Meiosis

Essential terms

GametogenesisGametogenesis SynapsisSynapsis Crossing overCrossing over Disjunction / Non disjunctionDisjunction / Non disjunction ChiasmataChiasmata

Key pointsKey points

Mitosis – Mitosis – diploid. Identical. Stability. Meiosis – Meiosis – haploid. Combination. Variation. Sexual reproduction ensures genetic continuity Sexual reproduction ensures genetic continuity

and genetic variety, producing offspring's that and genetic variety, producing offspring's that often differ greatly from patients.often differ greatly from patients.

Associated with gametogenesis.Associated with gametogenesis.Counterbalances fertilization - makes sure # of

chromosomes remains constant

Meiosis

Purpose :

production of sex cells (gametes)

Outcome 2 divisions 4 daughter cells with

1/2 (haploid) sets of chromosomes

1. Chromosomes found in homologous pairs identical but may have varying gene messages

b. Humans have 46 (23 pairs) 22 homologous pairs (autosomes) X and Y (23rd pair) sex chromosomes Females (XX); Males (XY)

c. Pairs separate during reproduction Offspring receives info from each parent Exchanging genetic material during crossing over

(Prophase I) d. Separated during meiosis (Anaphase I)

Homologous chromosomes form pairs in form Homologous chromosomes form pairs in form of tetrads (4 chromatids)of tetrads (4 chromatids)

Involves 2 divisions = Meiosis I and IIInvolves 2 divisions = Meiosis I and II

Interphase

chromosomes replicateConsists of 2 identical sister chromatids

attached at their centromerescentriole pairs also replicate into two pairs

Prophase IProphase I

Condenstaion. Chromatin thickens and coils. Become visible .Condenstaion. Chromatin thickens and coils. Become visible . Synapsis occurs Homologous chr. pair up forming tetrads.Synapsis occurs Homologous chr. pair up forming tetrads. Sister chromatids are attached at centromeres. Each pair = bivalent. Nonsister chromatids features crossing over resulting in

chiasmata. = More than 1 can form = genetic variation) • spindle forms from microtubules • nuclear envelope/nucleoli disperse

5 stages: Leptonema, zygonema, pachynema, diplonema and 5 stages: Leptonema, zygonema, pachynema, diplonema and diakenesis.diakenesis.

Metaphase I

Chromosomes have thickened.Chromosomes have thickened. Each tetrad interacts with spindle fibers.Each tetrad interacts with spindle fibers. Movement to the equitorial plate.Movement to the equitorial plate.Homologues are destined to separate towards

opposite poles

Anaphase I

sister chromatids remain attached while homologues move towards the opposite pole. (Mitosis =sister chromatids are moved apart)

One half of each tetrad ( one pair of sister One half of each tetrad ( one pair of sister chromatid) is pulled toward each pole of the chromatid) is pulled toward each pole of the dividing cell.dividing cell.

No attraction of the sister chromatids – No attraction of the sister chromatids – reduction in the number of chromosomes.reduction in the number of chromosomes.

Telophase I + Cytokinesis

Very short phase compared to mitosisVery short phase compared to mitosis May not always occursMay not always occurs Each pole now has a haploid set of chromosomes

composed of two sister chromatids attached at the centromere

cytokinesis occurs producing two daughter cells Nuclear membrane forms.Nuclear membrane forms. No interphase – No DNA replictaion. Already 2 No interphase – No DNA replictaion. Already 2

chromatids.chromatids.

2nd division (II)

Prophase IIProphase II = =spindle apparatus formsChromosomes move towards the metaphase II

plate

Metaphase IIMetaphase II = = chromosomes align on metaphase plate

kinetochores of sister chromatids point towards opposite poles

Anaphase II = Anaphase II = centromeres of sister chromatids separate

sister chromatids are pulled towards opposite sister chromatids are pulled towards opposite polespoles

Telophase II = I member of each homologue Telophase II = I member of each homologue pair is present at each pole.pair is present at each pole.

Each chromosome is referred to as monads.Each chromosome is referred to as monads.

Cytokinesis

4 haploid gametes results.4 haploid gametes results. Haploid state has been achieved.Haploid state has been achieved. If crossing over has occurred = each monad is If crossing over has occurred = each monad is

a combination of maternal and paternal genetic a combination of maternal and paternal genetic information ie. Receives info from information ie. Receives info from grandparents. = increases genetic variation. grandparents. = increases genetic variation.

Independent assortment

Mechanism that allows for genetic variation.Mechanism that allows for genetic variation. Reduction of diploid – diploid means that each Reduction of diploid – diploid means that each

gamete will only carry one form of gene for a gamete will only carry one form of gene for a particular characteristics.particular characteristics.

Crossing over results in the exchange of genetic info Crossing over results in the exchange of genetic info from maternal to paternal chromosomes = possibility from maternal to paternal chromosomes = possibility of new combination of genes.of new combination of genes.

Random assortment = mixture of maternal and Random assortment = mixture of maternal and paternal chromosomes = more combination.paternal chromosomes = more combination.

Nondisjunction and Translocation

Too few or too many chromosomes in gametes Translocation: attachment of all or part of chromosome to another

chromosome Nondisjunction: failure of chromosomes to separate

Down Syndrome 1. Translocation of chromosome #15 to #21, or 2. Nondisjunction with extra #21

Nondisjunction of sex chromosomes 1. XO- Turner Syndrome; sterile female 2. XXY- Kleinfelter Syndrome; sterile male 3. XXX- Metafemale; limited fertility