The Cell Cycle

Concept 8: The Cell Cycle & DNA Replication

Success Criteria & Vocabulary

Click this drop-down menu to see the Success Criteria.

  • Communicate the key stages of the cell cycle in words and a diagram.

  • Discuss the importance of DNA replication to the overall functioning of the cell and organisms (SA : V or cell type).

  • Discuss DNA replication in terms of where, when, and how it occurs.

  • Discuss the factors affecting the rate of DNA replication (relating to enzymes).

Click this drop-down menu to see the list of Vocabulary.

Chromosome: DNA in its compacted, conspicuous form. Only visible during mitosis.

DNA polymerase: Enzyme that fully extends and completes the daughter strands.

DNA replication: Process in which DNA is duplicated.

G1 phase: Part of interphase during the cell cycle where cells grow by producing more proteins and organelles.

G2 phase: Part of interphase during the cell cycle where cells continue to grow, and check for any errors in DNA.

Helicase: Enzyme that breaks hydrogen bonds to unzip DNA double helix during DNA replication.

Hydrogen bond: Weak bond between a hydrogen atom of one amino acid and an oxygen atom of another amino acid that contributes to the 3D shape of an enzyme/protein.

Lagging strand: DNA daughter strand that is built in fragments.

Leading strand: DNA daughter strand that is built continuously.

Ligase: Enzyme that forms the sugar-phosphate backbone between new nucleotides of the daughter strand.

Okazaki fragment: Small fragments of DNA produced on the lagging strand during DNA replication.

Primase: Enzyme that starts the daughter strands during DNA replication.

S phase: Part of interphase during the cell cycle where DNA replication occurs.

SA : V ratio: Ratio of the cell’s surface area and its volume.

Semi-conservative: Term that describes how DNA replication produces DNA consisting of one parent strand and one daughter strand.


Concept 8: Support Notes

Why do cells Divide?

Most somatic cells (so normal body cells, cells that aren’t eggs and sperm), need to be replaced and renewed constantly.

So for example, red blood cells only live for a three months, and so they need to be replaced to keep carrying oxygen around your body. Muscle cells can get damaged when you exercise, so they need to be replaced for your muscles to get bigger and stronger.

So the somatic cells in our body carry out their specialised tasks, and then when the time is right, they get ready to replicate and divide.

Cells divide when the distance between the cell membrane and centre of the cell becomes too large for diffusion to occur fast enough.

Therefore, cells divide to have a high SA : V ratio. This enables efficient chemical reactions.

The Cell Cycle has 2 Stages

The Cell Cycle

The cell cycle is the term given to the life-cycle of a cell. It’s an ordered sequence of events that occur in a cell as it prepares for cell division and duplication. There are 2 main stages in the cell cycle. These are interphase and the MITOSIS phase.

Interphase (G1, S, G2)

The cell spends the majority of its time in the interphase. Interphase can be further divided into 3 sub-phases.

The first growth phase is called the G1 phase. During this time, the cell carries out its functional role. This includes:

  • Carrying out metabolic processes (still making ATP etc).

  • Synthesising proteins (still making enzymes)

  • Replicating organelles.

Once a cell has replicated its organelles, it needs to replicate its DNA. This brings the cell to the synthesis phase (S) of its life. During this time, the DNA within the nucleus is replicated to create 2 identical copies of the genome. This is so that when the cell divides, each daughter cell will gain 1 copy of the genome each. The two daughter cells will therefore be genetically identical.

Once the cell has replicated its DNA, it carries out its final jobs to prepare the cell for division in the second growth phase (called G2). The cell synthesises the necessary proteins, and replicates more organelles in preparation for the mitosis phase. The chromosomes are also checked for errors and any required repairs to the DNA are made. This signifies the end of the interphase stage and the cell is now ready to divide.

Mitosis (M) Phase

The mitosis phase results in the division of the cell. At the end of this mitosis phase, the cell has divided to form two new cells, and the cell cycle will start again. The length of the cell cycle or the lifespan of a cell varies with different types of cells. For example, the cells lining the small intestine have a life-span of 2-4 days which means it’s cell cycle lasts 2-4 days, whereas red blood live for 3-4 months, which means its cell cycle lasts for 3-4 months!

Changes in cell volume during the cell cycle.

Graph A shows the changes in cell volume over the cell cycle. Through the three phases of interphase (G1, S, and G2), the volume increases. And then when the cell undergoes mitosis, the cell volume drops back to the original size. Each resulting daughter cell will be smaller and have an increased SA : V ratio.

Changes in quantity of DNA during the cell cycle.

