Cell Division Biology 105 Laboratory 8 THE MAJOR STEPS OF CELL DIVISION: When does DNA replicate? The first step of cell division is DNA replication: This occurs just prior to cell division. Cells need to divide for multicellular organisms to grow or to replace old cells, or for one-celled organisms to reproduce. 1
Chromosomes Chromosomes are made of DNA. DNA contains the code that makes proteins. Examples of chromosomal locations that correspond with various pathologies. Chromosomes DNA is wound up in coils and folded to form chromosomes. When the DNA replicates, it forms a chromosome in a duplicated state. This duplicated state has two strands: each strand is called a chromatid they are sister chromatids. The chromatids are held together at the centromere. 2
Chromosome Pairs After DNA replication, each chromosome will be made of two sister chromatids. But remember that chromosomes come in pairs! These are two chromosomes, each with two chromatids: 3
Cell Cycle: Stages of Cell Division Two main phases: interphase and the mitotic phase Interphase prepares for cell division DNA replicates during interphase. Mitotic phase: chromosomes separate and cell divides. In an average animal cell, the cell cycle takes about 24 hours. Mitosis take only about 30 minutes. Most of the time is spent in G 1, which is when the cell grows in size (about 12 hours). Interphase During this phase: The cell grows. The cell makes new cellular components. DNA is replicated. Check points will stop cell division if there is a problem. When the check points in interphase do not work properly, the cell divides when it should not! 4
End of Interphase: DNA has replicated Mitosis Divided into four phases: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase = PMAT! 5
Think about how hard it would be to divide the DNA into two equal parts if it was in the uncondensed form! Prophase 1. Chromosomes condense. 2. Nuclear membrane breaks down. 3. Two centrioles (one centrosome) begin to move to opposite ends of cell. 4. Microtubules form. 6
Microtubules Prophase Metaphase 1.Microtubules (mitotic spindles) attach to the chromosomes. 2.The microtubules line up the chromosomes at the center of the cell (the equator). 7
Metaphase Anaphase 1.Sister chromatids are pulled apart by the microtubules. 2. Chromatids move toward the poles of the cell. Now the chromatids are called chromosomes. Anaphase 8
Telophase 1.The chromosomes begin to uncondense. 2. Nuclear membrane forms. 3. Cell begins to divide in half. Telophase Cytokinesis The cell divides completely, forming two separate cells. 9
Cytokinesis 10
Videos Mitosis overview: http://www.youtube.com/watch?v=ahgrhxl7w _g Mitosis in an embryo http://www.youtube.com/watch?v=m73i1zk8e A0 Mitosis seen using contrast microscope: http://www.youtube.com/watch?v=dd3iqknc Edc Where does DNA replication take place? 1. Cytosol 2. Nucleus Cytosol Nucleus 11
When does DNA replication take place? 1. Metaphase 2. Anaphase 3. Prophase 4. Telophase 5. Interphase Cytosol Nucleus During this stage the nuclear membrane breaks down: 1. Metaphase 2. Anaphase 3. Prophase 4. Telophase 25% 25% 25% 25% Metaphase Anaphase Prophase Telophase During this stage chromosomes line up at the equator: 1. Metaphase 2. Anaphase 3. Prophase 4. Telophase 25% 25% 25% 25% Metaphase Anaphase Prophase Telophase 12
During this stage the chromosomes begin to uncondense: 1. Metaphase 2. Anaphase 3. Prophase 4. Telophase 25% 25% 25% 25% Metaphase Anaphase Prophase Telophase At the end of mitosis, how many cells are there? 1. One 2. Two 3. Three 4. Four 25% 25% 25% 25% One Two Three Four At the end of mitosis, how many chromosomes are there in each cell? 1. 23 chromosomes 2. 46 chromosomes 23 chromosomes 46 chromosomes 13
At the end of mitosis, are the chomosomes in the duplicated state? 1. Yes 2. No Yes No Cell Types Mitosis occurs in all the body s cells, except the cells that are responsible for reproduction. Gametes: cells that are responsible for reproduction. All the rest of the body s cells are called somatic cells. Gametes = Reproductive Cells Sperm and eggs are reproductive cells called gametes. They do not divide like the somatic cells. Instead, gametes undergo a process called meiosis. Recall that we have 23 pairs of chromosomes = 46 total chromosomes. If gametes (sperm and egg) combine with all these chromosomes, then the offspring will have 92 chromosomes! 14
How do gametes overcome this problem? Before the gametes combine, they need to reduce their number of chromosomes by half. So instead of 23 pairs (46 total chromosomes) they need to have 23 total chromosomes. The answer to this problem is meiosis this process halves the number of chromosomes. Terminology Diploid = cells that have 46 chromosomes or 23 pairs of chromosomes. All somatic cells are diploid (2n). Haploid = cells that have 23 chromosomes. Gametes are haploid (1n). Meiosis is when a diploid cell divides to produce haploid reproductive cells. 15
Meiosis First the DNA replicates during interphase. The chromosomes duplicate. Remember that there are pairs of chromosomes, and each chromosome has two chromatids after DNA replication. Then the chromosome pairs separate and the cell divides = 1 st cell division. Then the chromatids separate and the cells divide = 2 nd cell division. The figures are going to show only two pairs of chromosomes but there are really 23 pairs! 1 st step: DNA replicates chromosomes duplicate, the cell is diploid (2n). 2 nd step: homologous chromosomes separate and the cell divides, resulting in two haploid cells (1n). 3 rd step:the chromatids separate and the cells divide, resulting in four haploid cells (1n). Meiosis occurs in order to form gametes the reproductive cells (sperm and eggs). The cells that produce the gametes start out diploid before meiosis, and will end up haploid. There are two stages of meiosis: meiosis I and meiosis II. Each stage of meiosis has prophase, metaphase, anaphase, and telophase. 16
