chapter 3 part 2 (meiosis) Flashcards
is meiosis a cycle?
no
what does meiosis produce?
gametes for sexual reproduction that are genetically different
what happens big picture in meiosis 1?
-homologous chromosomes separate
-centromeres DO NOT split
-reduces diploid number of chromosomes to the haploid number
what happens in meiosis ii big picture?
-sister chromatids separate
-centromeres split
-produce four haploid gametes
-each gamete has one chromosome from the original diploid set of homologous chromosomes
what is unique to meiosis?
homologous chromosome pairing
crossing over between homologous chromosomes
what happens when we segregate the homologous chromosomes?
the chromosomes are reduced to the haploid number (n)
what are the means of generating genetic diversity by sexual reproduction?
crossing over between homologous chromosomes
segregation of homologous chromosomes
stages of meiosis I
-prophase 1
-metaphase 1
-anaphase 1
-telophase 1
when does pairing & recombination of homologous chromosomes take place?
in prophase 1
what are the 5 stages of prophase 1?
- leptotene
- zygotene
- pachytene
- diplotene
- diakinesis
when does chromosome condensation begin?
leptotene
leptotene & zygotene
-chromosome condensation begins in leptotene
-meiotic spindle forms as microtubules extend out from centrosomes
-nuclear envelope disintegrates during zygotene
-homologous chromosomes undergo synapsis
when does nuclear envelope disentegrate?
during zygotene
what is the synaptonemal complex?
-forms between homologous chromosomes as they align
-tri-layer protein structure binds to non-sister chromatids (of homologous chromosomes) together
-non-sister chromatids belong to different members of a homologous pair
where does chromosome condensation continue?
in pachytene
what are paired homologs called?
tedtrads (all 4 chromatids are visible)
-this is in pachytene
where do recombination nodules form?
in the synaptonemal complex
what are recombination nodules?
aggregates of enzymes & proteins needed for crossing over between homologs
where do chromosomes even further condense?
in diplotene- then the synaponemal complex behins to dissolve
when homologs slightly pull apart in finalizing crossing over, what is revealed at locations where crossing over has occured?
chiasma
what protein is present between sister chromatids?
cohesin
what does cohesin do?
resists the pulling force of kinetechore microtubules
do centromeres split in meiosis 1?
NO
what happens in diakinesis?
kinetochore microtubules move synapsed chromosome pairs toward the metaphase plate
-homologs align side by side.
CENTROMERES REMAIN INTACT
by metaphase 1, what has happened?
chiasmata between homologs are resolved, which completes crossing over
what happens in metaphase 1
-homologs align on opposite sides of the metaphase plate (different arrangement than what happens suring mitosis)
-kinetochore microtubules attach to both sister chromatids of one homolog pair (kinetochore microtubules from the opposite pole do the same for the other homolog)
what occurs in anaphase 1?
-sister chromatids remain firmly attached by cohesion
CENTROMERES REMAIN INTACT
when does anaphase 1 begin?
-when homologs separate from one another & are pulled to opposite poles of the cell (movement caused by shortening of kinetochore microtubules)
what occurs in telophase 1?
the nuclear membranes briefly reform around the separated haploid sets of chromosomes
what follows telophase 1 & what happens?
cytokinesis, it divides the cytoplasm to create two haploid cells (n=n)
-each cell carries two copies of each haploid set (sister chromatids)
-diversity of information has been reduced !!!!!!
Meiosis I is called the ________ _________ because the ploidy of the daughter cells is _______ compared to the original diploid parent cell
reductional division
halved
-but there are still two copies of each genome
what does meisos II divide each haploid daughter cell into?
two haploid cells, by separating sister chromatids from one another
what is meisos II similar to?
mitosis in a haploid cell
what is produced in meisos II?
four genetically distinct haploid cells, each carrying one chromosome from a homologous pair
what is the major means of generating genetic diversity?
independent assortment
what is the independent assortment of alleles illustrated by?
behavior of two pairs of homologues during meiosis
for an organism with genotype AaBb, how many arrangements can occur, what are they, and what is the likelihood of each?