The quantity of DNA also changes throughout the cell cycle. A cell that enters the G1 phase has 1 copy of the full genome in its nucleus. But then during the S phase, DNA gets replicated to create 2 copies of all the genetic material within the cell. So in the G2 phase, the DNA quantity has doubled. But when the cell divides, in the M phase, the quantity of DNA halves again (goes back to what it was in G1) as each daughter cell gets 1 copy of DNA each.

Steps in DNA Replication

DNA must make a copy of itself during the S phase of INTERPHASE, before the cell divides (M phase). This is because each daughter cell MUST be identical (have identical DNA).

Takes place in the nucleus.

DNA REPLICATION is a SEMI-CONSERVATIVE process. This is because each new molecule of DNA contains one strand that is originally from the parent molecule (is conserved), and one new strand. In other words, each new DNA double helix has one original template strand (blue) and one new daughter strand.

There are 5 steps during DNA replication that you must learn.

Excellent video explaining how DNA replication happens.

Step 1: Unwinding the double helix

An enzyme found in the nucleus (called HELICASE) unwinds the double helix of the DNA molecule. It does this by breaking the HYDROGEN BONDS between the nitrogen bases. The point of unwinding the DNA helix, is to expose the nitrogen bases, to expose the As, Ts, Cs, and Gs. The exposed strands are called template strands.

Step 2: Complementary Base Pairing

The daughter strands are started by an enzyme called PRIMASE, as it matches free nucleotides to the exposed bases on the template strands according to the complementary base pairing rules. Remember, A binds with T, and C binds with G.

Step 3: Extending the Daughter Strands

Another enzyme called DNA POLYMERASE fully extends and completes the daughter strands. The daughter strands are ALWAYS built in the 5’ to 3’ direction.

Therefore, the two new daughter strands of DNA get built in opposite directions to each other. One strand is called the leading strand and the other is called the lagging strand.

Another enzyme called LIGASE forms the sugar-phosphate backbone between new nucleotides of the daughter strand.

What’s the difference between leading and lagging daughter strands?

The LEADING STRAND is built continuously in the 5’ to 3’ direction. In the diagram below, the green arrow pointing to the left shows the direction of replication for the leading strand.

Whereas the LAGGING STRAND has to be built in fragments in the 5’ to 3’ direction. This is because only a small part of the template strand is exposed at a time. The small segments of new DNA are called OKAZAKI FRAGMENTS. The Okazaki fragments need to be connected together by an enzyme called LIGASE.

Step 4: Two identical DNA molecules

DNA replication on both the lagging and the leading strand continues until two new molecules of DNA are produced.

Each resulting DNA molecule has one strand that is from the original molecules, and one new strand of DNA (remember, SEMI-CONSERVATIVE REPLICATION). Both resulting DNA molecules are identical to the original DNA molecule.

Step 5: DNA Condenses into Chromosomes, in preparation for Cell Division.

Prior to the cell dividing, the two identical copies of the DNA molecule will coil and supercoil to form a replicated CHROMOSOME. So when the cell divides, each new daughter cell will have 1 copy of each chromosome and be genetically identical.

Video: Molecular visualisations of DNA I think this is a really cool video of how DNA is packaged, the difference between DNA, chromatin, and chromosomes. Highly recommended!

The Cell Cycle (and Cancers. Good introduction to the cell cycle. Names of specific stages not required. Details about Cdk and cyclin proteins are not required at all).

DNA Replication (Updated) (Amoeba Sisters) (Great overview of DNA replication and role of enzymes. You do not have to know the name of enzymes, but need to know their function/role).

Enzyme Function in DNA Replication

ENZYMES are involved in each step of DNA REPLICATION. During DNA replication:

  1. A specific enzyme (HELICASE) unwinds the DNA double helix, exposing nucleotide bases.

  2. Another enzyme (PRIMASE) starts to add free nucleotide bases to create the daughter strands.

  3. Another enzyme (DNA POLYMERASE) uses the bases of the parent template strand as a complementary template, matching bases by the complementary base pairing rule (i.e. A with T, C with G).

  4. Another enzyme (LIGASE) forms the sugar-phosphate backbone between nucleotides of the daughter strand.

During DNA replication, enzymes function using the INDUCED FIT MODEL. The active site continues to change until the SUBSTRATE is completely bound to it.

For efficient replication, factors must be at the optimum level for that enzyme.

  • Temperature

  • pH

  • Substrate concentration (e.g. deoxynucleotides)

  • CO-FACTORS (e.g. Magnesium ions are really important for DNA replication)

  • Enzyme concentration

  • Poisons/INHIBITORS

Featured Student Questions

Student's question:

In the Critical Thinking Booklet on pg 2, one of the definitions is "When a solute is more concentrated in one area compared to another". Now, I'm pretty sure the key word is hypertonic but I'm not 100% sure. Is the word "area" referring to the solution?

