9780198783688_Ch3

211

3.3 DNA replication

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Figure 3.12 Chromatin packing in eukaryotes. (a) Diagram of a nucleosome. (b) Beads on a string form, the 10 nm fibre. (c) A 30 nm fibre of chromatin. (d) Loops of the 30 nm fibre are attached to a central protein scaffold in a 360° array. Further looping and coiling then takes place to compact the chromatin into a chromosome. (e) and (f) Two views of the structure of the nucleosome core particle showing the DNA (ribbon structure) wrapped around the octamer histone core (space-filling model). Each histone protein is shown in a different colour, while DNA is grey. The histone proteins are tightly packed into the core of the octamer, as shown, but each histone also has a flexible ‘tail’ section that protrudes from the nucleosome. Only the tail of one of the histone H3 molecules is shown here. The flexible histone tails are involved in regulating transcription of genes, as discussed in Chapter 5. (g) Electron micrograph of a 30 nm fibre of chromatin. Source : (a–d) Snape, A. & Papachristodoulou, D. (2018). Biochemistry and Molecular Biology (6th ed.). Oxford University Press. (g) Lewin B. (1994). Genes V. Oxford University Press, Oxford. Photograph was provided by Prof. B. Hamkalo.

Replication is initiated at specific chromosome sequences For replication to occur the two strands of the DNA double helix must be separated. This does not occur simultaneously across the entire chromosome. Instead, the strands are initially separated at short sequences called replication origins , which are typically rich in A:T base pairs. Specific proteins aid in the process, which requires energy from ATP hydrolysis. Enzymes that unwind the DNA double helix are called helicases .

PAUSE AND THINK How does the presence of multiple A:T base pairs at replication origins aid the process of initiation?

ing A:T rather than G:C base pairs means it is easier to separate the two strands of DNA, as required for replication.

Answer: As A is linked to T by two hydrogen bonds rather than three, hav

© Oxford University Press