Lecture 2/18 Flashcards
what is a karotype?
a visual representation of an individual’s full set of chromosomes lined up by size and shape (during metaphase)
in a fertilized egg before the nuclei fuse together, how many chromosomes are there?
23 chromosomes in each nucleus, one set from each parent
how does one label a karotype?
total chromosome number and sex chromosome (ie 46, XX)
what is divided in meiosis one? is the resulting cell diploid or haploid?
homologous chromosomes, resulting cell is diploid
what is divided in meiosis two? is the resulting cell diploid or haploid?
a chromosome made of two sister chromatids, results in a haploid cell
what results from meiosis in a woman?
an egg
what results from meiosis in a man?
4 sperm
which two events create new combinations of alleles in meiosis?
crossing over between homologs and independent assortment of non-homologs
what is recombination?
2 homologous chromosomes exchange portions of DNA during meiosis i, creating new allele combos for linked genes and ensures proper chromosome segregation during meiosis
what is the result of a mistake in recombination?
nondisjunction
what are linked genes?
genes on the same chromosome
what does independent assortment of non-homologs mean?
different combinations of maternal and paternal chromosomes in gametes
what are some options for how homologous chromosomes find and align with each other in prophase i?
homologous chromosomes can be paired throughout devel (dipteran flies), non-coding RNAs can be used for homologue recognition (yeast), pairing can be based on genetic similarities among chromosomes (mammals)
what are the 2 kinds of similarities among chromosomes?
DNA sequence and size (a really big chromosome won’t pair up with a tiny one)
what forms when homologous chromosomes get close enough together?
synaptonemal complexes
what is associated with the synaptonemal complex and marks sites of recombination?
recombination nodules that contain enzymes for recombination
what disassembles synaptonemal complex and why?
phosphorylation by polo-like kinases, and it does this so the homol. chromosomes will only be connected at the sites of crossing over
what do exonucleases do?
cleave external phosphodiester bonds from overhangs/dsDNA
would a 3’ to 5’ or 5’ to 3’ exonuclease be used to get rid of a 3’ overhang?
5’ to 3’
why do bacteria have endo and exonucleases?
LOOK IN LECTURE
what are the two types of endonucleases?
to cleave internal phosphodiester bonds and make a blunt end OR a staggered end (overhang)
using bacteria grown in media with heavy or light isotopes, what would you expect to see if you expect no recombination
2 different bands
using bacteria grown in media with heavy or light isotopes, what would you expect to see if you expect recombination
DNA with hybrid densities/hybrid bands
what are the steps for homologous chromosome recombination
- all of the genes have to align
- Spo11 makes a double stranded break of one chromosome
- a complex does asymmetric 5’ chew back (5’ to 3’), leaving a 3’ overhang
- 3’ tail displaces and attaches to homologous chromosome
what is a 3’ tail invasion?
when the 3’ overhang in recombination displaces and attaches to the homologous chromosome
what does a 3’ tail invasion cause?
a displacement loop and then DNA synthesis
what holds open a D-loop?
a replication protein that stabilizes the displaced strand, opens up the helix
what happens to the gap created in the second strand of DNA not used in recombination?
DNA synthesis using the displaced strand as the template
what is a holliday junction
where four double-stranded arms meet in recombination and strands of DNA are exchanged between homologous chromosomes
what is the heteroduplex region?
the area between holliday junctions
what cuts holliday junctions?
an endonuclease (Gen1)
what direction does endonuclease Gen1 cut holliday junctions
either vertically or horizontally (equally likely)
which DNA strands do vertical cuts in the holliday junction cut
the outside ones
which DNA strands do horizontal cuts in the holliday junction cut
the inside strand
what reseals the cuts made by endonuclease Gen1?
ligase
when does non-crossover occur at the holliday junctions?
when the endonuclease cuts are made in the same direction, as only a small amount of DNA will be replaced
are recombination levels in mammals high or low?
relatively low
where in the genome is more likely to have recombination?
hotspots, whose positions are globally conserved
do females or males have higher crossover rates?
females
do longer or shorter chromosomes have higher rates of recombination?
shorter
what are chiasmata?
the visible points where 2 homologous chromosomes cross over during meiosis
what happens in metaphase i and anaphase i?
homologous chromosomes attach to microtubules from different poles and chiasmata are resolved to allow migration of homologous chromosomes to poles
are sister chromatids pulled to different poles in meiosis i?
no! the homologous chromosomes are separated
at what phase does recombination occur?
prophase
are longer or shorter chromosomes resolved first?
longer, as there are fewer recombinations
what is holding the homologous chromosomes together until resolved in anaphase?
the sites of recombination
at the end of meiosis i, what is n?
2n
do sister chromatids separate during meiosis ii?
yes!
what is n after meiosis ii?
n
is there a cohesin complex removed in meiosis i? ii? what is it removed by?
in both meiosis i and ii to separate the homol. chromosomes and then sister chromatids, it is removed by separase
what is crossing over between homologs?
different combos of paternal and maternal alleles within each chromosome
which meiosis is when most of the genetic diversity occurs?
meiosis i
what are some main differences between mitosis and meiosis?
mitosis is in somatic cells and homologous chromosomes don’t pair, and genetic exchange between homologous chromosomes is quite rare in mitosis
what is the difference in goals between mitotic proliferation of germ cells and meiosis?
mitotic proliferation is to make lots of diploid germ cells and meiosis is to generate gametes (sperm or eggs)
when is the switch from mitosis to meiosis in males?
puberty
when is the switch from mitosis to meiosis in females?
gestational week 10-20
in ovaries of female embryos, how many oocytes are generated during fetal devel? do they all stick around?
7 million are generated, most die before birth
what happens to the oocytes created before birth that don’t die before birth
they enter and arrest in meiosis i to become primary oocytes
are the chromosomes of primary oocytes condensed or de-condensed?
de-condensed
how many oocytes are still around at puberty
400,000
what happens to the oocytes once a month after a female goes through puberty?
hormonal stimulation causes one oocyte to re-condense chromosomes and complete meiosis i
do germ cells develop in the gonad?
no, they move there (they start out bipotential)
when do germ cells migrate to the gonad?
between 4 and 6 weeks depending on development of the gonad, the cells will either be sperm or egg
