Week 7 - Cell Divisions

Why is mitosis important?

It is important in eukaryotes, this is how they pass DNA information from cell to cell (in cell division).

How do eukaryotes reshuffle genetic information between each generation? Why is this important?

Through meiosis.

Meiosis produces genetically different offspring to the parent.

This produces genetic variety, speeding up evolution.

How do eukaryotes transfer genetic information?

• Mitosis: transfers genetic information between cells within an organism (growth, tissue maintenance, asexual reproduction).

• Meiosis: transfers genetic information between generations (produces gametes with half the chromosome number).

Describe the 2 distinct phases of the eukaryotic cell cycle

1. Interphase (which includes G1, S and G2 phase of the cell cycle) (In this period chromosomes are not visible in the nucleus).

2. Mitosis/Cytokenesis (M phase) (PMAT. This is the diving phase)

Describe the 3 distinct phases of interphase, what goes on in each?

G1: Cell grows and contributes to development, metabolism, behaviour, etc.
S: DNA is replicated

G2: preparation for mitosis.

What are homologous chromosomes?

Pairs of chromosomes in diploid organisms, with one chromosome inherited from each parent.

What holds sister chromatids together?

Cohesin proteins at the centromere.

How are homologous chromosomes different from sister chromatids?

Homologous chromosomes are similar but not identical and come from each parent; sister chromatids are identical copies formed during DNA replication.

What is the difference between sister chromatids and homologous pairs?

Sister chromatids are identical copies of a chromosome produced during DNA replication and joined at the centromere. Homologous pairs are two similar (but not identical) chromosomes—one from each parent—that carry the same kinds of genes but may have different alleles.

Describe 4 the steps of mitosis

Prophase: Chromatin condenses into visible chromosomes, each made of two sister chromatids joined at the centromere. The nucleolus disappears. The mitotic spindle begins to form as centrosomes move to opposite sides of the cell. The nuclear envelope starts to break down (this completes in prometaphase). Microtubules begin extending outward to prepare for chromosome attachment.

Metaphase: Homologous chromomses are randomly aligned on the metaphase plate at the equator of the cell. Each chromosome has microtubules from opposite spindle poles attached to the kinetochores of the sister chromatids.
Anaphase: The sister chromatids are pulled to opposite spindle poles (the cohesions are removed)

Telophase: Chromosomes arrive at opposite poles of the cell. Chromosomes begin to de-condense back into loosely packed chromatin. The nuclear envelope re-forms around each set of chromosomes, creating two nuclei. The nucleolus reappears in each new nucleus. The mitotic spindle breaks down. Cytokinesis usually begins during telophase, completing cell division.

Which 3 mechanisms act to move sister chromatids apart in anaphase?

1. Tubulin depolymerizes at the kinetochore, shortening the microtubule and pulling the chromatid toward the pole.

2. Kinetochore motor proteins “walk” the chromatid along the microtubule toward the centrosome.

3. Spindle microtubules slide past each other, powered by motor proteins, pushing the poles farther apart and increasing the separation of chromatids.

How does cytokinesis differ between animal and plant cells?

In animals:

• A contractile ring of actin filaments forms around the cell’s circumference.

• The ring tightens, creating a cleavage furrow.

• The cell is pinched into two daughter cells, each with one nucleus.

In plants:

• A rigid cell wall prevents cleavage.

• Golgi-derived vesicles move to the center of the cell along microtubules.

• Vesicles fuse to form a cell plate, which becomes a new membrane-bound compartment.

• A new cell wall forms inside the cell plate, separating the two daughter cells.

Why are cells cycle checkpoints needed?

To prevent uncontrolled divisions.

Ensure the process is error free.

Where are cell cycle checkpoints needed specifically?

They are particularly important in multicellular organisms, where unchecked divisions can lead to cancer.

What are the 3 checkpoints in mitosis and where are they found?

1. G1 checkpoint: Within the G1 phase.

2. G2 checkpoint: Between the G2 and the M phase.

3. M checkpoint: Within the M phase.

Describe how the G1 checkpoint works and how it is passed.

• The G1 checkpoint is the most critical checkpoint in the cell cycle.

• Whether a cell passes this checkpoint depends on external conditions (environment, nutrients) and internal signals(DNA damage status).

• If the cell passes the G1 checkpoint, it commits to DNA replication (S phase) and will proceed through at least one full cell division.

• If the cell does not pass, it exits the cycle and enters G0, a non-dividing, differentiated state. This can sometimes be reversed.

