3.3. & 10.1 Meiosis

Question 1

What is Meiosis?

Answer 1

meiosis is where one diploid nucleus divides to product four haploid nuclei
used to form gametes

Question 2

Meiosis I

Answer 2

The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated

Question 3

Prophase I

Answer 3

Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs

Question 4

Metaphase I

Answer 4

Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell

Question 5

Anaphase I

Answer 5

Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell

Question 6

Telophase I

Answer 6

Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells

Question 7

Meiosis II

Answer 7

The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)

Question 8

Prophase II

Answer 8

Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)

Question 9

Metaphase II

Answer 9

Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator

Question 10

Anaphase II

Answer 10

Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles

Question 11

Telophase II

Answer 11

Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells

Question 12

Allele Segregation

Answer 12

each pair of alleles is sorted into different gametes and subsequently into different offspring
this is because each allele is carried on separate homologous chromosomes that are separated during meiosis
* anaphase I

Question 13

Independent Assortment

Answer 13

is due to the random orientation of pairs of homologous chromosomes in metaphase I

Question 14

Crossing Over

Answer 14

it is the exchange of DNA material between non-sister chromatids
occurs during prophase I where sister chromatids undergo synapsis (line up side by side and tangle - chiasmata formation)
it produces new combinations of alleles on the chromosomes of the haploid cells
chiasmata formation between non-sister chromatids can result in an exchange of alleles
the chromosomes with a new combination of alleles are called recombinants
this makes all chromatids unique hence all gametes unique

Question 15

Asexual Reproduction

Answer 15

the daughter cells are exactly the same as the parent cells
is faster, requires less energy but results in the population having less genetic variation reducing the chance of survival

Question 16

Sexual Reproduction

Answer 16

the zygotes have a different set of chromosomes from the parents (promotes genetic variation)
takes longer, more energy but allows for more variation
species has a better chance of survival

Question 17

Aneuploidy

Answer 17

is a condition where one or more chromosomes are missing from or added to the normal somatic cell chromosome number
results from non-disjunction (homologous pairs haven’t separated)
one cell may have two copies of a homologous chromosome and the other has none
three types:
- autosomal aneuploidy
- sex chromosome aneuploidy
- polyploidy

Question 18

Down’s Syndrome

Answer 18

trisomy 21 (three copies of chromosome 21)
result as a cause of non-disjunction occurring in either stage of meiosis
studies have shown an increase in risk of down’s syndrome babies with age of mother

Question 19

Amniocentisis

Answer 19

taking a sample of amniotic fluid surrounding the baby
done at 16 weeks
carries a 0.5% chance of miscarriage
done to obtain cells for karyotyping

Question 20

Chorionic Villus Sampling (CVS)

Answer 20

sample taken from the placenta
done at 11 weeks carries a 1% chance of miscarriage
done to obtain cells for karyotyping