Sexual Division of Cells Meiosis Sexual Division of Cells

Gamete vs. Zygote What is a Gamete? Egg & Sperm What is a Zygote? When two gametes come together (fertilization)

Why your body can not use mitosis to make sperm or eggs? A typical cell in your body has 46 chromosomes (23 pairs of homologous chromosomes). Suppose that human sperm and eggs were produced by mitosis. How many chromosomes would each sperm or egg have? 46

Why your body can not use mitosis to make sperm or eggs? If a sperm of this type fertilized an egg of this type, and both the sperm and egg contributed all of their chromosomes to a zygote, how many chromosomes would the resulting zygote have? 92

Why your body can not use mitosis to make sperm or eggs? In humans, how many chromosomes should a zygote have, so the baby’s body cells will each have a normal set of chromosomes? 46

Why your body can not use mitosis to make sperm or eggs? Obviously, if the body used mitosis to make sperm and eggs, the resultant zygote would have too many chromosomes to produce a normal baby. TO produce a normal zygote, how many chromosomes should each sperm and egg have? 23

Gamete vs. Zygote What is a Gamete? Egg & Sperm What is a Zygote? When two gametes come together (fertilization)

To produce the needed number of chromosomes in the sperm and eggs… Meiosis reduces the number of chromosomes by half. A gamete (eggs and sperm) has half the number of chromosomes as a normal cell. During fertilization the sperm and egg unite to form a single cell called the zygote. The zygote contains 23 chromosomes from the sperm and 23 chromosomes from the egg. A zygote has 46 chromosomes . A zygote undergoes mitosis to begin to form an embryo and continue to grow and develop.

Diploid and Haploid Cells 46 23 Reproduction Haploid cells do not have all the DNA and therefore can not function like somatic cells

Gamete vs. Zygote What is a Gamete? Haploid What is a Zygote Diploid

Review! Discuss with your partner - Why do we need Meiosis to occur? What is the difference between a Gamete and a zygote? What is the difference between haploid and diploid?

Where do your genes come from? Meiosis Where do your genes come from?

Homologous Chromosomes Chromosomes are lined up from biggest to smallest (except the last pair). Each pair contains one pink (maternal) and blue (paternal) chromosome. The pairs are homologous. X and Y are the sex chromosomes – they determine the sex of the individual

Homologous Chromosomes 9. Each pair is homologous – meaning it is the same size, shape and carries the same genes. Homologous Chromosomes – Chromosomes that are the same size, shape and carry the same genes

10.

11. What are the two types of division shown? Mitosis and meiosis 12. Which type of division produces Diploid cells? Mitosis Haploid cells? (egg and sperm) meiosis 13. Which type of cell division, mitosis or meiosis, do you think is normally used to produce new cells for: Growing from a baby to an adult mitosis Healing a wound? mitosis 13. Making egg and sperm? meiosis

Review! Discuss with partner what we just spoke about What is the differences between Haploid and Diploid? What is a homologous pair? What are the differences between mitosis and meiosis that you know so far?

Cell Division with Meiosis Where do your genes come from?

The cell cycle remains the same except… G1 Division: meiosis & cytokinesis S G2

VIDEO Meiosis makes unique haploid cells. As you study meiosis focus on two things: How are we ‘mixing things up’ so that our offspring are unique? How are we moving and separating the chromosomes so that we end up with ½ the material? INTRO: Difference between meiosis and mitosis

Prophase I Prophase I is the longest and most complex phase. All of the events that occurred during prophase of mitosis occur + Homologous chromosomes come together to form a SYNAPSE (TETRAD). CROSSING-OVER occurs.

Prophase I Crossing Over Mixed Up DNA!

Metaphase I Homologous chromosomes line up randomly at the center of the cell. We call this independent assortment.

Anaphase I During anaphase the homologous chromosomes in the center of the cell divide.

Telophase I / Cytokinesis Telophase I two nuclei form and cytokinesis occurs resulting in 2 haploid daughter cells.