• Progression through the checkpoint is controlled by cyclin–CDK complexes (cyclin dependent kinase/cyclin complex).

Describe the G2 checkpoint and how it is passed.

• The G2 checkpoint occurs after the G1 checkpoint has been passed and DNA replication has taken place.

• Passing the G2 checkpoint commits the cell to enter mitosis.

• The decision to pass depends on whether DNA replication is fully completed and whether the DNA is undamaged and correctly replicated.

• Progression through this checkpoint is regulated by cyclin–CDK complexes.

Describe the M checkpoint and how it is passed.

• The decision to pass the M checkpoint depends on whether all chromosomes are correctly aligned at the metaphase plate and each sister chromatid is attached to microtubules from opposite spindle poles.

• This checkpoint is controlled by the anaphase-promoting complex (APC).

• Once activated, the APC targets cohesins and mitotic cyclins for degradation, allowing sister chromatids to separate and the cell to proceed into anaphase.

What is unique to meiosis?

Recombination occurring between homologous chromosomes in pairs (crossing over).

What is the main thing that happens in meiosis I?

Homologous chromosomes are separated.

What is the main thing that happens in meiosis II?

Sister chromatids are separated (same as in mitosis)

What is separated in mitosis?

Sister chromatids.

How do Prophase I, Metaphase I, and Anaphase I of meiosis differ from mitosis?

• Prophase I (Meiosis)

  • Homologous chromosomes pair up (synapsis) – this does NOT happen in mitosis, where chromosomes stay separate.

  • Crossing over occurs, exchanging DNA between homologues – no crossing over in mitosis.

• Metaphase I (Meiosis)

  • Homologous chromosome pairs line up together at the metaphase plate – in mitosis, individual chromosomes line up single file.

  • Microtubules attach to each homologue’s kinetochore, not to both sister chromatids independently as in mitosis.

• Anaphase I (Meiosis)

  • Homologous chromosomes separate and move to opposite poles – in mitosis, sister chromatids separate.

  • Sister chromatids remain together in meiosis I, unlike mitosis.

What is a synaptonemal complex?

A protein complex that holds the 2 homologous pairs (holds homologues together).

What happens during synapsis and crossing over in Prophase I of meiosis?

• Homologous chromosomes pair up, and their two non-sister chromatids align.

• Corresponding alleles line up gene-for-gene.

• DNA is cut at the same locations on each non-sister chromatid.

• The synaptonemal complex forms, stabilizing their alignment (synapsis).

• DNA breaks are repaired, swapping chromosome segments and producing crossing over.

• The synaptonemal complex dissolves after recombination is complete.

• Sister chromatids remain attached by cohesins, while homologous chromosomes stay linked by chiasmata, marking crossover sites.

How is genetic variety/diversity increased in meiosis?

Genetic variety is increased during meiosis through the crossing over of DNA molecules between homologous chromosomes.

This causes the mixing of the mothers and fathers genetic material.

What are the cell types of daughter cells from meiosis?

Haploid gametes (this is because in animals, meiosis reduces diploid cells to haploid gametes).

Describe the difference between sexual and asexual reproduction.

Asexual reproduction:

• Replication uses only mitosis

• Produces genetically identical offspring (The offsprings are exact clones of the diploid parent. The offspring are identical to one another, any diversity is seen through mutations only).

Sexual reproduction:

• Haploid gametes formed by meiosis

• Produces genetically unique offspring (There is a 50% contribution from each parent. Chromosome segregation and recombination occurs, creating diversity).

Describe the “Fisher-Muller argument”. What does it show?

It shows the long-term benefit of sexual reproduction.

It shows that with meiosis you get the better combination of genes (AB) sooner, because you’re not just reliant on mutation to produce genetic diversity. 

What evidence suggests that not reproducing sexually has evolutionary costs?

• Asexual lineages often disappear over time, shown by “twiggy” phylogenies where asexual groups appear as short branches that go extinct more frequently.

• Many organisms that could reproduce asexually still retain or occasionally use sex (e.g., aphids), even though maintaining sexual organs and engaging in sex is costly—suggesting sex provides an important long-term advantage.

What is the “two-fold cost of sex” in evolutionary biology?

Each parent only passes on 50% of their alleles to offspring, unlike asexual reproduction where 100% are transmitted.

To be favored, sex must provide a major advantage (roughly two-fold per generation)—such as producing offspring more genetically different from the parents.

