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Cell Division: Meiosis
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Recallyou have two different types of cells in your body
somatic cells (body cells) aka diploid cells 46 chromosomes “2n”
gamete cells (sex cells) aka haploid cells 23 chromosomes “n”
chromosomes are counted by the number of centromeres, not the number of chromatids
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Recall: Cell Cyclecell cycle starts with interphase (G1, S, G2)
stages of mitosis
prophase
metaphase
anaphase
telophase
cytokinesis
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Introduction• Sexual Reproduction
• combines genetic material from each parent to form a new, genetically unique individual
• genetic information is carried on the chromosomes and each cell contains two sets of chromosomes, one set derived from the father via the sperm and one set derived from the mother via the egg
• Egg cells and sperm cells are called gametes
• Every living species has a species specific chromosome number, ex) normal humans always have 46 chromosomes, then 23 come from the mother and 23 come from the father
• Therefore gametes carry half of the normal number of chromosomes (23) and they are said to be haploid and are formed by a type of cell division called meiosis
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• All other cells with two complete sets of chromosomes are said to be diploid. There are two sets of chromosomes that match i.e) each chromosomes of one set has a mate or homolog in the other set
• these pairs are called homologous chromosomes
• Each homologous chromosome carries information for the same trait or characteristic
• This information is found in specific regions on the chromosome called genes
• Therefore, there would be a copy of a gene for hair colour from the father and a gene on the other homologous chromosome from the mother also for hair colour
• But, these two genes may not carry exactly the same information. If so, they are said to be Alleles or allelic forms and can be either dominant or recessive
Introduction
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Meiosis: Sex Cell Reproduction
in meiosis, cell with half the chromosomes are formed (23)
these haploid (n) cells are the reproductive cells - sperm and egg cells
the result of fertilization is called a zygote and is diploid (46)
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Meiotic Phases
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https://www.youtube.com/watch?v=VzDMG7ke69g
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mitosis meiosis
1 division 2 divisions
2 identical diploid daughter cells
4 genetically different haploid daughter cellssomatic (body) cell
productiongamete (sex) cell production
iPMAT
begins with diploid cell
homologous pairs
crossing over
cytokinesis
DNA synthesis
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Genetic Variationgenetic variation comes from crossing over (recombinant chromosomes)
during prophase 1 & metaphase 1
crossing over assures that the cells produced as a result of meiosis will be different from and exhibit variations from the parent cell that produced them
this process is responsible for the variations seen in members of the same species of sexually reproducing organisms
These variations are the driving force for the process of natural selection
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Crossing Over
Crossing over creates genetic variation by exchanging DNA between two non-sister chromatids to produce genetically unique chromosomes
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Sources of Genetic Variability
Crossing Over Independent Assortment
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Random Fertilization Mutation
Sources of Genetic Variability
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Stages of MeiosisSame 3 phases… just as in asexual division (Interphase, Nuclear Division, Cytokinesis)
But, 2 Divisions Meiosis I and Meiosis II
46 23 23 23 23 23 23
meiosis 1 meiosis 2
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Daughter CellsDuring meiosis, chromosome number in each cell is reduced (reduction division)
Four daughter cells contain one chromosome each
These cells become gametes
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Summary
Mitosis
asexually reproduction creating two genetically identical somatic cells (46 chromosomes)
Meiosis
produces sperm and egg cells allowing for new gene combinations creating four genetically different haploid cells (23 chromosomes); diploid cell is formed following fertilization (sexual reproduction)
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TwinsTwins can be either
monozygotic (identical) they develop from one zygote, which splits and forms two embryos
dizygotic (fraternal) they develop from two different eggs
Conjoined twins are identical twins joined in utero at some part of the body
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GeneticsGenotype: The genetic chromosomal expression of that particular organism (letters)
The genetic composition of the allele
Male sex chromosome: XY
Female sex chromosome: XX
Phenotype: What you see (physical) expression
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AneuploidyWhen a chromosome fails to separate during meiosis.
Therefore the cell has an abnormal number of chromosomes.
3 Main types of chromosomal abnormalities include:
Turner Syndrome
Klinefelter Syndrome
Down Syndrome (Trisome 21)
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Turner SyndromeThe presence of a single chromosome rather than a normal pair
Prevalence: 1/2000 live births
Because these individuals have an X chromosome and no Y chromosome they are females
Although they lack ovaries and their sex characteristics are not fully developed
Genotype: written as XO, OR X045
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Klinefelter SyndromeMales born with an extra X chromosome
Prevalence: 1/600 live births
Genotype produced is XXY OR XXY47
Because a Y chromosome is present individuals are males
Phenotype: Usually taller than average, but their sex glands are sterile
Some men exhibit some degree (spectrum) of mental delay
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Down SyndromeA group of symptoms that results from “Trisomy of chromosome 21”
47 chromosomes instead of 46 OR 47XX/ 47XY
Trisomy: gamete with an extra chromosome is fertilized by a normal gamete
The zygote will have an extra chromosome
Prevalence: much higher in older mothers, especially those over age 40
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Chromosomal Mutation effects
Changes may occur at the level of the chromosome as well as the gene
Mutations can occur in a variety of ways
Serious as they affect the distribution of genes to gametes during meiosis
Note: Few chromosome mutations are passed on to the next generation because the zygote usually dies
In cases where zygote develops, the mature organism is often sterile, and thus incapable of producing viable offspring
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Phases of MeiosisName of Phase Description
1. Homologous chromosomes pair up and form tetrad
2. Spindle fibers move homologous chromosomes to opposite sides
3. Nuclear membrane reforms, cytoplasm divides, 4 daughter cells formed
4. Chromosomes line up along equator, not in homologous pairs
5. Crossing-over occurs
6. Chromatids separate
7. Homologs line up along equator
8. Cytoplasm divides, 2 daughter cells are formed
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Additional Resources
Mitosis in Action: http://www.phschool.com/science/biology_place/labbench/lab3/mitfilm.html; http://www.pbs.org/wgbh/nova/baby/divi_flash.html
Mitosis and Meiosis Comparison: https://www.youtube.com/watch?v=Ba9LXKH2ztU
Crossing Over Demo: http://www.youtube.com/user/maureensullivan#p/a/u/1/I9X_T-OYK4g
Meiosis Animation: http://www.sumanasinc.com/webcontent/animations/content/meiosis.html
Meiosis Animation: http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120074/bio19.swf::Stages%20of%20Meiosis
Meiosis Recap: http://www.youtube.com/watch?v=jdQeKjEsj0U&feature=related
Mitosis and Meiosis Comparison: http://www.pbs.org/wgbh/nova/body/how-cells-divide.html