Cell Division Day 3: Meiosis

Types of Cell Division

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Sexual Reproduction, What is it?

Involves 2 parents that will produce offspring that are genetically distinct from each parent.
Sex Cells/Gametes are needed to reproduce sexually
Males=Sperm
Females=Egg/Ovum
When the two cells are fused together ( Fertilization) the result is called a zygote

Some Important Vocab....

Karyotype: A picture of a person’s chromosomes

There are 22 numbered pairs of chromosomes called Autosomes; these code for other traits (not sex)

The 23rd pair of chromosomes are the sex chromosomes and they determine an individual’s sex.

Homologous Chromosomes make up a pair of Autosomes; One chromosome of each homologous pair comes from the Father and one comes from the Mother. Homologous Chromosome Pairs are the same size, same shape, and carry the same genes but, the alleles (choices/version) for each trait may not be the same (example: Dad's codes for brown eyes and mom's codes for blue eyes, but both are still coding for eye color.)

So, Meiosis is different than Mitosis?

Where Mitosis creates offspring that are identical to the parent, Meiosis allows for GENETIC VARIATION. Once the gametes combine, it results in offspring that are different from the parents and other siblings.
The ONLY Exception: identical twins Genetic Variation increases the chances that a population and/or species has of survival

EX: Some individuals may have inherited a trait making them resistant to certain diseases or tolerant of certain conditions which help them to better survive in changing conditions

Clutch of baby bull snakes, all full blood siblings, with varying colors

So, Meiosis is different than Mitosis?

Mitosis: Occurs in Somatic Cells, is Asexual Reproduction, Undergoes ONE round of division, makes TWO Identical Daughter Cells, goes from Diploid Parent Cells to Diploid Daughter Cells

Meiosis: ONLY occurs in gametes, is Sexual Reproduction, Undergoes TWO rounds of division, makes FOUR Genetically Distinct Daughter Cells, goes from Diploid Parent Cells to Haploid Daughter Cells.

Meiosis Phases

Meiosis is called reduction division because the
chromosomes replicate once (during interphase) but
2 cell divisions follow (Meiosis I & Meiosis II) to
reduce the number of chromosomes from diploid to haploid

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Meiosis I Separates Homologous Chromosomes

Meiosis II Separates Sister Chromatids

(kinda like mitosis but makes 4 haploids instead of 2 diploids)

Meiosis Increases Genetic Variation?

Yes, for a few reasons:
1. Independent Assortment: Each pair of chromosomes can be arranged in two ways when they pair up at the start of meiosis, mom on the "left" dad on the "right" or vice versa. The result= 8 million different possibilities PER CELL

2. Crossing Over: When homologous chromosomes form tetrads, the strands of the chromatids can twist around each other and may exchange segments.

3. Genetic Recombination:
Recombine means to combine again or differently.
When the sperm fertilizes the egg, the chromosomes are recombined, which means the diploid chromosome number has been restored in the zygote. However, there may be new combinations due to the variations that were passed down from mother and father.

*FACT: There are 64 TRILLION different possibilities when a sperm and egg cell unite