In your third video, you mentioned the cell's extracellular and intracellular environment. Can you please clarify what this means?

Ms. Adviento's answer:

Good question. The definition is just talking about one solute, and it is not a comparison of two solutes. An area of this ONE solute is more concentrated than a separate area of the SAME solute. Therefore, it’s talking about the concentration gradient within that solute.

In terms of tonicity (hyper, hypo, iso-tonic), it is a term(s) that compares TWO solutions. One solution is the solution INSIDE the cell, and can be called the intracellular environment. It’s like the cytosol and all the solutes within it. The other solution is the solution OUTSIDE the cell, and can be called the extracellular environment.

Student's question:

Do enzymes undergo DNA replication? If so, do other enzymes live within the enzyme?

Ms. Adviento's answer:

DNA replication is the process of DNA copying itself to make 2 copies. This MUST happen before a cell can divide into 2 copies. A cell MUST divide to maintain a low SA : V ratio or to replace cells that die off for whatever reason.

Enzymes are not DNA molecules. Enzymes are proteins. Proteins are made from the instructions in genes. Genes are functional sections of DNA – it is the sequence of DNA in genes that instruct cells how to make enzymes. This is a major topic for the 2.7 External.

So, DNA is not an enzyme, an enzyme is NOT a DNA molecule but they are very much related to each other.

Student's question:

In Video 8, you said that the "leading" strand and the "lagging" strand are built from 5' to 3'.

However, on Google, the "leading" strand is described as being built from 3' to 5'. The "lagging" strand is described as being built from 5' to 3' (which agrees with your statement).

I was wondering if the "leading" strand is described as being built from 3' to 5' because it is built towards the replication fork (despite being 5' to 3'), that is, the template strands are always read from 3' to 5'. I will really appreciate it if you can clarify this for me. I have attached a picture which should help you understand what I mean.

Ms. Adviento's answer:

The picture you have sent me corroborates what I said in the video – template DNA always read by the enzyme in the 3’ to 5’ direction, new DNA (BOTH leading and lagging strands) synthesised in the 5’ to 3’ direction. Just look at the dark blue arrows; BOTH arrows point from 5’ to 3’.

We talk about 5’ to 3’ or 3’ to 5’ directionality in terms of DNA polymerase reading it or making it. It’s simple because these rules ALWAYS apply:

  • (template) DNA is read in the 3’ to 5’ direction.

  • (new) DNA is made/synthesised in the 5’ to 3’ direction.

…regardless of whether it is the leading or lagging strand, or whether it is RNA (you’ll learn about RNA in 2.7 topic). Why? It’s because enzymes can only read in 3’ to 5’ direction, and they can only build in the 5’ to 3’ direction.

Student's question:

Where do the 'free nucleotides' come from?

Ms. Adviento's answer:

Free nucleotides come from the nucleus - they are 'free' because they are freely floating in the nucleus. When enzymes need them to create a new strand of DNA molecule during DNA replication, the correct nucleotide (A, T, C, or G) will fit into the active site of the enzyme and get added to the DNA strand.

Concept 9: The Cell Cycle & Mitosis

Success Criteria & Vocabulary

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  • I can describe mitosis and its purpose.

  • I can communicate the key stages of mitosis.

  • I can explain how the type of cell affects the rate of mitosis.

  • I can explain the factors that affect the rate of mitosis.

Click this drop-down menu to see the list of Vocabulary.

Anaphase: Third phase of mitosis where spindle fibres pull and separate sister chromatids.

Centromere: Region of a chromosome where the two sister chromatids attach.

Chromatin: DNA in its loose, inconspicuous form. Visible during the interphase of the cell cycle.

Chromosome: DNA in its compacted, conspicuous form. Only visible during mitosis.

Cytokinesis: Fifth phase of mitosis where the cell membrane pinches and buds off into two identical daughter cells,

DNA replication: Process in which DNA is duplicated.

Metaphase: Second phase of mitosis where chromosomes line up at the metaphase plate.

Metaphase plate/equator: Imaginary line in the middle of the cell, where chromosomes line up during metaphase.

Mitosis: Type of cell division that only occurs in somatic cells, resulting in two identical daughter cells.

Prophase: First phase of mitosis, where chromatin condenses into chromosomes and nuclear membrane breaks down.

SA : V ratio: Ratio of the cell’s surface area and its volume.

Sister chromatid: Replicated forms of a chromosome joined together by the centromere and eventually separated during mitosis or meiosis II.

Spindle fibres: Protein threads produced by centrioles/centrosomes during mitosis to pull chromatids apart.

