Transcript
Both meiosis and mitosis are preceded by replication of chromosomes
Meiosis 1
2 homologous pairs separate into 2 daughter cells
Each daughter ahs 1 set of chromosomes
2n to n
Known as reduction division
Meiosis 2
Sisters separate into 2 daughter cells
n to n
Results in 4 haploid cells
Meiosis 1 and Reduction
Diploid cells produce 4 haploid daughter cells that become eggs or sperm by gametogenesis
Fertilization results in a zygote that has restored number of chromosomes
Independent assortment of chromosomes
Early Prophase 1
Homologs pairs come together through protein binding to form tetrads
Bivalent (2 homologs)
Chromatids of the homologs are non-sister chromatids
Nuclear envelope beings to break down
Synapsis crossing occurs between non-sister chromatids
Late Prophase 1
Homologs begin to separate
Remain attached at points called chiasmata (this is where crossing over occurs)
Crossing over occurs between non-sister chromatids
Produces chromosomes with a combination of maternal/paternal alleles
Metaphase 1
Paired homologs line up on plate in tetrads
Alignment is random (this provides variation)
NOT SISTER CHROMATIDS
Homologous chromosomes
Anaphase 1
Paired homologs separate and migrate to opposite ends of the cell
Again, NOT SISTER CHROMATIDS
Telophase 1
Movement of homologous chromosomes continues until there is a haploid set at each plate
Chromosomes are in random assortment as a result of crossing over and random distribution of mom/dad genes
After telophase 1, the cell will undergo cytokinesis
Meiosis 1 Overview
Results in daughter cells with only 1 chromosome of each homologous pair
Haploid but still contain replicated chromosomes
Random assortment of chromosomes due to crossing over and random distribution of maternal/paternal homologs to daughter cells
NO CHROMOSOME REPLACTION OCCURS BETWEEN MEIOSIS 1 AND 2
Meiosis 2
Similar to mitosis
(n n)
Sister chromatids separate
Prophase 2
Spindle apparatus forms
Spindle fibers attach to kinetochores (one to each sister chromatid)
Metaphase 2
Replicated chromosomes line up at metaphase plate
Each chromosome is attached by spindle fibers to both spindle poles
Anaphase 2
Sister chromatids separate via centromeres
Resulting daughter chromatids move to opposite sides of the cell
Telophase 2
Chromosomes arrive at opposite sides of the cell
Nuclear envelope forms around each haploid set of chromosomes
After telophase 2 the cells undergo cytokinesis resulting in 4 haploid daughter cells
In meiosis, homologous chromosomes are separated during Anaphase I.
After telophase I of meiosis, the chromosomal makeup of each daughter cell is haploid, and the chromosomes are each composed of two chromatids.
Synapsis and Crossing Over
As chromosomes condense, sister chromatids stay joined together along their kinetochore’s entire length by cohesions
Homologs pair by synapsis and are held together by proteins called the synaptonemal complex
Breaks are made in the DNA and crossover occurs between corresponding segment of non-sister chromatids
Synaptonemal complex disassembles and homologs are held together only as chiasmata
Crossing Over
New combinations of alleles on the same chromosome that didn’t exist before
Increases genetic variability of gametes produced in meiosis
Produced by random assortment
Mitosis Promotes Genetic Variation
Independent assortment
Crossing over during Meiosis 1
Fertilization makes it even more unique and restores the chromosome number
Asexual no variation, based on meiosis, does not involve gametes (clones)
Sexual each offspring is different (shuffling of alleles)
Independent assortment
Random separation of homolog chromosomes results in variation
Genetics recombination creates genetic diversity
How to calculate number of unique gametes: 2^n when n=haploid chromosome number
How many different combinations of maternal and paternal chromosomes can be packaged in gametes made by an organism with a diploid number of 8 (2n=8)? 16
When Thing Go Wrong in Meiosis
Non-disjunction (homologs fail to separate)
Meiosis starts normally with bivalent lining up at center
Non-disjunction occurs with one set of homologs
Sister chromatids separate normally
Aneuploidy results in too many or too few
n+1 = trisomy
n-1 = monosomy
Why Do Mistakes Occur?
Age
Egg development (oogenesis)
Primary oocytes enter meiosis 1 during embryonic development
Oocytes arrest in prophase 1 till sexual maturity is reached
Don’t complete meiosis till ovulation
Why Does Meiosis Exist?
Sexual reproduction is uncommon, but offspring may lack deleterious alleles
Asexual reproduction is more efficient and produces more offspring, but a damaged gene will be inherited
Offspring genetically identical to parents are less likely to survive if environment changes
Plants produce more seeds when they reproduce asexually than sexually. Yet most plants reproduce sexually in nature. What is the probable explanation for the prevalence of sexual reproduction? Sexual reproduction…Mixes up alleles contributing to variation in species
The egg of a fly has 4 individual chromosomes. How many chromosomes are in a somatic cell of a fly? 8 – somatic cells have two times the number of chromosomes than gametes, multiply 4 by 2 to get the diploid number
Activity
Feature: Meiosis, Mitosis
Number of cell divisions: 2, 1
Number of chromosomes in daughter cells compared to parent: ½, Same number
Synapsis of homologs: Yes (prophase 1), No
Spindle fiber attachment: Yes, Yes
Number of crossing over events: 1 or more times per pair of homologous chromosomes, None
Make-up of chromosomes in daughter cells: Different, Same
DNA content amount of daughter cells compared with parent: ¼, ½
Role in organism life cycle: Gamete production, Somatic cell growth/repair