REVIEW! Do you understand how a haploid cell is created? Discuss with partner – what happens in Meiosis 1?

Meiosis II Meiosis II usually comes directly after cytokinesis. No growth (interphase) takes place. Meiosis II is broken into 4 events: prophase II metaphase II anaphase II telophase II. The steps of Meiosis II are identical to mitosis.

Prophase II Prophase II is the same as prophase in mitosis.

Metaphase II Metaphase II is the same as metaphase in mitosis.

Anaphase II Anaphase II is the same as anaphase in mitosis. Notice that the sister chromatids separate.

Telophase II Telophase II is the same as telophase in mitosis.

VIDEO Remember meiosis makes unique haploid cells. As you watch the video again pay attention to: How are we ‘mixing things up’ so that our offspring are unique? How are we moving and separating the chromosomes so that we end up with ½ the material?

Review! Discuss with your partner – What happens in Meiosis II?

What is the goal of meiosis? Goals – reduce the chromosome number & mix up the DNA Meiosis 2 Goals – separate sister chromatids and form 4 cells

What 2 important things are accomplished through meiosis 1?   reduce the chromosome number & mix up the DNA

What 2 important processes help mix up the DNA in meiosis 1? Crossing over (during prophase I) Independent Assortment (during Metaphase I) 

What is major difference between metaphase 1 and 2? Metaphase I – homologous pairs line up Metaphase II – individual chromosomes line up 

What is the major difference between anaphase 1 and 2? Anaphase I – homologous chromosomes separate Anaphase II – sister chromatids separate

Introduction to Meiosis Review Sheet What are the major differences between diploid and haploid cells? How similar are homologous chromosomes? How similar are sister chromatids? Cell 1 = Cell 2 = Cell 3 = Cell = 4 Arrow A = Arrow C = Would you expect the genetic information in Cell 4 to be identical to chromosome content of Cell 1? Explain.

Do you know meiosis? Why is meiosis necessary for sexually reproducing organisms? Needs to reduce number of chromosomes in order to have an offspring with the correct amount of DNA Genetic recombination – DNA is “mixed up” Are sister chromatids identical? YES! Are homologous chromosomes identical? Not identical. They are similar. One comes from female and one comes from male. They are the same shape, size and carry the same genes

Do you know meiosis? In Prophase I, how many homologous chromosome pairs are present? 23 What is crossing over? When does it occur and why is it important? It is when homologous chromosomes swap genes during prophase I Genetic recombination – it creates genetically unique offspring How can you tell by looking at a cell whether it is in metaphase I or II? Metaphase I – homologous chromosomes line up – double line Metaphase II - sister chromatids line up – one line

Do you know meiosis? What is independent assortment? When does it occur and why is it important? Homologous chromosomes randomly line up during Metaphase I. Genetically unique offspring How can you tell by looking at the cell whether it is in anaphase I or II? Anaphase I – homologous chromosomes are separated Anaphase II – sister chromatids are separated

Required Reading #3 What are homologous chromosomes? Each chromosome that come from the male parent has a corresponding chromosomes from the female parent. What is the difference between a haploid cell (gamete) and diploid cell (somatic)? DIPLOID = “two sets” (2N) – cell contains both sets of homologous chromosomes HAPLOID = “one set” (N) – cell has single set of chromosomes What is meiosis? Process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. What are the steps of meiosis? Meiosis 1, Meiosis 2

Required Reading #3 What is a tetrad? Pairing of the homologous chromosomes during Meiosis 1 What is crossing over? Why is it important? When tetrads exchange portions of their chromatids Look at figure 11-17. How is meiosis different in men and women? Male = produce 4 gametes (sperm) Female = only one gamete (out of 4) is actually involved in reproduction BECAUSE the division at the end of meiosis 2 is uneven so that a single cell receives most of the cytoplasm – known as egg Remaining 3 are known as polar bodies and usually do not participate in reproduction. Compare and contrast mitosis and meiosis. Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces 4 genetically different haploid cells.