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3 Information: genes and genomes

(a)

(b)

N

Interphase nucleus

Mitotic chromosomes

Dark staining heterochromatin

Light staining euchromatin

Figure 3.13 Different degrees of chromatin packing. (a) Light micrograph of onion root cells. Individual chromosomes are not visible in the interphase nucleus but can be seen in the cell that is undergoing mitotic cell division, due to compaction of the chromatin. (b) Transmission electron micrograph of an interphase nucleus, showing dark-staining compact heterochromatin and light-staining, more diffuse euchromatin. The dark structure marked ‘N’ is the nucleolus. (We discuss the nucleolus in more detail in Chapter 9.) Source : (a) agefotostock/Alamy Stock Photo. (b) Don Fawcett/Science Photo Library.

The separation of the two strands of DNA causes the appearance of a ‘bubble’ in the chromosome, with a structure at each end that is termed the replication fork . As replication proceeds the two forks move away from each other as shown in Figure 3.15. The circular chromosome of the bacterial species Escherichia coli ( E. coli ) with approximately 4.6 million base pairs, contains only one origin of replication. The two replication forks synthesize DNA at a maximum rate of around 1000 base pairs per second, with separation of parental DNA and synthesis of new DNA occur ring at the same time. The two forks meet at the opposite side of the circle (Figure 3.15a). Eukaryotic chromosomes are linear and longer than the E. coli chromosome. In eukaryotes the rate of DNA synthesis is slow er than in prokaryotes, perhaps because of their more complex chromatin structure. Rates vary but are estimated to be 4–40 base pairs per second. Typically, the replication of an animal cell genome takes around 8 hours, and this could not be achieved if each chromosome had only a single origin of replication. Therefore, multiple origins are spaced at intervals along the chromosome (Figure 3.15b).

REPLICATION

REPLICATION

REPLICATION

PAUSE AND THINK Human chromosome 1 consists of around 249 million base pairs. Assuming a replication rate of 40 base pairs per second, how many hours would it take to completely replicate the chromosome if there were only one repli cation origin placed centrally in the chromosome?

Figure 3.14 DNA replication is semi-conservative. Each of the two strands of the double helix acts as a template on which a new strand is synthesized, so the ‘daughter’ DNA molecules consist of one strand conserved from the previous generation and one newly made strand. Source : Cooper. The Cell , 8th edn. Oxford University Press/Sinauer Associates.

Answer: 1729.16 hours.

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