Читать книгу Cell Biology - Stephen R. Bolsover - Страница 83

1 DNA, the cell's database, contains the genetic information necessary to encode RNA and protein.

2 The information is stored in the sequence of four bases. These are the purines – guanine and adenine – and the pyrimidines – thymine and cytosine. Each base is attached to the l′‐carbon atom of the sugar deoxyribose. A phosphate group is attached to the 5′‐carbon atom of the sugar. The base + sugar + phosphate is called a nucleotide.

3 The enzyme DNA polymerase joins nucleotides together by forming a phosphodiester bond between the 5′‐phosphate group of one nucleotide and the hydroxyl group on the 3′ carbon of deoxyribose of another. This gives rise to the sugar‐phosphate backbone structure of DNA.

4 The two strands of DNA are held together in an antiparallel double‐helical structure because guanine hydrogen bonds with cytosine and adenine hydrogen bonds with thymine. This means that if the sequence of one strand is known, that of the other can be inferred. The two strands are complementary in sequence.

5 DNA binds to histone and nonhistone proteins to form chromatin. DNA is wrapped around histones to form a nucleosome structure. This is then folded again and again. This packaging compresses the DNA molecule to a size that fits into the cell.

6 The genetic code specifies the sequence of amino acids in a polypeptide. The code is transferred from DNA to mRNA and is read in groups of three bases (a codon) during protein synthesis. There are 64 codons; 61 specify an amino acid and 3 are the stop signals for protein synthesis.