MEIOSIS & Genetic VariationMEIOSIS & Genetic Variation

Goal: Divide up chromosomes & cytoplasm to sex cells so that when they unite the zygote has the correct # of chromosomes.

•Only in eukaryotic cells•Only is gametes

2 Cell Divisions (Meiosis I & II)2 Cell Divisions (Meiosis I & II)1 Duplication of chromosomes1 Duplication of chromosomes

• If Meiosis did not occur the chromosome # in each new generation would double…. The offspring would die.

Organisms that reproduce sexually are made up of two different types of cells.

1. Somatic Cells – “body” cells – “Normal” # of chromosomes (46 for humans)– Diploid # (the symbol is 2n). – Humans have 23 pairs of chromosomes for a total of 46

chromosomes. Diploid #: 46

2. Gametes – “sex” cells (sperm & eggs)– ½ the normal number of chromosomes

– Haploid # (the symbol is n)– Humans: n = 23 (23 pairs of chromosomes)

Making GametesMaking Gametes• Meiosis in males = spermatogenesis = make sperm

in testes

• Meiosis in females = oogenesis = make ova in ovaries

Ovum+ Sperm Zygote (fertilized egg)

2n=46zygote

n=23egg

Spermn = 23

SpermatogenesisSpermatogenesis

2n=46

humansex cell

diploid (2n)

n=23

n=23

meiosis I

n=23

n=23

n=23

n=23

sperm

haploid (n)

meiosis II

4 sperm cells are produced from each

primary spermatocyte.

Primary Spermatocyte

Secondary Spermatocyte

Secondary Spermatocyte

Oogenesis

*** The polar bodies die… only one ovum (egg) is produced from each primary oocyte.

Homologous Chromosomes2n = 46

n = 23 … pairs of chromosomes

1 set from mom; 1 set from dad

Homologous Chromosomes =

Matching pairs of chromosomesLocuscus: position of a geneposition of a gene

eye color locus

eye color locus

hair color locus

hair color locus

Paternal Maternal

Humans have 23 Sets of Homologous Chromosomes

Homologue

Homologue

Sex Chromosomes(Pair 23)XY (male)XX (female)

Autosomes Chromosomes # 1 – 22; code for most traits

23

Homologous ChromosomesHomologous Chromosomes

Homologous ChromosomesHomologous ChromosomesTetrad = Tetrad = homologous pairs when they line up and touchhomologous pairs when they line up and touch consists of consists of 4 4 chromatidschromatids

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

Stages of Meiosis2 cell divisions2 cell divisions with only 1 duplication of chromosomes.with only 1 duplication of chromosomes.

Interphase IInterphase I• Similar to mitosismitosis

interphase.• ChromosomesChromosomes replicate

(S phase).(S phase).• Each duplicated

chromosomechromosome consist of two identical sister chromatidschromatids attached at their centromerescentromeres.

• CentrioleCentriole pairs also replicate.

nuclear membrane

nucleus

nucleolus

Meiosis IMeiosis I

• Cell division Cell division that reduces the chromosomechromosome number by one-half (2n one-half (2n n) n)

• Stages:Stages:

a.a. prophase Iprophase I

b.b. metaphase Imetaphase I

c.c. anaphase Ianaphase I

d.d. telophase Itelophase I

Prophase IProphase I

• Longest; most complex phase.Longest; most complex phase.• 90% of meiosis90% of meiosis• Chromosomes condense.• SynapsisSynapsis occurs: homologous

chromosomes come together to form a tetradtetrad.

• TetradTetrad is two duplicated chromosomes (4 chromatids)

• CROSSING OVER occurs!CROSSING OVER occurs!

Also: Nuclear membrane disappears, spindle fibers form, etc.

Prophase IProphase I - - SynapsisSynapsis

Homologous chromosomes

sister chromatids sister chromatidsTetrad

Prophase I : “Crossing Over”Prophase I : “Crossing Over”

• Segments of nonsister chromatidschromatids break and reattach to the adjacent chromatidchromatid.

• RESULTS IN GENETIC RECOMBINATION! (New mixture of genetic material; Creates diversity in gene pool)

Crossing OverCrossing Over

nonsister chromatids

chiasmata: site of crossing over

variation

TetradHumans: 1 – 3 crossover events per chromosome pair in meiosis

Prophase IProphase I

centriolesspindle fiber

asterfibers

Metaphase IMetaphase I• Shortest phaseShortest phase

• TetradsTetrads align in middle of cell.

• INDEPENDENT INDEPENDENT ASSORTMENT OCCURS:ASSORTMENT OCCURS:1. Orientation of homologous pair to poles is random.2. Results in VariationFormula for genetic combinations: 2n

Example: 2n = 4then n = 2;thus 22 = 4 combinations

Metaphase IMetaphase I

metaphase plate

OR

metaphase plate

Anaphase IAnaphase I

• Homologous chromosomes separate and move towards the poles.

