dna

Meiosis

The form of cell division by which GAMETES, with HALF the number of CHROMOSOMES, are produced.

DIPLOID (2n) → HAPLOID (n)

Meiosis is SEXUAL reproduction.

TWO divisions (MEIOSIS I and MEIOSIS II).

Sex cells divide to produce GAMETES (sperm or egg).

Gametes have HALF the # of chromosomes.

Occurs only in GONADS (testes or ovaries).

Male: SPERMATOGENESIS -sperm

Female: OOGENESIS - egg or ova

Cell division that reduces the chromosome number by one-half.

Interphase I

Similar to mitosis interphase.

CHROMOSOMES (DNA) replicate in the S phase

Each duplicated chromosome consist of two identical SISTER CHROMATIDS attached at their CENTROMERES.

CENTRIOLE pairs also replicate.

Nucleus and nucleolus visible.

Prophase I

Longest and most complex phase (90%).

Chromosomes condense.

Synapsis occurs - Homologous chromosomes come togetherto form a tetrad.

Tetrad is two chromosomesor four chromatids (sister and non-sister chromatids).

Nucleus & Nucleolus disappear

Spindle forms

Chromosomes coil & Synapsis (pairing) occurs

Tetrads form & Crossing over Occurs

Non-Sister Chromatids-HOMOLOGS

Homologs contain DNA that codes for the same genes , but different versions of those genes

Genes occur at the same loci

Homologous Chromosomes

Pair of chromosomes (maternal and paternal) that are similar in shape and size.

Homologous pairs (tetrads) carry GENES controlling the SAME inherited traits.

Each locus (position of a gene) is in the same position on homologues.

Humans have 23 pairs of homologous chromosomes:

a. First 22 pairs of autosomes

b. Last pair of sex chromosomes

Crossing Over

Crossing over may occur between non-sister chromatids at sites called chiasmata.

Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.

Chiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over.)

Causes Genetic Recombination

Metaphase I

Shortest phase

Tetrads align on the equator.

Independent assortment occurs – chromosomes separate randomly causing GENETIC RECOMBINATION

formula

Formula: 2n

Example: 2n = 4

then 1n = 2

thus 22 = 4 combinations

In terms of Independent Assortment -how many different combinations of sperm could a human male produce?

Formula: 2n

Human chromosomes: 2n = 46

n = 23

223 = ~8 million combinations

Anaphase I

Homologous chromosomes separate and move towards the poles.

Sister chromatids remain attached at their centromeres.

Telophase I

Each pole now has haploid (1n) set of chromosomes.

Cytokinesis occurs and two haploid daughter cells are formed.

Meiosis II

No Interphase II

No DNA Replication

Remember: Meiosis II is similar to mitosis

Prophase II

Same as Prophase in mitosis

Nucleus & nucleolus disappear

Chromosomes condense

Spindle forms

Metaphase II

Same as Metaphase in mitosis

Chromosomes (not homologs) line up at equator

Anaphase II

Same as Anaphase in mitosis

SISTER CHROMATIDS separate

Telophase II

Same as Telophase in mitosis.

Nuclei and Nucleoli reform, spindle disappears

CYTOKINESIS occurs.

Remember: FOUR HAPLOID → DAUGHTER cells are produced.

Called GAMETES (eggs and sperm)

1n Sperm cell fertilizes 1n egg to form 2n zygote

Variation

Also known as GENETIC RECOMBINATION

Important to population as the raw material for NATURAL SELECTION.

All organisms are NOT alike

Strongest “most fit” survive to reproduce & pass on traits

What are the 3 sources of genetic recombination or variation?

CROSSING OVER (prophase I)

INDEPENDENT ASSORTMENT (metaphase I)

RANDOM FERTILIZATION

A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

10 chromosomes (haploid or 1n)

Karyotype

An organized picture of the chromosomes of a human arranged in pairs by size from largest to smallest.

Pairs 1-22 called AUTOSOMES

Last pair are SEX CHROMOSOMES

Fertilization

The fusion of a sperm and egg to form a zygote.

A zygote is a FERTILIZED EGG

A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

10 chromosomes