Sex has high short-term fitness costs, including investment in finding and competing for mates.

There are additional costs such as time, energy, and increased risk of predation during mating.

Describe the 3 mechanisms that contribute to genetic variation (produced in meiosis/sexual reproduction).

1. Independent assortment of chromosomes in meiosis I (The number of possible combinations is 2^n, where n is the number of chromosomes).

2. Recombination (crossing over) (In humans, approx 1-3 crossovers per chromosome arm during Prophase I).

3. Random fertilisation (In human, fusion of male and female gametes creates a variety of chromosome combinations).

Which is more prone to errors, mitosis or meiosis. Where do these errors generally take place. Why is this not that bad though?

Meiosis is more prone to errors that mitosis, especially during separation of tetrads in anaphase I.

High error rate is less critical than in mitosis as most bad gametes won’t achieve fertilisation (as they won’t be carried forwards).

Which takes longer, mitosis or meiosis? How long roughly?

Meiosis takes a long time compared to mitosis (24 days vs 90 minutes in human males).

What is it meant by non-disjunction (a type of meiotic error)?

Nondisjunction is the incorrect separation of homologues in meiosis I

What is Aneuploidy? what is it caused by? What is its effect?

The gain or loss of chromosomes.

Caused by non-dysfunction in meiosis.
It is often embryo lethal.

Describe what it is meant by translocation and deletion (2 types of meiotic errors).

Translocation: To transfer a piece of one chromosome to another.

Deletion: loss of fragment of a chromosome.

What are meiotic errors important for?

Meiotic erros are central to the generation of new genes.

What can duplication of a gene lead to? Why is this?

Unduplicated genes are constrained by selection pressures.

However, gene duplication events lead to more genetic material, not under a selection pressure. Tehn, sub or neofunctionalisation of a duplicated gene, can then lead to a new gene function.

Why does evolution by natural selection happen (3 reasons)?

1. Overproduction (hence there are finite resources)

2. Variation between individuals in the ability to use these resources.

3. (Some of) this variation can be passed on to the next generation.

How are evolutionary changes linked with meiosis and natural selection?

• New combinations of alleles (genotypes) are generated by mutation and recombination (which occurs in meiosis).

• Differences in the transmission rate of these alleles leads to evolution by natural selection.

• Adaptation is based on changes in allele frequency resulting from natural selection.

What is the function of mitosis?

Growth, maintenance, and repair of an organism.

What is the function of meiosis?

Production of gametes.

To produce genetic diversity through recombination and independent assortment.

How many divisions are there in mitosis and meiosis?

In mitosis: 1

In meiosis: 2

What is the chromosome number of daughter cells in mitosis, how does this compare to the mother cell?

Diploid (46 chromosomes) – Identical to mother cell

What is the chromosome number of daughter cells in meiosis, how does this compare to the mother cell?

Haploid (23 chromosomes) – reduced by half compared to mother cell

When does DNA replication occur in meiosis?

Only before meiosis I.

Why is the regulation of mitosis and the cell cycle important?

• To prevent cancer

• For plant development

• For the evolution of multicellularity

Which 2 external cues controls the cell cycle of most mammalian cells?

• Density-dependent inhibition → cells stop dividing when they perceive that they are surrounded by other cells.

• Anchorage-dependent inhibition → cells will only divide if they are attached to a matrix.

What are proto-oncogenes? Give examples

Examples include genes coding for growth factors.

Proto-oncogenes are “growth-promoting” genes that are normal in function (they promote cell division when the growth factor is present).

What mutants of proto-oncogens can cause cancer?

Gain-of-function mutants (If a proto-oncogene undergoes a gain-of-function mutation, it becomes an oncogene).

What are tumour suppressor genes? Give examples.

Examples: Retinoblastoma (Rb) gene.

There normal function is to stop the cell cycle if errors occur (Tumour suppressor genes are genes that normally act to prevent uncontrolled cell growth and division, maintaining the integrity of the genome).

What mutants of tumour supressor genes can cause cancer?

Loss-of-function mutants of tumour suppressor genes cause cancer/

What do growth factors do?

They stimulate cell division.

Why is the orientation of the cell division plane important in development?

Plant cells cannot migrate around the body of the plant, when produced they remain in the same place.

Which 3 factors cause cell cycle misregulation, leading to cancer?

1. Misidentification of external cues

2. Upregulation of proto-oncogenes

3. Downregulation of tumour suppressor genes