Mrs. Stewart Biology I Stewarts Creek High School MITOSIS VS. MEIOSIS Mrs. Stewart Biology I Stewarts Creek High School

Objectives: Differentiate between the process of mitosis and meiosis Demonstrate the movement of chromosomes throughout meiosis Analyze how meiosis leads to genetic variation

Face Partners:

Review 2 ways for animals/cells to reproduce Asexual reproduction Mitosis Binary fission These are used to create daughter cells that are identical to parent cells Sexual reproduction This creates daughter cells that are genetically different from parent

REVIEW: Cell Cycle Mitosis Interphase M-phase (mitosis) Cytokinesis G1, S, and G2 M-phase (mitosis) P-M-A-T Cytokinesis Mitosis Asexual reproduction Produces 2 identical daughter cells Daughter cells are diploid Daughter cells are identical to parent/mother cell

What differences can you see? How many sets of chromosomes are in the cells that Meiosis produces? How many cells does Meiosis produce? How many divisions occur in Meiosis?

Final Products: Mitosis 2 identical daughter cells Somatic cells Diploid Meiosis 4 genetically different daughter cells Gametes Haploid

Mitosis vs. Meiosis Animation

Mikey: Explain to Raph how meiosis differs from mitosis

Why are gametes haploid? Because two gametes fuse to create an offspring during sexual reproduction Sperm (23) + Egg (23) = Offspring (46)

What happens in fertilization? Fertilization of an egg Zygote = the intial cell created from the fusion of a sperm and an egg

Fertilization to Implantation

Raph: summarize the process of fertilization for Mikey

MEIOSIS: The process of creating haploid gametes for sexual reproduction

Vocabulary Two categories for chromosomes: Sex chromosomes (2 out of 46) the 23rd pair determine sex (gender) Autosomes (44 out of 46) – all the rest Homologous chromosomes (homologues) The two copies of each autosome Where did they come from?

Homologues come from mom and dad

Mikey: Explain to Raph what a homologous chromosome pair is and where they came from.

How did babies get one homologue from each parent? Meiosis Creates haploid sex cells Each sex cell has 23 chromosomes that are randomly assorted This occurs through two cell divisions

What are the steps? Phases of Meiosis I Interkinesis Prophase I Metaphase I Anaphase I Telophase I Cytokinesis Interkinesis Phases of Meiosis II Prophase II Metaphase II Anaphase II Telophase II

Let’s see it in action! Meiosis animation

Interphase DNA replicates Makes the diploid (2n) cell now be (4n) This process results in sister chromatids that will be attached by a centromere

Here is a karyotype showing homologous chromosome pairs

Here is a karyotype after DNA replication has occurred Notice how each chromosome has duplicated itself.

Prophase I Homologous chromosomes pair up (forming a tetrad) - Mom & Dad go on a date Crossing over occurs Chromatids MAY exchange portions of DNA Leads to genetic variances

Prophase I Homologues (homologous pairs of chromsomes) form Tetrads Crossing Over occurs

Raph: Explain to Mikey the difference between homologous chromosomes and a tetrad

Mikey: Explain to Raph the process of crossing over and how that leads to genetic variation

Telophase I and Cytokinesis Metaphase I Spindle fibers attach to the chromosomes Tetrads line up in the middle of the cell Anaphase I Fibers pull the homologous chromosomes toward opposite ends of the cell Telophase I and Cytokinesis Nuclear membranes form Cell separates into two new cells

Interkinesis Resting period between Meiosis I and Meiosis II DNA DOES NOT REPLICATE AGAIN HERE!

Meiosis II The daughter cells from Meiosis I divide again WITHOUT replicating their chromosomes That leads to 4 gametes, each with half the number of chromosomes (haploid) as the original “mother” cell

Telophase II and Cytokinesis Prophase II Spindle fibers form and move chromosomes to center Metaphase II Spindle fibers attach to the chromosomes chromosomes line up in the middle of the cell – similar to how they do in Mitosis Anaphase II Fibers pull the sister chromatids toward opposite ends of the cell Telophase II and Cytokinesis Nuclear membranes form Both cells separate – forming 4 new haploid cells

How are the daughter cells genetically different?

Genetic Variation Sexual reproduction can lead to genetic variation between the offspring and the parents in 3 ways: Crossing over – exchanging pieces of DNA leads to new DNA combinations on the chromosome Random assortment of chromosomes – the parent’s homologues are randomly sorted into the gametes each time meiosis occurs Random fertilization – which gametes combine to form baby is a random, unpredictable process

How is it that my daughter looks more like me but my son looks more like my husband?

Let’s see it in action! Meiosis animation 1 Meiosis animation 2

Explanatory Videos Meiosis – Crossing Over Random, Independent Assortment of Chromosomes

Raph and Mikey together: Name two ways that meiosis leads to genetic variation

Oogenesis – meiosis in human female reproductive cells – makes eggs (ovum) Total of 4 cells produced: Occurs in the ovaries Forms one usable egg cell with a large supply of stored nutrients. The other 3 cells, called polar bodies, disintegrate.

Oogenesis

All 4 gametes produce a long whip-like tail Spermatogenesis – meiosis in human male reproductive cells to make sperm (spermatazoa) Occurs in the testes Produces 4 viable sperm All 4 gametes produce a long whip-like tail

Mikey: Explain to Raph why only one egg is usable but all 4 sperm are usable.

Meiosis: Cell division necessary for sexual reproduction Produces 4 daughter cells Daughter cells are Haploid Daughter cells are gametes (sexual repro. cells) 2 nuclear/cellular divisions Vital to maintain correct number of offspring in sexually reproducing organisms Crossing over = opportunity for genetic variability

Differentiate Mitosis Meiosis Used for sexual reproduction Produces 4 daughter cells Daughter cells are Haploid Daughter cells are genetically different from each other, and from parent cell Produces gametes Two nuclear/cellular divisions Asexual reproduction Produces 2 daughter cells Daughter cells are diploid Daughter cells are identical to each other and to parent cell Produces somatic cells One cell/nuclear division

Human chromosomal diseases **Mistake in meiosis can lead to an incorrect chromosomal number, causing consequences for offspring** Down’s syndrome (extra chromosome #21) Turner’s syndrome (missing or incomplete X chromosome in girls) Klinefelter’s syndrome (males that have an extra X chromosome [XXY])

As a table group: How does a baby with Down’s Syndrome end up with 3 chromosome #21s?

Video Meiosis square dance