• SEGREGATION of chromosomes.

• Sister chromatids remain attached at their centromeres.

Sister chromatids attached

Homologous chromosomes separate

Telophase ITelophase I• Each pole now has

haploidhaploid set of chromosomeschromosomes.

• CytokinesisCytokinesis occurs and two haploid

daughter cells are formed.

Meiosis II:Meiosis II:• Sister chromatids separateSister chromatids separate• No interphase II No interphase II (or very short - no more DNA DNA

replicationreplication)• similar to mitosismitosis

MeiosisMeiosisMeiosis I: Meiosis I: Homologous Homologous chromosomes chromosomes divide (2n divide (2n n) n)

Meiosis II: Meiosis II: Sister Sister chromatids chromatids separateseparate

Mitosis vs. MeiosisMitosis vs. Meiosis

Mitosis Meiosis

1 parent produces 2 DIPLOID daughter cells (2n 2n)

1 parent produces 4 HAPLOID daughter cells (2n n)

Somatic cells Gametes

Parent & daughter are genetically identical

Daughter cells are not genetically identical (They have different chromosomes)

Prophase: Homologous chromosomes do not line up

Prophase 1: Homologous chromosomes SYNAPSE (Tetrads); Cross-over occurs

Metaphase: Chomosomes line up in middle of cell

Metaphase 1: Homologous chromosomes line up together.

Anaphase: Sister chromatids split Anaphase 1: Homologous chromosomes split; sister chromatids still attached.

Sources of Genetic Variation in Sexual Reproduction.

• 1. Mutation = original source of variation (in asexual & sexual)

• 2 Random fertilization = offspring are a combination of paternal + maternal chromosomes. There different versions of genes (alleles) for each trait (BB, Bb, bb; B = brown fur; b = black fur) Different versions = diff. nucleotide sequences on chromosomes.

Sources of Genetic Variation in Sexual Reproduction.

• 3. Independent arrangement of homologous chromosomes at metaphase plate in Metaphase I of meiosis. (Do you get maternal or paternal chromosome?)– 2n = # combinations n=23 chromosomes– Over 8 million combinations of chromosomes

4. Crossing over: Exchange of corresponding segments b/w 2 homologous chromosomes (Prophase I)

Sources of Genetic Variation: Crossing Over

• Produces new combinations of traits that did not exist in maternal or paternal genome.

New combinations of genes = Genetic Recombination

Genetic Variation: Who Cares?

Variations of traits w/in populations is what natural selection acts on in the process of evolution.

What traits are favorable? Unfavorable?

Nondisjunction in Meiosis• Failure of chromosomes to separate during Meiosis I

or II• Leads to abnormal # of chromosomes in offspring

(usually fatal)• Ex: Downs syndrome

Nondisjunction in Meiosis I vs. II

Nondisjunction in meiosis I all resulting gametes have incorrect chromosome # (fertilization will result in trisomy or monosomy in zygote)

Nondisjunction in meiosis II Half of resulting gametes have incorrect chromosome #

MonosomiesHaving 45 Chromosomes•Lethal if autosomal•Turner Syndrome (XO) - infertile females, short stature, “webbed” neck, possible cognitive impairments

http://en.wikipedia.org/wiki/Monosomy

http://web.udl.es/usuaris/e4650869/docencia/segoncicle/genclin98/recursos_classe_(pdf)/revisionsPDF/chromosyndromes.pdf

Trisomies: 47 chromosomesAutosomes•Down’s Syndrome (#21) – most common•Edwards (#18) •Patau (#13)

Sex Chromsomes (Y determines maleness)•XXY Klinefelter males (infertile, tall, long limbs, some secondary female characteristics, possible cognitive issues)•XYY normal male •XXX normal female

http://en.wikipedia.org/wiki/Monosomy

http://web.udl.es/usuaris/e4650869/docencia/segoncicle/genclin98/recursos_classe_(pdf)/revisionsPDF/chromosyndromes.pdf

Nondisjunction in Meiosis

Chromosomal mutations: change in an entire chromosome; may involve loss or duplication of multiple genes

Types of Chromosomal Mutations1.Deletion2.Duplication3.Inversion4.Translocation (balanced & unbalanced)

Types of Chromosomal Mutations

Partial deletions, duplications, translocations

http://en.wikipedia.org/wiki/Monosomy

Nondisjunction in Meiosis

Links• http://www.youtube.com/watch?v=Ba9LXKH2ztU&feature=related• http://www.youtube.com/watch?v=sJCWVTnFf5o&feature=related

• http://www.youtube.com/watch?v=QwmpD0OB3AQ&NR=1&feature=endscreen• http://www.youtube.com/watch?v=kVMb4Js99tA&feature=related