Telophase: Fourth phase of mitosis where nuclear membrane re-forms, and chromosomes decondense into chromatin.


Concept 9: The Cell Cycle & Mitosis

MITOSIS is the type of cell division that occurs only in SOMATIC CELLS, where a parent cell divides to form two genetically identical daughter cells.

It’s needed for:


Growth of the organism (especially during germination from seed or when plants are young, or when animals grow from just a tiny embryo to an organism with trillions of cells).


Repair of Injuries such as cuts, grazes, burns, and broken bones.


Replacement of old/damaged cells with new cells. Scientists are not sure why exactly, but older cells just don’t work as well as young cells. For this reason, our bodies are constantly killing off old cells and replacing them with new ones. This keeps the tissues within our organs in the best condition possible.

Asexual reproduction

For example, bacteria reproduce by replicating its DNA and undergoing mitosis.

Maintaining a large SA : V ratio.

When a cell grows too large, the SA : V ratio decreases, which also decreases the rate of cell transport processes like diffusion and osmosis. So to maintain a high SA : V ratio, cells divide into two.

Can you determine which stage of mitosis each of these cells are in?

Preparatory stage: Interphase

Before mitosis starts, the parent cell has to replicate its DNA so that the parent cell has two copies of its DNA. This is important because both daughter cells need a full copy of the DNA to function properly.

During this stage, the DNA in a cell is stored as long, loose strands called CHROMATIN that are not easily visible. So chromatin is DNA but in long, loose strand form.

How DNA replication allows mitosis to occur.

It’s important for you to understand why DNA REPLICATION absolutely needs to happen before mitosis. A full set of DNA, a full set of 23 pairs of CHROMOSOME needs to be passed onto daughter cells, so that the new daughter cells carry out the same function as parent cells.

If DNA fails to replicate fast enough or if DNA fails to replicate at all, the cell will recognised this problem in the G2 part of the Interphase.

Remember that the G2 phase is the part of the cell cycle where cells check for errors before mitosis. Failure to replicate DNA will stop the cell cycle until the cell fixes the issue, or the cell will decide that the issue is beyond repair and it will self-destruct.

Either way, growth and repair of an organism may be seriously affected.

Mitosis stage 1: PROPHASE

During the first step of mitosis, CHROMATIN (the long loose strands of DNA) condense into short, thick structures called CHROMOSOMES. The identical strands of DNA called SISTER CHROMATIDS stay joined together at a point called the CENTROMERE, forming an X shape. This is so that the sister chromatids are kept together and can move to where they need to go together.

Condensing the long strands of chromatin into shorter chromosomes helps keep the DNA safe and organised as the cell goes through the stages of mitosis. If DNA stayed in long chromatin structures, it would get all tangled up, and DNA wouldn’t be able to move around to where it needs to go during mitosis.

Also during this first stage of mitosis, the nuclear membrane breaks down and disappears.

Mitosis stage 2: METAPHASE

In the second stage of mitosis, SPINDLE FIBRES, which are these long proteins in dark blue, extend from small organelles called centrioles (orange).

These spindle fibres attach to the CENTROMERE part of the chromosomes and move the CHROMOSOMES around so that they line up along the cell equator. The cell equator or the METAPHASE PLATE is just the midline or the invisible line down the middle of a cell.

Mitosis stage 3: ANAPHASE

In the 3rd stage of mitosis, the CENTROMERES keeping the SISTER CHROMATIDS together releases, so the sister chromatids are separated from each other.

The SPINDLE FIBRES pull these strands towards the poles of the cell. During this stage, the cell itself also begins to elongate.

Mitosis stage 4: TELOPHASE

During the 4th stage of mitosis, the SPINDLE FIBRES break down and release the CHROMOSOMES.

A new nuclear membrane forms around each set of chromosomes while they decondense back into long, loose strands of CHROMATIN.

Mitosis stage 5: CYTOKINESIS

The cell membrane pinches in the middle and buds off into two separate cells. After this, one cell has finally become two daughter cells. This is the end of mitosis.

Video: Mitosis [Updated] (Amoeba Sisters) Good overview of mitosis, number of chromosomes, different stages of cell division. You do not have to know the names of the stages).

Video: Becoming (Aeon). Awesome time-lapse video of how a salamander undergoes mitosis many many times to grow from a single cell to an organism with many cells.

Video: Mitosis vs. Meiosis: Side by Side Comparison (Amoeba Sisters) Good video to help you understand the difference between mitosis and meiosis. You’ll never be asked to compare mitosis and meiosis.)

Video: Chromosome Numbers During Division (Amoeba Sisters) (Thoroughly explains the number of chromosomes after DNA replication, during mitosis and meiosis).

Comics Corner!