3.3 meiosis

Question 1

n

Answer 1

haploid number of chromosomes; 23 in humans; sex cells

Question 2

2n

Answer 2

diploid number of chromosomes; 46 in humans; somatic cells;

Question 3

what does meiosis start and end with?

Answer 3

starts with 1 diploid parent cell, ends with 4 haploid daughter cells

Question 4

bivalent

Answer 4

pair of homologous chromosomes

Question 5

prophase 1 (4 parts)

Answer 5

1. crossing over b/w non-sister chromatids, resulting in genetic variation
2. chromosomes condense, nuclear membrane dissolves
3. genetic recombination
4. centrioles travel to poles

Question 5

interphase

Answer 5

DNA is replicated to produce two genetically identical copies (sister chromatids held together at the centromere)

Question 6

crossing over

Answer 6

Crossing over can happen between non-sister chromatids when the homologous pairs line up.
It results in the recombination of alleles and leads to genetic variation in gametes.

Question 7

chiasmata

Answer 7

X-shaped points of attachment between two non-sister chromatids of a homologous pair

Question 8

draw crossing over

Answer 8

see ipad

Question 9

what happens in first meiotic division (meiosis 1)?

Answer 9

reduction division (diploid → haploid) in which homologous chromosomes are separated

Question 10

what happens in meiosis 2?

Answer 10

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

Question 11

metaphase 1

Answer 11

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

Question 12

anaphase 1

Answer 12

Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell (sister chromatids stay together, but homologous pairs are pulled apart)

Question 13

how does random orientation lead to genetic variation?

Answer 13

The homologous pairs do not always line up in the same way. So the different combinations always occur resulting in further variation.

Question 14

telophase 1 (3 parts)

Answer 14

1. chromosomes decondense, nuclear membrane reforms
2. cleavage furrow, cytokinesis
3. reduction division

Question 15

draw meiosis 1

Answer 15

see ipad

Question 16

draw meiosis 2

Answer 16

see ipad

Question 17

draw both meiosis 1 and 2

Answer 17

see ipad

Question 18

prophase 2 (3 parts)

Answer 18

1. Nuclear membrane dissolves
2. Chromosomes (with two sister chromatids connected by a centromere) condense.
3. Centrioles travel to the poles.
   *no crossing over occurs

Question 19

metaphase 2 (2 parts)

Answer 19

1. Microtubule spindle fibres connect to the centromeres.
2. Chromosomes (two sister chromatids) line up along the equator

Question 20

anaphase 2 (2 parts)

Answer 20

1. Spindle fibres contract and separate the sister chromatids, centromere divides
2. chromatids (now called chromosomes) move to opposite poles

Question 21

telophase 2 (3 parts)

Answer 21

1. Nuclear membrane reforms,
   chromosomes decondense
2. Cytokinesis occurs
3. 4 genetically unique nuclei created

Question 22

purpose of meiosis

Answer 22

to produce sex cells for sexual reproduction

Question 23

how does genetic variation happen in meiosis?

Answer 23

crossing over (prophase 1), or random orientation (metaphase 1 & 2)

Question 24

why is genetic variation important?

Answer 24

Increased genetic variation results in a more resilient population. The population as a whole will be better able to withstand environmental changes, such as disease. Genetic variation is essential for successful change by evolution.

Question 25

karyogram

Answer 25

a photograph of chromosomes in the nucleus. It is obtained by staining a cell undergoing cell-division (this division is halted).

Question 26

how are human adult karyograms usually obtained?

Answer 26

from white blood cells that still divide in culture

Question 27

how are karyograms obtained from unborn fetus’s?

Answer 27

obtained from interphase cells from the amniotic fluid (this is obtained by an amniotic synthesis or a chorionic villus sample).

Question 28

karyotype

Answer 28

a property of a cell and describes its number of chromosomes and their appearance (banding, centromere position, size, and shape)) in an individual/species.

Question 29

what can a karyogram be used for?

Answer 29

determine sex, chromosomal disorders such as Down syndrome (trisomy 21)

Question 30

non-disjunction

Answer 30

the failure of homologous chromosomes or sister chromatids to separate properly during meiosis (anaphase I or anaphase II).

Question 31

what can non-disjunction lead to?

Answer 31

Down syndrome and other chromosomal disorders

Question 32

3 methods to obtain cells for karyotype analysis

Answer 32

1. nuchal translucency scan
2. amniotic centesis
3. chorionic villus sample

Question 33

nuchal translucency scan

Answer 33

• performed at week 11-13
• Non-invasive. Uses ultrasound to view the nuchal fluid behind the neck of the fetus. Large amounts correlate with Down Syndrome.
• 0% chance of miscarriage

Question 34

amniotic centesis

Answer 34

• performed at week 14-16
• A needle is inserted through the mother’s abdominal wall and punctures the amniotic sac (ultrasound guided). Amniotic fluid is collected
• 0.5% chance of miscarriage

Question 35

chorionic villus sample

Answer 35

• performed at week 10-12
• Placental tissue is obtained either through the vagina/cervix or the abdominal wall.
• 1-2% chance of miscarriage