoded information is called transcription. Translation is decoding of mRNA information into a sequence of amino acid in a protein.”
2. Translation: The messenger RNA’s message is for the protein synthesis. Its base sequence is translated into a sequence of amino acids of a particular protein.