3.3 Meiosis

Question 1

Types of cell division: Asexual vs Sexual reproduction

Answer 1

Asexual: Mitosis

Sexual: Meiosis

Question 2

Number of daughter cells produced, genetic variation, haploid or diploid: Asexual vs Sexual reproduction

Answer 2

Asexual: 2, genetically identical, diploid

Sexual: 4, genetically varied, haploid

Question 3

Function: Asexual vs Sexual reproduction

Answer 3

Asexual: Growth, repair, cloning

Sexual: Produce gametes

Question 4

What is synapsis?

Answer 4

The process during prophase I where homologous chromosomes come together

Question 5

How are homologous chromosomes connected?

Answer 5

By a protein-RNA complex called the synaptonemal complex

Question 6

Why is genetic variation an advantage?

Answer 6

Evolution, natural selection, adaptation

Question 7

What are the 3 methods by which meiosis creates genetic variation?

Answer 7

1. Crossing over - prophase I
2. Independent assortment - metaphase
3. Random fusing of gametes (fertilisation)

Question 8

What is crossing over? Where does it occur? What is the result?

Answer 8

Crossing over can occur multiple times on the same homologous pair.

It occurs in prophase I when homologous chromosomes undergo a process synapsis, whereby they pair up to form a bivalent (or tetrad). The homologous chromosomes are held together at points called chiasmata (singular: chiasma)

Then crossing over of genetic material between non-sister chromatids can occur at these chiasmata

• As a result of this exchange of genetic material, new gene combinations are formed on chromatids (recombination)
• Chromatids of each chromosomes are no longer genetically identical

Question 9

What does the exchange of alleles form?

Answer 9

Recombinant chromatids

Chromatids of each chromosomes are no longer genetically identical

Question 10

How are recombinant chromatids different?

Answer 10

Recombinant chromatids still contain the same genes but a different combination of alleles

Question 11

Where does independent assortment occur? How does it produce genetic variation?

Answer 11

During metaphase I, the orientation of pairs of homologous chromosomes is random, as is the subsequent assortment of chromosomes into gametes (metaphase II)

• This affects which chromosome of the pair (maternal or paternal) goes into which daughter so when they are pulled apart during anaphase I
• Will occur for each homologous pair

Question 12

What is the number of possible gamete combinations dependent on?

Answer 12

As this random assortment will occur for each homologous pair, the number of possible gamete combinations are dependent on the number of homologous pairs

• Gamete combinations = 2ⁿ (where n represents the haploid number)

Question 13

What needs to happen for the formation of a diploid zygote that can grow via mitosis?

Answer 13

Fertilisation of two haploid gametes (egg + sperm)

Question 14

What happens if chromosome number was not halved in gametes?

Answer 14

The total chromosome number would double each generation (polyploidy)

Question 15

How does random fusing of gametes produce genetic variation?

Answer 15

Haploid gametes undergo random fusion with gametes derived from another organism (of the same species) at fertilisation.

The resulting zygote will therefore contain genetic material from two unrelated individuals, introducing further genetic variation.

Question 16

What are two ways genetic variation can be introduced by mutations?

Answer 16

Gene mutations e.g. point mutations

Chromosome mutations e.g. non-disjunction - failure of homologous chromosomes or sister chromatids to separate properly during anaphase

Question 17

What are examples of non-disjunction:

Answer 17

Down syndrome - triplets in chromosome 21

Klinefelter syndrome - triplet in heterosome (male also with female attributes, wider hips etc)

Turner syndrome - 1 heterosome chromosome (female but ovaries not developed properly, developmental issues)

Question 18

What does the age of the parent have to do with chances of non-disjunction?

Answer 18

Studies show that the chances of non-disjunction increase as the age of the parents increase

• There is a particularly strong correlation between maternal age and the occurrence of non-disjunction events
• This may be due to developing oocytes being arrested in prophase I until ovulation as part of the process of oogenesis

Question 19

What two procedures are used for obtaining cells containing the fetal chromosomes needed for producing a karyotype.

Answer 19

1. Amniocentesis - passing a needle through the mother’s abdomen wall, suing ultrasound to guide the needle. The needle is used to withdraw a sample of amniotic fluid containing fetal cells from the amniotic sac. Risk of miscarriage from amniocentesis is 1%. Usually carried out around week 16 of preganancy
2. Chronic Villi sampling - the sampling tool that enters through the vagina is used to obtain cells from the chorion, one of the membranes from which the placenta develops. This can be done earlier in the pregnancy than amniocentesis (around week 11). Risk of miscarriage is 2%.

Question 20

Why would parents produce a karyotype?

Answer 20

So they can choose not to carry on pregnancy, because they can choose not to carry on the pregnancy because some can’t afford the medical bills to support a child with a syndrome

Question 21

What is a recombinant

Answer 21

Chromosomes that consist of genetic material from both homologues are called recombinant chromosomes

Question 22

What is a chiasmata

Answer 22

A region where non-sister chromatids remain physically connected at points of exchange during crossing over

Question 23

What can happen while in synapsis?

Answer 23

Non-sister chromatids may break and recombine with their homologous partners (crossing over)