Chromosomes & InheritanceMEIOSIS

-two successful divisions of a diploid nucleus following only one DNA replication cycle- the original diploid cell contained a haploid set of chromosomes from mom and a haploid set from dad (except for self-fertilizing organisms - plants)- produces a haploid gamete

Chromosomes & InheritanceINTERPHASE

- G1, S, G2

- chromosomes are duplicated- produces a temporary 4N- each “chromosome” consist

of 2 sister chromatids attached at a centromere

Chromosomes & InheritanceMEIOSIS I

-chromosome number is reduced

- PROPHASE I-similar to Prophase of

Mitosis except for crossing over of homologous pairs of chromosomes

-4 stages

-produces greater chromosomal individuality of offspring

Chromosomes & InheritanceMEIOSIS I

- PROPHASE I-Leptonema – chromosomes

begin to coil and become visible-Telomere – end of

chromosomes that do not code for proteins; protect and secure the chromosome;

Chromosomes & InheritanceMEIOSIS I

- PROPHASE I-during Meiosis I these

are clustered on the nuclear envelope to produce a bouquet of the chromosomes

-Zygonema – chromosomes continue to shorten (coil)

Chromosomes & InheritanceMEIOSIS I

- PROPHASE I-homologous pairs align

and undergo synapsis (formation of the synaptonemal complex) which aligns them base pair for base pair

-made possible because of the telomeres and bouquet

Chromosomes & InheritanceMEIOSIS I

- PROPHASE I-Pachynema – follows

completion of synapsis

-in this tetrad is where crossing-over now occurs

-each synapsed set of homologous chromosomes has 4 chromatids, this is called a tetrad

Chromosomes & InheritanceMEIOSIS I

- PROPHASE I-resulting genetic

differences between the homologs can produce new gene combinations -there is usually no loss or gain of genetic material here, just exchange – each homolog gives and receives genes

Chromosomes & InheritanceMEIOSIS I

- PROPHASE I-recombinant

chromosome - chromosome that has a different combination of genes than what it began meiosis with

-genetic recombinantion - process by which progeny receives genotypes different from the parents

Chromosomes & InheritanceMEIOSIS I

- PROPHASE I-Diplonema - synaptonemal

complex is disassembled and homologous chromosomes move apart

-chiasma – cross-shaped structure resulting from the

overlapping of chromatids

Chromosomes & InheritanceMEIOSIS I

- PROPHASE I-at these locations the

homologous chromosomes are very tightly connected

-in most organisms the remainder of meiosis goes very quickly, except for egg development in animals

-Humans: diplonema is completed by the 7th month of fetal development, then they wait, wait, wait

Chromosomes & InheritanceMEIOSIS I

- PROPHASE I-Humans: The onset of

puberty begins a cycle of one oocyte per month completing meiosis I and is ovulated.

-Humans: If fertilized, it quickly completes meiosis II

as it passes down the fallopian tube and then a functional zygote is produced

Chromosomes & InheritanceMEIOSIS I

- PROPHASE I-Diakinesis – nucleolus and

nuclear envelope break down-spindle is assembled-chromosomes can be

counted fairly easily* Crossing-over occurs between homologous chromosomes, mainly autosomes

Chromosomes & Inheritance

Chromosomes & InheritanceMEIOSIS I

- METAPHASE I-nuclear envelope is

completely broken down-tetrads become aligned at the equator of the cell-spindle is completely formed-microtubules are attached to

the homologs

PowerPoint® Layered Art

Chromosomes & InheritanceMEIOSIS I

- ANAPHASE I-chromosomes of the

homologous pair (dyads) disjoin and move to opp. poles

-centromeres segregate randomly

-move to opp. poles is completed

-sister chromatids remain joined- TELOPHASE I

-new nuclear envelope forms-cytokinesis is happening

concurrently

PowerPoint® Layered Art

Chromosomes & InheritanceMEIOSIS II

- PROPHASE II-chromosomes condense

-spindle is organized and attaches to centromere (still connecting sister chromatids)

- METAPHASE II

-centromeres line up on the equator of the cell

PowerPoint® Layered Art

Chromosomes & InheritanceMEIOSIS II

- ANAPHASE II-centromeres split-chromatids are pulled to opp. sides

- TELOPHASE II-nuclear envelope forms

around each set of chromosomes

-sister chromatids of each pair move to opp. poles

-cytokinesis takes place-chromosomes become

elongated

PowerPoint® Layered Art

Chromosomes & InheritanceMEIOSIS II

- after TELOPHASE II-chromosomes become

elongated and no longer easily visible -end product is 4 haploid cells

(gametes)-each gamete has one

chromosome (not exact copies) from each homologous pair