ch. 3 meiosis Flashcards
is meiosis a cycle?
no
what does meiosis produce?
4 haploid cells (gametes)
where does mitosis occur
somatic cells
where does meiosis occur
germ-line cells
overview of meiosis 1
- homologous chromosomes separate
- centromeres don’t split
- reduces diploid # to haploid #
overview of meiosis 2
- sister chromatids separate
- centromeres split
- 4 haploid gametes
- each gamete has one chromosome from the original diploid set of homologous chromosomes
mechanisms of diversity in meiosis 1
- homologous chromosome pairing
- crossing over between homologous chromosomes
- segregation (separation) of homologous chromosomes (reduces chromosomes to haploid #)
stages of meiosis 1
- prophase 1
- metaphase 1
- anaphase 1
- telophase 1
five substages of prophase 1
- leptotene
- zygotene
- pachytene
- diplotene
- diakinesis
where does pairing and recombination of homologs take place
prophase 1
leptotene
- chromosome condensation begins
- centrosomes begin to migrate toward opposite poles
- meiotic spindle forms as microtubules extend from centrosomes
zygotene
- nuclear envelope disintegrates
- homologous chromosomes undergo synapsis
synaptonemal complex (protein bridge_
tri-layer protein structure that binds non-sister chromatids of homologous chromosomes together
- forms between homologous chromosomes as they align
non-sister chromatids
belong to different members of a homologous pair
pachytene
- chromosome condensation continues
- tetrads form
- crossing over begins (recombination nodules form)
- kinetochore microtubules attach to kinetochores
tetrads
paired homologs
- all 4 chromatids visible
recombination nodules
aggregates of enzymes and proteins needed for crossing over between homologs
- form at intervals in synaptonemal complex
diplotene
- crossing over completes
- synaptonemal complex begins to dissolve
- chiasma
- nucelar envelope breakdown finishes
chiasma
locations where crossing over has occured
what is present between sister chromatids?
cohesin protein
what does cohesin resist
resists pulling forces of kinetochore microtubules
do centromeres split during meiosis 1?
no
diakinesis
- kinetochore microtubules move synapsed chromosome pairs toward metaphase plate
- homologs align side by side
- centromeres remain intact
metaphase 1
- chiasmata between homologs resolved, completes crossing over
- homologs align on opposite sides of metaphase plate
- kinetochore microtubules attach to both sister. chromatids of one homolog pair
anaphase 1
- homologs separate from one another and are pulled to opposite poles of cell
- movement caused by shortening of kinetochore microtubules
- sister chromatids remain attached by cohesin
- centromeres remain intact
telophase 1
- nuclear membranes briefly reform around haploid sets of chromosomes
- followed by cytokinesis
cytokinesis in meiosis
- divides cytoplasm to create 2 haploid cells (n+n)
- each cell carries 2 copies of each haploid set
why is meiosis I called reductional division
because the policy of the daughter cells is halved compared to the original diploid parent cell
- still 2 copies of each genome
meiosis 2 is similar to…
mitosis in a haploid cell
what does meiosis 2 divide each haploid daughter cell into
2 haploid cells by separating sister chromatids from one another
what is the independent assortment of alleles illustrated by
behavior of 2 pairs of homologues during meiosis
for an organism with genotype AaBb, 2 equally likely arrangements of paired homologs can occur:
- gametes AB and ab
- games Ab and aB
sex determination
involves genetic and biological processes that produce the male and female characteristics of a species
chromosomal sex
presence of chromosomes characteristic of each sex and is determined at the moment of fertilization
phenotypic sex
internal and external morphology of each sex, and results from differences in gene expression
what chromosomes do placental mammals have
X and Y
male mammals
XY (normal), XXY, or XYY
female mammals
XX (normal), XO, or XXX
pseudoautosomal regions (PAR1 and PAR2)
two small regions of homology that exist between the X and Y chromosomes
what do PAR1 and PAR2 allow?
homologous pairing between X and Y and meiosis
- there is evidence that crossing over occurs within this regions
undifferentiated gonads
clusters of tissue in early mammalian embryos that can develop as ovaries or testes
what does sex determination depend on
presence of absence of single gene, SRY, found on Y chromosome
SRY
transcription factor needed for male-specific gene expression
what does the expression of SRY initiate
testicular development of undifferentiated gonads
what does absence of SRY result in
allows default, female state to develop
sex determination for birds, some reptiles, some fish, butterflies, and moths
Z/W system
- females: 2 different sex chromosomes (ZW)
- males: 2 same sex chromosomes (ZZ)
sex chromosomes of platypus
5 pairs of sex chromosomes:
- 5 XY pairs in males
- 5 XX pairs in females
sex determination in Drosophila
- females: 2 X chromosomes (XXY, XX)
- males: 1 X chromosome (X0, XYY, XY)
what does the dosage of the X chromosome determine in Drosophila
outcome of a sex determination gene regulation cascade
X/A ratio (X:autosome)
determines gender based on number of X chromosomes to sets of autosomes
- males: 0.5
- females: 1.0
dosage compensation
any mechanism that compensates for the difference in number of copies of genes between males and females
how many mechanisms of dosage compensation
4
what happens early in mammalian development to 1 of X chromosomes
randomly inactivated
Lyon hypothesis
random X inactivation hypothesis named after Mary Lyon (1962)
Barr body
inactive condensed X chromosome visible near nuclear wall
once X inactivation has occurred in a cell…
it is permanent in al the descendants of that cell
how can some female mammals be mosaics of 2 populations of cells
one cell expresses maternal X, other paternal X
what does the X chromosome carry a gene for in cats
coat color
tortoiseshell cats and mosaicism
random X inactivation in heterozygous females leads to pattern of orange/black patches unique to each ind.
random X inactivation requires what?
X-linked gene called Xist (X-inactivation-specific-transcript)
what does the Xist gene do
- produces large RNA moelcules that spread out and cover/pain chromosome to be inactivated
what can Xist only act on
the chromosome from which it is being transcribed, not the homolog
do all genes on the inactivate X undergo inactivation?
no, some escape
X-linked recessive inheritance
females homozygous for recessive allele or males homozygous for it display recessive phenotype
hemizygous
when only one copy is present
hemizygous males
display any allele on their single X whether the allele is recessive or dominant in females
X-linked dominant traits
heterozygous females and hemizygous males for dom allele express the dominant phenotype
hallmark of recessive X-linked inheritance
expression of trait much more often in males than females
do more males or females have X-linked recessive trait
males due to hemizygosity
hemophilia A
X-linked recessive trait caused by mutation in factor VIII gene on X chromosome
what does the mutant allele of hemophilia A produce
nonfunctional blood-clotting protein
frequency in sexes of X-linked dominant traits
appears relatively equally in males and females
congenital hypertrichosis (CGH)
rare X-linked dominant disorder in humans that leads to large increased in number of hair follicles on body
Y-linked trait transmission
exclusively male-to-male pattern
how many genes on Y chromosome in males
fewer than 50
do genes on the Y chromosome have a copy on the X chromosome?
no
