topic 3.3/10.1- meiosis

Question 1

describe meiosis

Answer 1

when one diploid nucleus divides to produce 4 haploid nuclei

Question 2

what are the 2 divisions in meiosis known as

Answer 2

meiosis I and meiosis II

Question 3

define a diploid nucleus

Answer 3

has two chromosomes of each type

Question 4

chromosomes of the same type are known as….

Answer 4

homologous chromosomes

Question 5

define a haploid nucleus

Answer 5

has one chromosome of each type

Question 6

meiosis is also known as…because…

Answer 6

a reduction division because it involves a halving of the chromosome number

Question 7

the halving of the chromosome number allows

Answer 7

a sexual life cycle with fusion of gametes

Question 8

what does sexual reproduction involve in eukaryotic organisms?

Answer 8

fertilisation (the fusion of gametes)

Question 9

when in the sexual life cycle does meiosis occur?

Answer 9

any stage, but in animals it happens during the process of creating gametes

Question 10

main events in meiosis

Answer 10

x2:
- prophase
- metaphase
- anaphase
- telophase

Question 11

prophase I

Answer 11

-Chromosomes condense
- nuclear membrane dissolves
- homologous chromosomes form bivalents (synapsis)
- crossing over occurs

Question 12

metaphase I

Answer 12

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

Question 13

orientation of paternal and maternal chromosomes on either side of the equator is…

Answer 13

random and independent of other homologous pairs

Question 14

anaphase I

Answer 14

• homologous pairs separated by spindle fibres
• one chromosome of each pair moves to each pole

Question 15

telophase I

Answer 15

• chromosomes decondense
• nuclear membrane may reform
• cell divides (cytokinesis) to form two haploid daughter cells.

Question 16

prophase II

Answer 16

• Chromosomes condense
• nuclear membrane dissolves
• centrioles move to opposite poles

Question 17

metaphase II

Answer 17

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

Question 18

anaphase II

Answer 18

centromeres separate and chromatids are moved to opposite poles

Question 19

telophase II

Answer 19

• chromatids reach opposite poles
• nuclear envelope forms
• cytokinesis occurs

Question 20

what is meiosis preceded by?

Answer 20

interphase- DNA is replicated to produce chromosomes consisting of two sister chromatids

Question 21

diploid human cell

Answer 21

46 chromosomes in 23 pairs

Question 22

n

Answer 22

=23=haploid no of chromosomes

Question 23

what three things promote genetic variation in meiosis?

Answer 23

• random orientation
• crossing over
• fusion of gametes from different parents

Question 24

random orientation

Answer 24

in metaphase I, the orientation of bivalents is random and the orientation of one bivalent does not influence the orientation of the others.
- this generates genetic variation among genes that are on different chromosome types

Question 25

for every additional bivalent, the no of possible chromosome combinations in a cell produced by meiosis...

Answer 25

doubles

Question 26

crossing over

Answer 26

in prophase I: - new gene combinations are formed on chromatids (recombination)

Question 27

gametes fuse to form a

Answer 27

zygote

Question 28

define crossing over

Answer 28

the exchange of DNA material between non-sister homologous chromatids

Question 29

what are chiasmata?

Answer 29

connection points where non-sister chromatids continue to adhere after crossing over has occurred

Question 30

chiasmata formation between non-sister chromatids can result in

Answer 30

an exchange of alleles

Question 31

what causes Down syndrome?

Answer 31

non-disjunction- this can cause other chromosome abnormalities as well

Question 32

define non-disjunction

Answer 32

when homologous chromosomes fail to separate at anaphase - both of the chromosomes move to one pole and neither to the other pole, resulting in a gamete that either has an extra chromosome or is deficient in a chromosome (fertilization= individual w either 45/47 chromosomes)

Question 33

Describe: - Down syndrome

Answer 33

- trisomy 21: non-disjunction event leads an individual with three of chromosome 21 rather than 2 - hearing loss, heart and vision disorders, mental and growth retardation

Question 34

give 3 other common non-disjunction disorders

Answer 34

trisomy 18 and 13 Klinefelter's syndrome (caused by having the sex chromosomes XXY) Turner's syndrome (caused by having only one sex chromosome, X)

Question 35

State two methods used to obtain cells for karyotype analysis

Answer 35

- amniocentesis : passing a needle through the mother's abdomen wall, using ultrasound to guide the needle which withdraws a sample of amniotic fluid containing metal cells from the amniotic sac - chorionic villus sampling; sampling tool enters the vagina is used to obtain cells from the chorion, one of the membranes from which the placenta develops

Question 36

describe the risks associated with chorionic villus sampling and amniocentesis

Answer 36

amniocentesis- risk of miscarriage is 1% Chronic villus sampling- risk is 2%

Question 37

why does independent assortment of genes occur?

Answer 37

due to the random orientation of pairs of homologous chromosomes in meiosis I - the pole to which each chromosome in a pair moves depends on which way the pair is facing - this is random - the direction in which one pair is facing does not affect the direction in which any other pairs are facing

Question 38

bivalent =

Answer 38

tetrad, composed of 4 chromatids - the combination of homologous chromosomes when they align beside each other

Question 39

describe the process of crossing over w a diagram

Answer 39

1. at one stage in prophase I all of the chromatids of two homologous chromosomes become tightly paired up together (synapsis) 2. the DNA molecule of one of the chromatids is cut. A second cut is made at exactly the same point in the DNA of a non-sister chromatid 3. the DNA of each chromatid is joined up to the DNA of the non-sister chromatid. this has the effect of swapping sections of DNA between the chromatids 4. in the later stages of prophase I the tight pairing of the homologous chromosomes ends, but the sister chromatids remain tightly connected. The X shaped structure after crossing over is called a chiasma

Question 40

give 4 ways in which meiosis I differs from mitosis and meiosis II

Answer 40

- sister chromatids remain associated w each other - homologous chromosomes behave in a coordinated fashion in prophase - homologous chromosomes exchange DNA leading to genetic recombination - meiosis I is a reduction division