Week 2: cell reproduction Flashcards
What’s a gene?
A gene is a portion of a chromosome that codes for a trait
How many chromosomes do you have? Are they all identical? Are they the same as your parents?
You have 23 pairs so 46 chromosomes
- Each pair contains one chromosome from your mum and one from your dad. (Although the chromosome your parent gave you isn’t identical to any of theirs). They both have genes in the same loci but may be different alleles (aka dominant and recessive) so they form a homologous pair.
For each pair you pass on a blend of these two chromosomes to your offspring: via crossing over genes are exchanged by non-sister chromatids to create new unique combinations of those genes so both the whole chromosomes in the pair have been re-designed but also the sister chromatids are no longer identical because some have had certain genes shuffled and others not.
–> You do a mashup of these two chromosomes (two versions of the gene, each version in two identical copies because two sister chromatids) then each cromatid gets recombined… forms into a set of 4 unique chromosomes.
What happens during meiosis?
START: 2 different chromosomes, one maternal one paternal. Each has 2 identical sister chromatids (2 copies of each allele (4 when homozygous)).
P1: crossing over of non-sister chromatids. Leads to 4 unique chromatids no longer two pairs of two identical chromatids.
M1: alignment of jumbled homologs either side of metaphase plate. (Like an original chromosome but sister chromatids are non-identical).
A1: whole chromosomes (comprised of 2 unique chromatids) from the homologous pair are pulled apart.
T1: 2 daughters, each with a chromosome with two unique chromatids.
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ROUND 2: THIS is where you get haploid which needs to meet with another gamete to form a whole genome.
Start: daughter cell with two unique chromatids.
P2: no replication
M2: alignment of chromosomes with non-identical chromatids either side of midline.
A2: non-identical sister chromatids are pulled apart. NOW you’re haploid.
T2: 4 unique cells formed, each with only one version/allele of each gene: haploid, 23 chromosomes not 46.
END: 1 allele on one unique chromatid (now called chromosome) (instead of two copies of same thing.
What are the four general events of cell division? Applicable to all replication types
The four events of cell division are
1) A reproductive signal
2) Replication of DNA
3) Segregation of DNA
4) Cytokinesis
Crossing over errors rant
T1: 2 daughters, each with a chromosome with two unique chromatids. So in theory, instead of having two copies of the same allele (as you would have on identical chromatids of one member of a pair), it has one copy of each allele so basically is the condensed version of all the possible alleles. Because of duplicate chromatids you start off with a duplicate of everything so if you have Aa you actually have AAaa because each is duplicated across chromatids. So the end result is each X -shaped chromatid you end up with after this round has only an Aa because the duplicates are in the other X. Theoretically if meiosis stopped here and then the non-identical chromatids split, becoming chromosomes with their own identical chromatides, you’d go back to having a pair of chromosomes with the original genome: essentially a clone, still diploid, of your parent.
If you then add another gamete to this you get a trisomic child (2 from you and 1 from other parent), or uniparental disomy (2 from you and 0 from other parent)
- -> Down syndrome is trisomy 21: you get an extra copy of chromosome 21: for this particular chromosome maybe it duplicated when it wasn’t meant to, or somehow the non-identical chromatids weren’t pulled apart in A2, so instead of getting one unique half of the set of genes, you get a copy of every allele on the chromosome that your parent had (disomic).
- Then add your other parents gamete? You become trisomic (3 copies: a pair (2) from one parent and 1 from other. It is possible that one of these chromosomes is lost, meaning you’ll either go back to having a normal 1 copy from each parent, or you’ll get uniparental disomy: you lose the copy from your other parent, and have two copies from one parent.
http: //hihg.med.miami.edu/code/http/modules/education/Design/CoursePageContent.asp?ID=11
Disomic: two copies of one chromosome
Diploid: two copies of every chromosome.
Trisomic: three copies of one chromosome.
Triploid: three copies of every chromosome.
Note: having three copies of a particular gene (not whole chromosome) could be due to crossing over not happening right: in theory, if crossing over happens right, instead of each chromosome being a set of two copies of half it’s portion of the genome, is a set of one copy of every gene in it’s portion of the genome (each chromosome obviously doesn’t represent each portion of the genome, but 1/46 of it). If this isn’t right then you may end up with a double up of a particular gene and a lack of another gene.
Say it happened in EVERY chromosome, not just one. You’d not be trisomic, but triploid. Instead of getting a set of half the alleles in the genome, you’d get one of every allele in your parent’s genome. If you then combine this with your other parents gamete you’d be triploid. If you lost their copies of the chromosome somehow for EVERY chromosome, you’d end up back to having only the pair of chromosomes from one parent: you’d be a CLONE.
How does crossing over work in too much questionable detail rant? (Skip card)
So in theory, instead of having two copies of the same allele (as you would have on identical chromatids of one member of a pair), it can have one copy of each allele so basically is the condensed version of all the possible alleles. Because of duplicate chromatids you start off with a duplicate of everything so if you have Aa you actually have AA on one pair of sister chromatids and aa on another because each is duplicated across chromatids. So the end result is each X -shaped chromatid you end up with after this round has only an Aa because the duplicates are in the other X. Note, this is all random, swapping over might not yield a perfect Theoretically if meiosis stopped here and then the non-identical chromatids split, becoming chromosomes with their own identical chromatides, you’d go back to having a pair of chromosomes with the original genome: essentially a clone, still diploid, of your parent.
NOW: you don’t have two copies of the same allele on each chromosome,
because although you’ve mixed up which chromosome has which allele, you haven’t lost or gained anything, so overall genotype is the same. You still would have two alleles
