Meiosis

Heredity Heredity

Passing of genetic traits from parent to offspring

Chromosome theory of heredity Chromosomes

carry genes Gene = unit of

heredity

What Meiosis is all About Meiosis allows the creation of unique

individuals through sexual reproduction.

In The Beginning Two

Reproduction in which there is a re-mixing of the genetic material is called sexual reproduction

Two cells, a sperm and an egg, unite to form a zygote, the single cell from which the organism develops

Meiosis is the process of producing sperm and eggs (gametes) – the number of chromosomes are halved

Gametes Are Haploid Gametes must have half the

genetic material of a normal cell If the genetic material in the

gametes was not halved, when they combined the zygote would have more genetic material than the parents!

Gametes have exactly one set of chromosomes, this state is called haploid (1n)

Regular cells have two sets of chromosomes, this state is called diploid (2n)

Fertilization Results in a Diploid Zygote

Sperm1nHaploid

nucleus

Egg1n Haploid

nucleus

Fertilization Results in a Diploid Zygote

Sperm1n

Egg1n

Haploidnucleus

Haploidnucleus

Fertilization Results in a Diploid Zygote

Sperm1n

Egg1n Haploid

nucleus

Haploidnucleus

Fertilization Results in a Diploid Zygote

Sperm1n

Egg1n Haploid

nucleus

Haploidnucleus

From Zygote to Embryo

Diploid Zygote2n

From Zygote to EmbryoCleavage

From Zygote to Embryo

Cleavage

From Zygote to Embryo

Cleavage

From Zygote to Embryo

Cleavage

From Zygote to Embryo

Morula

Meiosis 1Prior to division, amount of DNA

doubles

Stages of Meiosis

http://www.sumanasinc.com/webcontent/animations/content/meiosis.html

Independent Assortment during Prophase I

The key difference between Mitosis and Meiosis is the way chromosomes uniquely pair and align in Meiosis

Mitosis The first division of Meiosis

Synapsing UNLIKE in mitosis,

homologous chromosomes line up next to each other during prophase

This process is called synapsing

Lined up homologues are called tetrads

Anaphase 1 During anaphase 1,

each homologous chromosome is pulled to opposite sides of the cell. Unlike mitosis, THE CENTROMERES DO NOT BREAK.

Nuclei may or may not reform following division.

Cytokenesis may or may not occur.

Check out the Tetrads

Meiosis I

In Telophase 1, two daughter cells areformed. They are NOT identical! (Why?)

Meiosis II

Chromosomes are NOT

duplicated again between

Meiosis 1 and Meiosis 2Why not?

Meiosis II Prophase 2: spindle reforms and

chromosomes move toward the metaphase plate

Metaphase 2: sister chromatids lined up on the metaphase plate

Anaphase 2: sister chromatids are separated and pulled toward opposite poles of the cell

Telophase 2 and Cytokinesis: nuclei form at either pole and each cell is finally divided into two identical daughter cells

All Together Now

Independent Assortment

Independent Assortment

Example:2 chromosomes in haploidn = 22n = 22 = 4 possible combinations

In Humans

Example: 23 chromosomes in haploidn = 23

2n = 223 = ~ 8 million possible combinations!

Prophase 1 – Crossing Over Homologous

chromosomes come together

Areas of homologous chromosomes connect at areas called chiasmata

Crossing Over Segments of

homologous chromosomes break and reform at similar locations.

Results in new genetic combinations of offspring.

This is the main advantage of sexual reproduction

Crossing-Over

Random fertilization At least 8 million combinations from Mum

and another 8 million from Dad … >64 trillion combinations for a diploid

zygote!!!

Boy or Girl? The Y Chromosome Decides

X chromosome

Y chromosome

Oogenesis The female gamete is

called an ovum At birth each female

carries a lifetime supply of developing oocytes, each of which is in Prophase I.

A developing egg (secondary oocyte) is released each month from puberty until menopause, a total of 400-500 eggs.

Oogenesis Only one ovum is

produced during meiosis Oogenesis places most of

the cytoplasm into the large egg. The other cells, the polar bodies, do not develop.

All the cytoplasm and organelles go into one egg for nourishment for the young organism that will develop after fertilization.

Spermatogenesis The male gamete is

called a sperm Four spermatocytes

are formed during meiosis

Men are busy - meiosis produces roughly 250,000 sperm a day.

Any Questions?

ReferencesSlides, information and images were taken from

the following presentations:

cchs.churchill.k12.nv.us/marshk/Notes/meiosis.ppt bioweb.wku.edu/Faculty/Bowker/120/mitosis.html edweb.sdsu.edu/ltca/Mitosis_Meiosis_files/slide0001.htm waukesha.k12.wi.us/South/Bio1/MEIOSIS.htm my-ecoach.com/coaching/meiosis_files/meiosis.ppt www.grisda.org/tstandish/teachers/presentations/High%20S

chool/Meiosis.ppt www.biology.usu.edu/courses/biol1010-podgorski/

PPpage.htm