2
AB and ab
Ab and aB
-they are all equally likely to occur
what does sex determination involve?
the genetic & biological processes that produce the male & female characteristics of a species
what is chromosomal sex
the presence of chromosomes characteristic of each sex & is determined at the moment of fertilization
what is phenotypic sex?
the internal & external morphology of each sex, & results from differences in gene expression
what type of chromosomes do placental mammals have?
X & Y chromosomes
mammal female chromosomes
normal: XX
XO or XXX
mammal male chromosomes
normal: XY
XXY or XYY
psuedoautosomal regions
(PAR1 & PAR2)
exist between X & Y chromosomes
what allows homologous pairing between X & Y at meisosis
psuedoautosomal regions
-also evidence that crossing over occurs within these regions during meiosis
what in the mammal can either develop as testes or ovaries?
undifferentiated gonads
-(are clusters of tissue in mammalian embryos)
what is SRY?
a transcription factor needed for male specific gene expression
what is the single gene that its absence or presence determines the sex?
SRY, found on the Y chromosome
what does expression of SRY initiate?
testicular development of undifferentiated gonads
what does the absence of SRY allow?
the ‘default’ female state to develop
what chromosomal system do birds, some reptiles, some fish, & moths use?
the Z/Y system
in the systems that birds, some reptiles, some fish, & moths use, what makes up a female?
ZW (two different sex chromosomes)
in the systems that birds, some reptiles, some fish, & moths use, what makes up a male?
ZZ (same two sex chromosomes)
how many pairs of sex chromosomes do platypuses have?
5 XY pairs in males
5 XX pairs in females
drosophila female chromosomes
XXY or XX (normal)
drosophila male chromosomes
X0, XYY or XY (normal)
what ratio do male dropshila have?
X/A ration of 0.5
what ratio do female drosophila have?
1.0
what determines the outome of a sex determination gene regulation is drosophila?
the dosage of the X chromosome
how many different mechanisms of dosage are there?
4
Any mechanism that compensates for the difference in number of copies of genes between males and females is called
dosage compensation
what is the random X inactivation process also called?
the Lyon hypothesis
in mammalian development, what is randomly inactivated?
one of the two X Chromosomes
what is condensed as a Barr Body?
the inactive X chromosome that is visible near the nuclear wall
once X inactivation has occured in one cell, it is what
permanent in all the descendants of that cell
in cats, the X chromosome carries a gene responsible for what?
coat color
what does random x inactivation of heterozygous females lead to in cats?
a pattern of orange & black patches that is unique to each individual
what does random X-inactivation require?
an x-linked gene called Xist
what does Xist produce?
large molecules that spread out & cover (or paint) the chromosome to be inactivated
Xist can only act on what?
on the chromosome from which it is being transcribed
what sometimes escaped X inactivation
some genes on the inactive X
what is something common/expected with recessive X-linked inheritance?
the expression of the trait much more often in males than females
hemizygous
a diploid organsim is hemizygous when only one copy is present
the cell or organism is called a hemizygote
hemizygosity is also oberserved when one copy of a gene is deleted, or in the heterogametic sex when a gene is located on a sex chromosome
hemizygous males
-they display any allele on their single X whether the allele is recessive of dominant in females
who displays the recessive phenotype for X-linked recessive?
males hemizgyous for it or females homozygous for the recessive allele
in X-linked dominant traits, who expresses the dominant phenotype
heterozygous females & hemizygous males for the dominant allele
what does the mutant allele of hemphilia A produce?
a nonfunctional blood-clotting protein
what is de-novo
newly occurring mutation
what is hemophilia A caused by?
mutation in the factor VIII gene on the X chromosome
how is hemophilia A inherited?
as an x-linked recessive trait
for dominant x-linked traits it is equally likely for males & females to have it
true
what type of disorder is congenital hypertrichosis?
a rare x-linked dominant disorder
what does congenital hypertrichosis lead to??
large increase in the number of hair follicles on the body
Y linked traits transmission
exclusively male-to-male
do genes on the human Y chromosome have a copy on the X chromosome?
no
