Lecture 5: Mitosis And Meiosis

Question 1

Why are mitosis and meiosis important in genetics?

Answer 1

They are the methods by which a eukaryotic cell transmits its genetic information to daughter cells (mitosis) or the next generation (meiosis).

The differences between these mechanisms explain the different genetic outcomes.

Question 2

What is the cell cycle made up of?

Answer 2

Interphase (G1, S, G2) and mitosis.

Question 3

What is G0?

Answer 3

Quiescence, the phase when cells enter the period of dormancy/inactivity. Most of the cells in our body are in this phase (they wait until they need to be activated).

Question 4

What are cell cycle checkpoints?

Answer 4

Control mechanisms in the eukaryotic cell which ensure its proper progression.
Decide whether or not a cell can proceed to the next stage of the cycle based on factors.

Question 5

What will happen if there is a defective checkpoint?

Answer 5

Disease will form.

Question 6

How does DNA content change?

Answer 6

It doubles during S phase when the DNA replicates, therefore cells in G2 have double the DNA content of a G1 cell.

In mitosis, half of the DNA goes into each daughter cell, taking the DNA content back down to G1 levels.

Question 7

How does a cell prepare for mitosis?

Answer 7

• Centrioles duplicate during S phase.
• Nuclear envelope breaks down.
• DNA condenses into mitotic chromosomes.
• Centrioles migrate to opposite poles and form spindle fibres.
• Condensed chromosomes align on spindle fibres.

Question 8

How does approximately 2m of DNA fit into a 5-10 micron nucleus?

Answer 8

It gets compacted and packaged into chromosomes.

Question 9

What are somatic cells?

Answer 9

All but reproductive cells in diploid organisms.

Question 10

What are some several layers of compaction?

Answer 10

1. Nucleosomes – 147 bp DNA (which is negatively charged) wrapped around positively charged octamer histone core - ‘beads on a string’ with a diameter of 11 nm.
2. Solenoid – coiling of nucleosomes giving 6-fold compaction that has a 30nm diameter.
3. Chromosome – solenoids coil into 300 nm fibres, which condense into a 700nm (diameter of a chromatid) chromosome.

Question 11

What is a chromatosome?

Answer 11

Consists of a nucleosome plus the H1 histone.

Question 12

How is the chromatin fiber formed?

Answer 12

Solenoid fibres form loops (averaging 300nm in length).

Question 13

What is a diploid cell?

Answer 13

Containing two complete sets of chromosomes, one from each parent.

Made up of two haploid (single set of unpaired chromosomes n=23) cells.

Question 14

What is a homologous chromosome?

Answer 14

Similar paired chromosomes that may differ in alleles. One maternal copy and one paternal.

Question 15

What is the difference between homologous chromosomes and sister chromatids?

Answer 15

Sister chromatids are the duplicated forms of each homologous chromosome.

While homologous chromosomes have same genes, their alleles differ. Sister chromatids have the same genes and alleles.

Question 16

What happens during anaphase to sister chromatids?

Answer 16

They split, one into each daughter cell.
M-P, M-P.

Question 17

Why does DNA condense when it is about to split?

Answer 17

A messy ‘soup’ of loose DNA would be very difficult to split accurately between daughter cells.

Question 18

What is a karyotype?

Answer 18

An individual’s complete set of chromosomes.

Question 19

What stain is used to look at the banding pattern of chromosomes and pair them up?

Answer 19

Giemsa stain.

Question 20

What is the difference between heterochromatin and euchromatin?

Answer 20

Heterochromatin is highly condensed, gene-poor, and transcriptionally silent.

Euchromatin is less condensed, gene-rich, and more easily transcribed.

Question 21

Which chromosome material stains darker? Why?

Answer 21

Heterochromatin, because it is denser.

Question 22

How many pairs of autosomes do humans have? How many of sex chromosomes?

Answer 22

22 pairs of autosomes, 1 pair of sex chromosomes.

Question 23

What is trisomy?

Answer 23

The presence of three chromosomes instead of the usual pair of chromosomes.

For example, Down Syndrome in chromosome 21.
Trisomy 18 and 13 are fatal genetic disorders.

Question 24

What is fragile X syndrome?

Answer 24

When X-chromosome 23 is abnormally susceptible to damage, especially by folic acid deficiency.

This causes severe limited intellectual functions in a person.

Question 25

What is missing X in chromosome 23 also known as? What are its characteristics?

Answer 25

Turner Syndrome, it affects females only and causes infertility.

Question 26

What is chromosome translocation?

Answer 26

A genome abnormality in which a chromosome breaks and either the whole or a portion of it reattaches to a different chromosome. It causes acute myeloid leukaemia.

Question 27

What are the stages of Mitosis? Describe them.

Answer 27

1. Interphase: Chromosomes are extended and uncoiled, forming chromatin. 2. Prophase: Chromosomes coil up and condense; centrioles divide and move apart. 3. Prometaphase: Chromosomes are clearly double structures; centrioles reach the opposite poles; spindle fibres form. 4. Metaphase: Centromeres align on metaphase plate. 5. Anaphase: Centromeres split and daughter chromosomes migrate to opposite poles. 6. Telophase: Daughter chromosomes arrive at the poles; cytokinesis commences.

