S2) Mitosis and Meiosis

Question 1

Identify all the stages in the cell cycle

Answer 1

• G₁
• S Phase
• G₂ Phase
• M Phase

Question 2

Describe what happens the G₁ phase and when it begins

Answer 2

• Begins immediately after mitosis
• Synthesis of RNA, proteins, and organelles

Question 3

Describe what occurs in the S Phase

Answer 3

DNA replication occurs, i.e. chromosomes form sister chromatids

Question 4

Describe what occurs in the G₂ Phase

Answer 4

• Mitochondria divide
• Synthesis of precursors of spindle fibres

Question 5

Identify all the stages of M Phase (mitosis)

Answer 5

• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase

Question 6

What occurs in prophase?

Answer 6

• Chromatin condenses due to supercoiling and chromosomes become visible
• Nucleoli disappear
• Nuclear envelope breaks down
• Centrioles divide and migrate to opposite poles

Question 7

What occurs in prometaphase?

Answer 7

• Microtubules produce spindle fibres form
• Spindle fibres attach to centromere of chromosomes

Question 8

What occurs in metaphase?

Answer 8

Chromosomes, with spindle fibres attached, line up on the equator of the dividing cell

Question 9

What occurs in anaphase?

Answer 9

• Centromeres divide, converting each sister chromatid into a chromosome
• Chromosomes migrate toward opposite ends of the cell until there is a complete set of chromosomes at each end of the cell

Question 10

What occurs in telophase?

Answer 10

• Chromosomes reach the opposite poles of the cell
• Chromatin decondenses & spindle fibres break down
• Nuclear membrane reforms
• Nucleoli reappear

Question 11

What occurs in cytokinesis?

Answer 11

• Cleavage furrow forms at equator of cell and deepens
• Cytoplasm divides
• New nuclear envelope forms

Question 12

Describe the structure of chromosomes

Answer 12

• Each chromatid consist of one identical DNA molecule
• Each chromatid has a p (short) and q (long) arm
• Telomeres at the end of each chromatid
• Centromeres link sister chromatids

Question 13

Identify the different types of chromosome structures

Answer 13

Question 14

What occurs in meiosis?

Answer 14

• Members of a chromosome pair separate from each other
• Each daughter cell receives a haploid (n) set of 23 chromosomes
• These haploid cells form gametes (sperm and egg)

Question 15

Explain the reduction that occurs in meiosis

Answer 15

• Before cells enter meiosis, the chromosomes replicate during interphase
• Meiosis I reduces the chromosome number accomplished by two rounds of division (meiosis I and meiosis II)

Question 16

How does fertilisation affect the overall chromosome number?

Answer 16

• Fusion of two haploid gametes in fertilisation restores the chromosome number to the diploid number of 46
• Fertilised egg has a full set of genetic information

Question 17

Explain what is meant by the following terms:

• Homologous chromosomes
• Sister chromatids
• Non-sister chromatids

Answer 17

• Homologous chromosomes have the identical DNA but may have different alleles
• Sister chromatids have identical DNA and the same alleles
• Non-sister chromatids have identical DNA, but may have different alleles

Question 18

How does recombination occur in meiosis I?

Answer 18

Meiosis produces new combinations of parental genes in two ways:

• Random assortment which puts random combinations of maternal and paternal chromosomes into gametes
• Crossing over which is the exchange of chromosome segments between non-sister chromatids of homologous chromosomes

Question 19

What occurs in prophase I of meiosis?

Answer 19

• Chromatin condenses & chromosomes are visible
• Homologous chromosomes pair & crossing over occurs
• Newly forming spindle microtubules become attached to each chromosome

Question 20

What occurs in metaphase I of meiosis?

Answer 20

• Paired chromosomes align at equator of cell
• Random assortment of chromosomes occurs

Question 21

What occurs in anaphase I of meiosis?

