Transcript
Molecular Biology Chapter 13
Meiosis
Cell division
Includes a reduction in chromosome number
Why would this be useful?
2n 4n 8n 16n 32n
Also provides new genetic variation
Chromosomes
In many species, chromosomes come in pairs
1 from mom and 1 from dad
Homologous chromosomes- homologs
Homologous chromosomes carry the same genes
But genes can have different forms
Alleles= variants of genes
Homozygote: alleles identical
Heterozygote: alleles different
Ploidy
# of sets of chromosomes (n)
Humans
1 set from mom and 1 set from dad
2n= diploid
Not all human cells are diploid
Gametes only have 1 set of chromosomes
1n= haploid
Many bacteria, algae, and fungi are predominantly haploid
Some species have many sets of chromosomes
Polyploidy
4n= tetraploid, 8n= octoploid, etc…
Meiosis
Many similarities with mitosis
Before division, chromosome must be doubled
Sister chromatids= exact copies of a chromosome
Outcome: 4 haploid cells from 1 diploid cell
2 divisions
1st and 2nd meiotic divisions
Meiosis 1
1 diploid cell 2 haploid cell
Each chromosome in the haploid cell is doubled
Meiosis 2
Sister chromatids separate into their own cells
Divisions can be broken into similar stages as mitosis
Meiosis 1
Prophase 1
Metaphase 1
Anaphase 1
Telophase 1
Meiosis 2
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2
Meiosis- chiasma
During late prophase, individual sister chromatids may cross over with their homologs
Net result= new chromosome that is made of parts of both parent’s chromosomes
Meiosis- crossing over
In humans, more common in egg formation than in sperm formation
1.7 crossovers/ chromosome vs 1.1
Might be genetic variation between people, too
Meiosis- metaphase 1
Homologous chromosomes line up side-by- side on the metaphase plate
Each pair of sister chromatids is attached to ONLY 1 side
Meiosis- anaphase1/telophase1
Homologous chromosomes move to opposite poles
Mom’s chromosome to one side, dad’s to the other
New cells are haploid
Nuclear envelop may reform
Meiosis
Meiosis 1
Homologous chromosomes separate
Crossing over
Meiosis 2
Sister chromatids separate
Meiosis 2 is similar to mitosis
Consequences of meiosis
Formation of gametes- sexual reproduction
Required for maintaining proper ploidy level
Introduces genetic variation
Chromosome assort independently during metaphase 1
With 2 chromosomes, you can produce gametes that are:
MM, DD, MD, DM
2 chromosomes= 4 combinations
2# of chromosomes
Variation created by alignment during metaphase 1
Humans= 23 chromosomes
Number of possible alignments= 2^23
About 8.4 million different gametes possible even without crossing over
# of possible gamete combinations between 2 people: 2^23 X 2^23 === about 70 trillion combinations
Variation of chromosome number
Lots of variation that can happen
Aneuploidy= normal #of chromosomes +/- individual chromosomes
Euploidy = multiple sets of chromosomes
Consequences of aneuploidy
Missing a chromosome is usually lethal in animals
Exceptions
45, X- Turner syndrome in humans
Drosophila can survive having only 1 chromosome 4
In plants, individuals with a single chromosome can survive, but are usually less viable
Pollen grains don’t function as well
Trisomy- 1 extra chromosome
In humans, generally not survivable except trisomy 21
Down syndrome
1: 800 births
About 250,000 in the USA
Down syndrome
12-14 possible phenotypic characteristics, most only have 6-8
Slowed motor, psychomotor, mental development
Average life expectancy = about 50 years
Trisomy 18- Edward syndrome
1: 6,000
Mental and physical retardation, facial abnormalities, early death
Trisomy 13- Patau syndrome
1: 15,000
Paradox of sex
A species with asexual reproduction will grow 2x as fast as one with sexual reproduction
Limiting step is the number of females
In an asexual species, everyone is female, everyone can reproduce on their own
In a sexual species, takes 2 individuals (male and female) to make an offspring
Males are slowing us down!!!!!!!!
2 hypotheses
Purifying selesction
If oyu are an asexual species, mutation in the mother ets passed to all her offspring
Bad news, evolutionarily
In a sexual species, contributions from the male and/or crossing over can get rid of mutations
Compare closely related sexual and asexual species
Compare mutations
Silent substitutions
Amino acid substitutions
Changing- environment
“good” genotypes are defined by how they interact with the environment ]
What happens when the environment changes? Like new pathogens/ parasites?
For an asexual species, offspring are weak against environmental change/ new disease/ new parasite because they are a;; genetically identical
For a sexual species, new genotypes are always being created, likely to find a “good” one for the new environment
What would this look like over the long term?
Q: How do parasites affect reproductive strategy?
H: Parasites coevolve to match most common genotypes.
P: Areas with high parasite loads will be more likely to have sexually reproducing species
So how can asexual species persist?
Hypothesis: Clonal lineages will reproduce faster than sexual species, but only until parasites become adapted to them
Test: Follow individual clonal lineages in a mixed population that includes sexual species
Prediction: Clonal lineages that are successful at the beginning will not be successful over the long term
Clonal species do well at first, but when parasites catch up, they are at a net disadvantage