Question 28

What is a kinetochore?

Answer 28

Large protein assemblies that connect chromosomes to microtubules of spindles.

Question 29

What is the genetic outcome of mitosis?

Answer 29

Two daughter cells receive an **exact copy of the chromosomal content of the parent cell**.

Question 30

What is a chromatid?

Answer 30

One half of a duplicated chromosome.

Question 31

How do mitosis and meiosis compare?

Answer 31

They both start with a diploid cell which undergoes cell replication.

Question 32

What happens in meiosis?

Answer 32

In meiosis I, homologous chromosomes pair up to form **tetrads** [*when 2 homologous chromosomes line up together, 4 sister chromatids] *(important for recombination)*. Sister chromatids split into separate cells (**dyads**, 2 chromatids, duplicated chromosomes) via reductional division. In meiosis II the dyads (sister chromatids) separate into **monads** (chromosome consisting of just one chromatid), resulting in 4 **haploid cells** (gametes) with *half the DNA* complement of a parental cell.

Question 33

If females with Turner syndrome could reproduce, how many children would be affected?

Answer 33

On average, half of the children will be 0X or 0Y. Females with Turner syndrome will generate half of their gametes with a single X chromosome and the other half with no sex chromosomes. 50% of male gametes will have an X chromosome and 50% will have the Y chromosome. Therefore, we can expect half of the fertilised female gametes to result in embryos 0X or 0Y, depending on the sex chromosome present in the male gamete that succeeded in fertilising the “0” female gamete.

Question 34

How do homologous chromosomes behave? Why?

Answer 34

They behave **independently** of each other. Because you want both daughter cells to be identical! Therefore, you want one copy of each homologous chromosome per daughter cell because you want both daughter cells to be identical! There is no requirement for the homologous chromosomes to pair up.

Question 35

How many gamete combinations can you form with a diploid number of 6 chromosomes?

Answer 35

8. 6 chromosomes = 3 pairs of homologous chromosomes. 2 possibilities per chromosome = 2 x 2 x 2 = 8.

Question 36

How does meiosis compare to mitosis?

Answer 36

In meiosis there are **two** rounds of cell division, each with the **same phases as mitosis**. However, the difference in chromosome processing in *meiosis I* results in **genetic diversity** in gametes. Meiosis II produces *gametes*.

Question 37

List the phases of Meiosis I – Prophase and describe their key events.

Answer 37

1. Leptonema • Chromosomes condense and **search** for their homologous chromosome. 2. Zygonema • Loose pairing between homologous chromosomes (approx. 300nm) results in **synaptonemal complex** formation. 3. Pachynema • Chromosomes approx. 100nm apart (**synapsis**/pairing) and non-sister chromatids **‘cross-over’** to exchange genetic material. 4. Diplonema • Non-sister chromatids attempt to separate but are held together at cross-over points (**chiasmata**), which must be resolved. 5. Diakinesis • Spindles form and **centromeres attach**.

Question 38

What is the synaptonemal complex? What is its characteristic?

Answer 38

It is found only in chromosomes of cells undergoing meiosis. It is the vehicle for pairing of homologues (zips them together) and brings them close enough to ‘cross-over’ to generate genetic diversity.

Question 39

What is the difference between meiosis I and meiosis II?

Answer 39

During meiosis I, the centromeres holding each pair of *sister chromatids* together **do not divide**. The pairs of each tetrad are pulled towards opposite poles, separating the homologous chromosomes. In meiosis II, *sister chromatids* are **separated** as they are in mitosis, converting chromosome compliment from diploid number (2n) to haploid (n) in gametes. Sex chromosomes **behave** as homologues during meiosis.

Question 40

What is independent assortment?

Answer 40

Random alignment of homologous chromosomes (tetrads) in the metaphase of meiosis I, leading to a random combination of maternally and paternally derived chromosomes in the gametes.

Question 41

How many possible arrangements can exist for 2 homologous chromosomes in the gametes?

Answer 41

2 possibilities for one pair (pair 1) by 2 possibilities for another (pair 2) = 4 different combinations of chromosomes in the gametes.

Question 42

In what form do chromosomes exist in diploid organisms?

Answer 42

In homologous pairs.

Question 43

What is each homologous pair of chromosomes made up of?

Answer 43

One chromosome copy from a maternal parent, another copy from a paternal parent.

Question 44

How many copies of each autosomal gene do diploid organisms contain?

Answer 44

At least two.

Question 45

What are alleles?

Answer 45

Alternative forms of the same gene.

Question 46

What is the genome of a species?

Answer 46

The whole DNA content of the **haploid** complement.

Question 47

What is reductional division?

Answer 47

Meiosis is sometimes called “reductional division” because it reduces the number of chromosomes to half the normal number. The chromosome number is reduced from diploid (46 chromosomes) to haploid (23) state.