Answer 21

• Paired homologous chromosomes separate
• Sister chromatids move to opposite poles

Question 22

What occurs in telophase I of meiosis?

Answer 22

• Chromosomes uncoil
• Spindle fibres break down
• Nucleoli & nuclear envelop reform

Question 23

What occurs in cytokinesis of meiosis I?

Answer 23

• Cytoplasm divides
• 2 haploid daughter cells are formed
• All chromosomes are still in the duplicated state

Question 24

Outline Meiosis II

Answer 24

• Prophase II: Chromosomes re-coil and shorten
• Metaphase II: Unpaired chromosomes become aligned at equator of cell
• Anaphase II: Centromeres separate. Daughter chromosomes, which were sister chromatids, pull apart
• Telophase II: Chromosomes uncoil and nuclear envelope re-forms
• Cytokinesis: Cytoplasm divides forming 4 haploid daughter cells

Question 25

What is spermatogenesis?

Answer 25

**Spermatogenesis** is the production of sperm which occurs in the testes of males

Question 26

Outline the process of spermatogenesis

Answer 26

Question 27

What is oogenesis?

Answer 27

**Oogenesis** is the production of gametes (ova) which occurs in the ovaries of females

Question 28

Outline the process of oogenesis

Answer 28

Question 29

Describe the phenomena of missegregation in meiosis

Answer 29

- 30% of human meiosis is faulty - Consequences: miscarriages and infertility

Question 30

How can chromosomes be analysed?

Answer 30

Metaphase spread: - Stained metaphase chromosomes - Actively dividing cells are needed

Question 31

State the standard format used to describe the karyotype

Answer 31

**Karyotype** = Chromosome number, sex complement, structural changes (no spaces) ## Footnote * E.g. 46,XX - normal female* * E.g. 47,XY,+21 - male with trisomy 21*

Question 32

What is mitotic non-disjunction?

Answer 32

**Mitotic nondisjunction** is when daughter cells have abnormal chromosome numbers (aneuploidy) due to the failure of sister chromatids separating in anaphase of mitosis

Question 33

Aneuploidy can result fron non-disjunction. Define this term

Answer 33

**Aneuploidy** is the presence of an abnormal number of chromosomes in a cell

Question 34

What is mosaicism?

Answer 34

**- Mosaicism** is the presence of 2/more cell lines in an individual - It can occur throughout the body or be tissue limited - Non-disjunction can cause mosaicism

Question 35

What influences the degree of mosaicism?

Answer 35

- When the first post-zygotic division occurs - When later mitotic divisions occur

Question 36

What is the role of the mitotic spindle in mitosis?

Answer 36

The mitotic spindle ensures the accuracy of chromosome segregation

Question 37

The mitotic spindle is a proven target for successful cancer treatment. Explain how

Answer 37

- Cancer cells are aneuploid (not diploid) and exhibit chromosome instability - Treatments interfere with microtubules, preventing cell division in rapidly-dividing cancer cells

Question 38

Cancer cells often display numerical chromosome instability (CIN). What causes this?

Answer 38

- Mitotic errors promote CIN and tumour heterogeneity - **Amplified centrosomes**, often seen in cancer, produce multipolar spindles which also promote CIN

Question 39

What regulates spindle polarity?

Answer 39

Spindle polarity is regulated by centrosome number

Question 40

What is the role of TSG?

Answer 40

**Tumour suppresor genes** (TSG) stop cancer growth

Question 41

What is the role of oncogenes?

Answer 41

**Oncogenes** promote cancer growth

Question 42

What results from TSG and oncogene mutations?

Answer 42

**Centrosome amplification** results from TSG and oncogene mutation

Question 43

What is centrosome clustering and what does it do?

Answer 43

- Cancer cells have mechanisms to **cluster amplified centrosomes** - Clustering extra centrosomes allows cancer cells to survive

Question 44

Identify a therapeutic strategy targeting centrosome clustering

Answer 44

Inhibition of centrosome clustering to lead to cancer cell death