Lecture #5 Meiosis and Gamete Formation

Unit: Cellular Reproduction

The Birds & The Bees Humans and most otheranimals reproducesexually. • This means that two

cells fuse to make a new ‘baby’ organism.

• These specialized cells are called gametes, or sex cells.

• Sperm and egg cells are human gametes

Sexual Reproduction 101

Question: Where do sperm & egg come from?

Answer: Meiosis!

Who does it? • All sexually reproducing organisms.

Where does it happen? • Males- in the testes • Females- in the ovaries

Why does Meiosis happen?

Answer

It is the only way someorganisms (like humans!)can reproduce.

Meiosis allows sex cells tojoin and form a completeset of chromosomesnecessary to make a neworganism.

Why is a process other than mitosis needed inorder to make sperm and egg?

• Mitosis results in two daughter cells that are diploid.

• Egg and sperm need to be haploid. The chromosome number needs to be cut in half in order for fertilization to occur!

Take 2 minutes to summarize the purpose ofmeiosis.

Meiosis

Meiosis is a special kind of cell division thathalves the number of chromosomes in gametes.

This ensures that the correct number ofchromosomes are passed on to offspring.

There are two parts to meiosis (Meiosis I & Meiosis II)

Meiosis is sometimes referred to as ‘reduction-division’.

Meiosis I (‘reduction’) – starts with 1 diploid cellbut the end result is 2 cells that have had theirchromosome # reduced by half (2 haploid cells).

Meiosis II (‘division’) – end result is 4 haploid cells

Meiosis I

Meiosis 1 (reduction-division’)Prophase I

Homologous chromosomes pair and exchangeSegments (crossing over)

Meiosis KM 11

Crossing-OverDefinition- process during meiosis in whichhomologous chromosomescross and exchange pieces.

• This leads to great genetic diversity of the egg and sperm that are produced.

Metaphase I: spindle moves pairs of Homologous chromosomes to the equator of the cell • Different than in Mitosis where they lined up

Anaphase I: Homologous pairs separate with sister chromatids remaining together. * Different than Mitosis! *

Telophase I: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair. • This is the stage where the diploid number is reduced to the haploid number!

Meiosis II (‘reduction-division’)

** The chromosomes do NOT replicate between meiosis I and meiosis II **

** The steps of Meiosis II are just like what happens in mitosis (PMAT)

** The end result is very different however, due the fact that the cells starting it out are

haploid!

End Result of Meiosis II

Four haploid cells that are genetically unique!

Take 2 minutes to summarize what you havelearned about the events of Meiosis.

Be sure to mention how many cells start off theprocess and how many cells are present at theend.

How is the cell at the start of meiosis differentthan the cells at the end? How is it similar?

Question

Is Meiosis different in males and in females?

Answer: YES; it ends differently.

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Oogenesis in ovaries

Polar Bodies

(not functional)

Egg

Meiosis in Females Oogenesis - the process by which gametes are produced in female animals

The cytoplasm divides unequally during meiosisI and meiosis II in females.

• One of new cells produced receives almost all of the cytoplasm and will give rise to (meiosis I) or become (meiosis II) the egg (ovum).

• The other cells are very small (polar bodies) and will NOT survive.

Why does the egg need all of that cytoplasm?

Answer

Because the surviving cell has a largeshare of cytoplasm, it has a rich storehouse ofnutrients to nourish the young organism thatdevelops if the ovum is fertilized.

In summary: Meiosis in females starts with onediploid cell, and ends with one unique haploidegg.

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Spermatogenesis in testes

SPERM

Meiosis in Males

Spermatogenesis - the process by which spermare produced in male animals. • After meiosis II, the four gametes change in form

and develop a tail to become male gametes called sperm.

In summary: Meiosis in males starts with onediploid cell and ends with 4 unique haploid cells

Take 2 minutes to summarize what you have learned regarding the similarities and differences in meiosis in males and females