All the cells in multicellular organisms which are capable of division go through a cell cycle – a sequence of phases which incorporates cell growth, DNA replication, and cell division by mitosis. However, remember that not all cells are capable of division e.g. red blood cells.
The Cell Cycle
The cell cycle begins just after mitosis with gap phase 1 (G1). During G1, the cell grows and synthesises new organelles and biological molecules such as proteins. At the end of G1 comes the G1 checkpoint. This is a chance for the cell to check that everything is going to plan. If it detects any DNA damage, finds that enzymes and proteins needed for DNA replication are missing, or is low on energy reserves, the cycle is stopped and the cell enters a resting state.
If everything is good, it carries on to the next stage in the cycle – the synthesis phase (S) – and becomes committed to division. During the S phase, the DNA in the cell is replicated. Take a look at the next section for more detail on exactly how that works. After the S phase comes gap phase 2 (G2). Here, the cell continues to grow and synthesise proteins needed for mitosis.
At the end of G2 there is another checkpoint (the G2 checkpoint) where the cell checks that the DNA has been replicated fully and without error. If the DNA passes quality checks, mitosis can begin. If the DNA is not up to scratch, repairs are attempted, or the cell will program its own death if the damage is irreversible.
We’ll take a detailed look at mitosis in the next article, but for now just note that there is another checkpoint during metaphase of mitosis, bringing the total number of checkpoints to three. Also note that G1, S and G2 combined are called interphase.
DNA replication is sometimes easy to confuse with transcription, but the key difference here is that we are synthesising two new strands of DNA rather than a strand of mRNA. DNA replication happens in the nucleus of a eukaryotic cell, or in the cytoplasm of a prokaryotic cell. Here is the process step by step:
- An enzyme called DNA helicase breaks the hydrogen bonds between the bases to separate the two DNA strands. Now both strands have exposed bases and can each act as a template strand.
- Free DNA nucleotides complementary base pair with the exposed bases and hydrogen bonds form.
- DNA polymerase (an enzyme) joins the nucleotides using condensation reactions to form phosphodiester bonds. This forms the sugar-phosphate backbones of the new strands. DNA polymerase can only add nucleotides at the 3′ end of a strand due to the specificity of its active site, so it synthesises DNA in the 5′ to 3′ direction.
- Each new DNA molecule has one original strand and one new strand – this is called semi-conservative replication.
Next, let’s take a look at the experiment carried out by Meselson and Stahl which provided evidence that DNA replication is semi-conservative. They used two isotopes of nitrogen (14N and 15N), two samples of bacteria, and a centrifuge.
- One sample of bacteria was grown in growth medium containing 14N (light nitrogen), and the other grown in growth medium containing 15N (heavy nitrogen). The bacteria used the nitrogen in the growth medium to synthesise nucleotides, which were incorporated into DNA as the bacteria grew and replicated.
- DNA was extracted from a sample of each of the bacteria, and each DNA sample spun in a centrifuge. DNA containing 14N settled near the top of the tube, whereas DNA containing 15N settled near the bottom of the tube (because it is heavier).
- The bacteria grown in 15N were transferred to growth medium containing 14N. So now they were incorporating 14N into any new DNA, but any old DNA would contain 15N.
- When the DNA was extracted and spun in a centrifuge, it settled in the middle of the tube. This showed that the DNA must have one strand containing 14N and one strand containing 15N. If DNA replication had been conservative, the scientists would have seen some DNA settle at the top of the tube and some at the bottom.
Of course, this experiment only provided evidence that DNA replication was semi-conservative in bacteria, but subsequent work showed that it is universal to all living organisms.
If you want to know even more detail about DNA replication, check out this great article.
We’ve covered a lot of material today to introduce the topic of cell division.
- The cell cycle consists of mitosis and interphase (gap phase 1, synthesis, and gap phase 2). DNA replication happens during the synthesis part of interphase.
- DNA replication is semi-conservative, meaning that the new molecules contain one old strand and one new strand.
- DNA helicase and DNA polymerase are two important enzymes in this process.
- Work by Meselson and Stahl using nitrogen isotopes provided evidence for semi-conservative replication.