Meiosis – The spice of life

Meiosis – The SPICE of life

Meiosis

• A cell division that separates homologous pairs of chromosomes forming haploid sex/germ cells

•Whaaaa?• Let’s back it up a few steps

Parents give you your genesA. Gene – a unit of hereditary info consisting of a specific nucleotide sequence of DNA, via a gamete

Genes are packaged into chromosomes.

B. A Karyotype• is a display of all a

cell’s chromosome pairs in order of shape and size

C. Chromosomes come in pairs…in somatic cells that is!

-Homologous chromosomes (one from mom and one from dad) have the same -size -centromere location -shape -gene type location (locus) -humans have 46 chromosomes, or 23 pairs!

mom dad

These are all double stranded chromosomes

Normal Male Karyotype

These are all double stranded chromosomes

Normal Female Karyotype

Typical Karyotypes

1. How many pairs of homologous chromosomes are found in a typical human cell?2. Which chromosome pair is not homologous in humans?3. Why are chromosomes found in pairs? (hint – sexual reproduction)4. What must happen to homolog chromosomes in cells used for sexual reproduction?

Sex chromosomes – 23

Autosomes 1-22

• This karyotype has 2 copies of every chromosome, and we call that diploid (2n). The “n” stands for a set of chromosomes. This means it came from a somatic (or body) cell.

• If two somatic cells fertilized each other, what would the offspring’s karyotype look like, or what would the chromosome number be?

YIKES!

***ALMOST ALL your cells are diploid – skin, muscle, bone, fat, nerve

What does meiosis do to the # of chromosomes?

• It reduce the chromosome number to one chromosome from each homologous pair! The chromosome number is now haploid (n). The type of cell formed is a gamete (aka germ, sex, reproductive , sperm, egg)

• Humans – somatic cell = 46 chromosomes• Humans – germ cell = ____ chromosomes

• When is a human cell’s ploidy (chromosome set) restored to diploid?

23

Fertilization

There are 3 ways to define MEIOSIS

1. What happens to homologous chromosomes?

2. What cell type is formed?

3. What happens to the ploidy/chromosome number?

***Assume you start meiosis with a somatic cell***

Meiosis separates homologous pairs of chromosomes into new cells

Meiosis makes gametes (sex, reproductive cells)

Meiosis reduces the diploid # (2n) to haploid # (n), aka (reduction division)

II. Meiosis follows the cell cycle• Interphase (G1, S, G2) followed by TWO cell

divisions – Meiosis I and Meiosis II.• Don’t forget that chromosomes duplicate

during the S phase!

Overview of MeiosisMEIOSIS I: Separates

homologous chromosomes

Prophase I Metaphase I Anaphase ITelophase I

andCytokinesis

Prophase II Metaphase II Anaphase IITelophase II

andCytokinesis

Sisterchromatidsremainattached

Sisterchromatids

Chiasmata

Spindle

Homologouschromosomes

Microtubulesattached to kinetochore

Homologouschromo-somesseparate

Cleavagefurrow

Sisterchromatidsseparate

Haploiddaughtercellsforming

MEIOSIS II: Separatessister chromatids

Meiosis I = “reductional division” as chromosome number goes from diploid to haploid

Meiosis II = “equational division” as sister chromatids separate (start double-stranded, centromeres break and become single-stranded)

A. Importance of Meiosis I

• In Prophase I of Meiosis I, crossing over occurs. This is when the double-stranded homologous pairs connect and exchange DNA (some of mom’s genes go onto dad’s chromosomes and vice versa). This is also known as recombination.

Crossing Over and Synapsis in Prophase I

Paternalsisterchromatids

Crossover – DNA exchanged

2. Synapsis occurs – When homologous pairs “associate” (loosely bond together)

1

2

3

4

4. Chiasmata - points where crossing over occurred

B. Steps of Meiosis I

Prophase IUnique Events:1. Synapsis: Homologous Chromosomes Pair up 2. Crossing over: Homologous chromosomes exchange parts

Metaphase I Homologous Chromosomes line up opposite one another Centromeres are not lined up on the metaphase plate (different arrangement than mitosis)

Anaphase I Complete DS Chromosome moves the poles. CENTROMERES DO NOT SPLIT

Telophase I Cytokinesis

Meiosis II - ***The two cells are haploid! But theyhave double-stranded chromosomes!

Prophase II May be skipped (human males) Will not start until fertilization (human females)

Metaphase II Chromosomes line up at equator ***Centromeres line up on the metaphase plate (like mitosis)

Anaphase II CENTROMERES SPLIT ***Double Stranded chromosomes become single stranded

Telophase II Cytokenesis Maturation of gametes into 4 HAPLOID daughter cells

III. How Meiosis Increases Genetic Variety in Offspring

A. Crossing over during Prophase 1 – recombination of genes!

B. Independent Assortment of chromosomes during Metaphase 1 & II.

-this means how they align on the ‘metaphase plate)

C. Random fertilization to form zygote

IV. Who does Meiosis?

1. Adults always diploid

2. Meiosis only used to make gametes

1. Meiosis used to produce haploid adult

2. Gametes made by mitosis

3. Only a zygote is diploid

1. Adults can be diploid or haploid

2. Gametes made by mitosis

3. Spores made by meiosis

TAKE-HOME POINTS

Ѻ Sexual reproduction GREATLY INCREASES genetic variation!

Ѻ Sexual reproduction is possible via meiosis, formation of gamete cells with half the chromosomes (haploid) than somatic cells (diploid)

Crossing over in Prophase I

Slide 16

Independent Assortment of Homologous Chromosomes

Slide 16

Meiosis Cell Cycle

Meiosis Phase

Meiosis

Slide 10