Cell Division: Meiosis

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

Recall: Cell Cyclecell cycle starts with interphase (G1, S, G2)

stages of mitosis

prophase

metaphase

anaphase

telophase

cytokinesis

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

• 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

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)

Meiotic Phases

https://www.youtube.com/watch?v=VzDMG7ke69g

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

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

Crossing Over

Crossing over creates genetic variation by exchanging DNA between two non-sister chromatids to produce genetically unique chromosomes

Sources of Genetic Variability

Crossing Over Independent Assortment

Random Fertilization Mutation

Sources of Genetic Variability

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

Daughter CellsDuring meiosis, chromosome number in each cell is reduced (reduction division)

Four daughter cells contain one chromosome each

These cells become gametes

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)

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

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

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)

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

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

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

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

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

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