topic 3.3/10.1- meiosis Flashcards
describe meiosis
when one diploid nucleus divides to produce 4 haploid nuclei
what are the 2 divisions in meiosis known as
meiosis I and meiosis II
define a diploid nucleus
has two chromosomes of each type
chromosomes of the same type are known as….
homologous chromosomes
define a haploid nucleus
has one chromosome of each type
meiosis is also known as…because…
a reduction division because it involves a halving of the chromosome number
the halving of the chromosome number allows
a sexual life cycle with fusion of gametes
what does sexual reproduction involve in eukaryotic organisms?
fertilisation (the fusion of gametes)
when in the sexual life cycle does meiosis occur?
any stage, but in animals it happens during the process of creating gametes
main events in meiosis
x2:
- prophase
- metaphase
- anaphase
- telophase
prophase I
-Chromosomes condense
- nuclear membrane dissolves
- homologous chromosomes form bivalents (synapsis)
- crossing over occurs
metaphase I
Spindle fibres from opposing centrioles connect to bivalents (at centromeres) and align them along the middle of the cell
orientation of paternal and maternal chromosomes on either side of the equator is…
random and independent of other homologous pairs
anaphase I
- homologous pairs separated by spindle fibres
- one chromosome of each pair moves to each pole
telophase I
- chromosomes decondense
- nuclear membrane may reform
- cell divides (cytokinesis) to form two haploid daughter cells.
prophase II
- Chromosomes condense
- nuclear membrane dissolves
- centrioles move to opposite poles
metaphase II
Spindle fibres from opposing centrioles attach to chromosomes (at centromere) and align them along the cell equator
anaphase II
centromeres separate and chromatids are moved to opposite poles
telophase II
- chromatids reach opposite poles
- nuclear envelope forms
- cytokinesis occurs
what is meiosis preceded by?
interphase- DNA is replicated to produce chromosomes consisting of two sister chromatids
diploid human cell
46 chromosomes in 23 pairs
n
=23=haploid no of chromosomes
what three things promote genetic variation in meiosis?
- random orientation
- crossing over
- fusion of gametes from different parents
random orientation
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
for every additional bivalent, the no of possible chromosome combinations in a cell produced by meiosis…
doubles
crossing over
in prophase I:
- new gene combinations are formed on chromatids (recombination)
gametes fuse to form a
zygote
define crossing over
the exchange of DNA material between non-sister homologous chromatids
what are chiasmata?
connection points where non-sister chromatids continue to adhere after crossing over has occurred
chiasmata formation between non-sister chromatids can result in
an exchange of alleles
what causes Down syndrome?
non-disjunction- this can cause other chromosome abnormalities as well
define non-disjunction
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)
Describe:
- Down syndrome
- 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
give 3 other common non-disjunction disorders
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)
State two methods used to obtain cells for karyotype analysis
- 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
describe the risks associated with chorionic villus sampling and amniocentesis
amniocentesis- risk of miscarriage is 1%
Chronic villus sampling- risk is 2%
why does independent assortment of genes occur?
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
bivalent =
tetrad, composed of 4 chromatids - the combination of homologous chromosomes when they align beside each other
describe the process of crossing over w a diagram
- at one stage in prophase I all of the chromatids of two homologous chromosomes become tightly paired up together (synapsis)
- 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
- 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
- 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
give 4 ways in which meiosis I differs from mitosis and meiosis II
- 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
