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3.3 DNA replication

SUMMARY OF KEY CONCEPTS

■ All cellular organisms have a genome that is made up of DNA. ■ DNA and RNA are polymers of nucleotides (deoxyribose or ribose sugar, base and phosphate) that are linked to each other by phosphodiester bonds. ■ The four bases in DNA are adenine, cytosine, guanine, and thymine (A, C, G, and T), while in RNA uracil (U) replaces thymine. ■ DNA forms a double helix of two antiparallel strands, in which the sugar phosphate backbone gives 5 ′ to 3 ′ polarity, and complementary bases are paired by hydrogen bonds (A:T and G:C). ■ Prokaryotes have a single circular double-stranded DNA chro mosome, while eukaryotic chromosomes are long linear double stranded DNA molecules, which are packaged with histones and other proteins to form compact chromatin. ■ Eukaryotic genes are split into non-coding introns and coding exons. Exons make up only around 1.6 per cent of the human genome sequence. ■ Around 25 per cent of the human genome sequence consists of introns and exons—perhaps 21,000 protein-coding genes. The rest consists of a variety of non-coding sequences, some regula tory, much of viral origin, and much of it repetitive and of unknown function.

■ DNA replication is semi-conservative: each strand acts as a tem plate for polymerization of complementary nucleotides (cata lysed by DNA polymerase) so that two identical molecules are made simultaneously at each replication fork. ■ The main polymerase enzymes in prokaryotic DNA replication are DNA Pol III and DNA Pol I. DNA Pol I plays a specific role in replacing RNA primers, which are necessary to start off the new strands. ■ New DNA strands are synthesized in the 5 ′ to 3 ′ direction, so at each replication fork the leading strand is made continuously and the lagging strand is made discontinuously as Okazaki frag ments, which are then linked together by DNA ligase. ■ DNA replication is extremely accurate, and fidelity is improved by proofreading. Incorrectly incorporated nucleotides may also be removed by mismatch repair, to achieve a final error rate of less than or equal to one in 10 10 . ■ DNA replication, proofreading, and mismatch repair processes are essentially similar in prokaryotes and eukaryotes, with dif ferences in the detail. ■ DNA that is damaged can be repaired by direct repair and exci sion repair mechanisms. Double-stranded breaks in DNA can also be repaired but the repairs are error prone. Defects in DNA repair can lead to cancer.

Use the flashcards in the e-book to test your recall of key terms introduced in this chapter.

QUESTIONS

Looking for answers? Once you’ve answered these questions, follow the link in the e-book to the answer guidance and check your work.

Concepts and definitions 1. Name the bases found in DNA and RNA that are purines, and those that are pyrimidines. 2. The sequence of one strand of a DNA molecule is: ¢- - ¢ 5 3 ATTGATCCGTGA What would be the sequence of the complementary strand in the double helix, and how many full turns of the double helix would this sequence make?

3. Approximately how many nucleosomes would be formed by a DNA molecule that is 2000 base pairs in length? Assume the linker sequence between nucleosomes is 35 base pairs. 4. Which enzyme is responsible for each of the following roles in prokaryotic DNA replication? (a) Unwinding the two strands of the DNA double helix.

(b) Synthesis of the RNA primer. (c) Synthesis of the leading strand.

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