Meiosis

Process of making

gametes.

(Sperm and egg)

• Gametes (gametic cells) contain ½ the genetic material as somatic cells and are genetically unique.

• They are called haploid. Examples are sperm and egg. (Not somatic or body cells)

• They are used in sexual reproduction to form the new offspring. Sperm + egg = zygote

• The goals of meiosis are two-fold 1. to create diversity within the chromosomes

and 2. to reduce the number of total chromosomes

in the cell to ½ that of the parent cell.

Check your understanding

•What is the difference between haploid and diploid?

•What is a somatic cell?

•What is a zygote?

•What are the 2 main goals of meiosis?

Names for the processes

• Spermatocytes- special cells that create

sperm (testes)

• Oocytes- special cells that create eggs

• (ovaries)

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2 goals = 2 rounds of phasesWhich goal is achieved in each

phase?

• Interphase ONLY occurs before Meiosis I

•Both Meiosis I and Meiosis 2 have a cytokinesis

•Between Meiosis 1 and Meiosis 2 there is a process called interkinesis

What do you think is the main difference between interphase and

interkinesis?

• In every phase but prophase 1, things occur just like in mitosis.

• However, in P1, we allow homologous chromosomes to come in contact with each other. They form a structure called a tetrad.

Crossing Over • Allowing 2 pairs of

chromosomes to physically touch allows the process of crossing over to occur.

• Crossing over is the actual exchange of genes (pieces of DNA) from one chromosome pair to the other.

DNA recombination

• The result of crossing over leads to new

gene sequences on the chromosomes!

Planned potential catastrophe!

• This is called “DNA recombination”

• What does this do to diversity?

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•This allows for DIVERSITY to occur in the chromosomes of the cells made during meiosis.

Q: Why didn’t we do this in mitosis?

• After Prophase 1 all other phases of meiosis are the same as mitosis, followed by cytokinesis.

• BUT, we are not done at the end of meiosis 1

• So, we have to have a meiosis 2. Second round of the same steps as mitosis!

!

Polar Bodies• In males, all 4 haploid cells become sperm

• In females, only 1 of the haploid cells become an egg. That one egg takes all of the cytoplasm and organelles from the other 3 cells

•The other 3 cells produced are waste that is re-absorbed. These cells are known as polar bodies.

Revisit Earlier question

Which stage of meiosis accomplishes each of the 2 goals?

•Meiosis 1 = Genetic Diversity

•Meiosis 2 = Reduce chromosome number

Compare Mitosis and MeiosisSIMILARITIES: Both involve similar phases, replicate

cells, occur in eukaryotic organisms.

DIFFERENCES: Mitosis Meiosis

# of DNA replications:

# of times through the stages:

# of daughter cells produced:

Chromosome number in daughter cells:

Chromosome number in parent cell:

Purpose:

1 1

1 2

2 4

46 23

46 46

Growth and repair

Gamete production

(sexual reproduction)