Читать книгу Cell Biology - Stephen R. Bolsover - Страница 111

1 During replication each parent DNA strand acts as the template for the synthesis of a new daughter strand. The base sequence of the newly synthesized strand is complementary to that of the template strand.

2 Replication starts at specific sequences called origins of replication. The two strands untwist and form the replication fork. Helicase enzymes unwind the double helix, and single‐stranded DNA‐binding proteins keep it unwound during replication. In prokaryotes, DNA polymerase III synthesizes the leading strand continuously in the 5′ to 3′ direction. The lagging strand is made discontinuously in short pieces in the 5′ to 3′ direction. These are joined together by DNA ligase. DNA polymerase is a self‐correcting enzyme. It can remove an incorrect base using its 3′‐ to 5′‐exonuclease activity and then replace it with the correct base.

3 DNA repair enzymes can correct mutations. Uracil in DNA, resulting from the spontaneous deamination of cytosine, is removed by uracil – DNA glycosidase. The depyrimidinated sugar is cleaved from the sugar‐phosphate backbone by AP endonuclease, and DNA polymerase then inserts the correct nucleotide. The phosphodiester bond is reformed by DNA ligase.

4 In eukaryotes protein‐coding genes are split into exons and introns. Only exons code for protein. The human genome has a large amount of DNA whose function is not obvious. This includes much repetitious DNA, whose sequence is multiplied many times.

5 Protein‐coding genes may be found in repeated groups of slightly diverging structure called gene families, either close together or scattered over the genome. Some of the family members have lost the ability to operate – they are pseudogenes.