Crossing Over End of interphase: DNA has replicated Prophase I: Duplicated chromosomes condense and intertwine, and this produces genetic variation = crossing over Genetic material is exchanged between the homologous chromosomes. Prophase I: 1. Duplicated chromosomes condense and crossover this produces genetic variation. 2. Nuclear membrane begins to break down. 3. Two centrioles (centrosomes) begin to move to opposite ends of cell. 4. Microtubules form. Metaphase I: 1. Microtubules (mitotic spindles) attach to the chromosomes. 2. Microtubules line up the chromosomes at the center of the cell (along the equator). 3. The chromosomes arrange themselves randomly, and this also gives genetic variation = independent assortment. 17
Anaphase I: Chromosomes in the pairs separate. *** This is different from mitosis! Here the chromosomes in a pair are separated from each other. So now there are 23 duplicated chromosomes on each side. Telophase I: 1. Nuclear membrane forms. 2. Cell divides in half. We now have two haploid cells (1n), which means there are 23 chromosomes total. The chromosomes are in the duplicated form. Now the cell enters meiosis II. 18
Meiosis II Meiosis II Prophase II: the 23 chromosomes are already condensed, and the nuclear membrane dissolves. Metaphase II: chromosomes line up at the equator. Anaphase II: chromatids separate. Telophase II: cells separate. Now there are four haploid cells: each has 23 chromosomes (not in the duplicated state). 19
Videos Meiosis overview: http://www.youtube.com/watch?v=d1_- mqs_fz0 At the end of meiosis I, how many cells are there? 1. One 2. Two 3. Three 4. Four 25% 25% 25% 25% One Two Three Four At the end of meiosis I, are these cells haploid or diploid? 1. Haploid 2. Diploid Haploid Diploid 20
At the end of meiosis I, how many chromosomes are there in each cell? 1. 23 2. 46 23 chromosomes 46 chromosomes At the end of meiosis I, are the chomosomes in the duplicated state? 1. Yes 2. No Yes No At the end of meiosis II, how many cells are there? 1. One 2. Two 3. Three 4. Four 25% 25% 25% 25% One Two Three Four 21
At the end of meiosis II, are these cells haploid or diploid? 1. Haploid 2. Diploid Haploid Diploid At the end of meiosis II, how many chromosomes are there in each cell? 1. 23 chromosomes 2. 46 chromosomes 23 chromosomes 46 chromosomes At the end of meiosis II, are the chromosomes in the duplicated state? 1. Yes 2. No Yes No 22
Genetic Diversity through Meiosis There are two points in this process that contribute to the genetic diversity of the offspring. Prophase I: the pairs of chromosomes undergo crossing over. Metaphase I: the way the chromosomes line up on the equator is random = independent assortment. Meoisis and Gender The gametes now contain 23 chromosomes (not in the duplicated form). One of these chromosomes is a sex chromosome. Eggs contain an X chromosome. Sperms contain either an X or a Y chromosome. 23
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Spermatogenesis In the male testes, sperm are produced. One cell produces 4 sperm. Each sperm has 23 chromosomes, and they are not in the duplicated form. The sperm can have either an X or a Y sex chromosome. The sperm have a small head and a long tail (= flagellum) for locomotion. Spermatogenesis The sperm need to contain the genetic material and deliver it to the egg. The head contains the chromosomes and lots of mitochondria to power the flagellum. About 400 million sperm are produced each day. 25
Egg Formation The ovaries in females produce eggs. One cell will produce one egg and three nonfunctioning polar bodies. The one egg gets most of the cytoplasm, leaving the other three cells unable to survive. It contains 23 chromosomes, with an X sex chromosome. It is large enough to support the embryo. Egg Formation All of the cells that produce the eggs are made before the female is even born. When a female is born, her ovaries contain all the cells that produce her eggs. Each month one of these cells will leave the ovary and go on to mature and produce the egg and polar bodies. 26
Review of Mitosis versus Meiosis Mitosis and meiosis both start with a diploid cell (46 chromosomes, 23 pairs). Before both mitosis and meiosis, the DNA replicates. Duplicated chromosomes are formed, each containing two chromatids. Mitosis occurs in somatic cells (cells other than those that produce the gametes), and meiosis produces gametes. 27
Mitosis During mitosis: The chromatids separate to produce two cells, each with 46 chromosomes = 23 pairs of non-duplicated chromosomes. These cells are diploid (2n) cells. There is no exchanging of genetic material. Meiosis Two Stages Meiosis I: the pairs of chromosomes line up and the chromosomes separate, resulting in 2 cells, each with 23 chromosomes in the duplicated state. Meiosis II: the chromatids separate, producing two haploid cells that each contain 23 non-duplicated chromosomes. One original cell produces four haploid cells. 28
17-17 Important Concepts Know the two phases of the cell cycle (interphase and mitosis) and what happens in each phase. For mitosis: know each stage, the order of the stages, and what happens in each stage. Be able to recognize each stage of mitosis from pictures, models, and microscope slides. Understand the importance of check points in interphase. Important Concepts Know what state the cell and the chromosomes are in at the beginning and end of mitosis, meiosis I, and meiosis II. For example: Are the cells haploid or diploid? Are the chromosomes duplicated, or not duplicated? How many chromosomes are there in the cell? Are they in pairs? Know which cells undergo mitosis vs meiosis. How is genetic diversity introduced during meiosis? What are the events that add diversity and when does each event occur? 29
Important Concepts How is the gender of the offspring determined? How many functioning sperm are produced from one spermatocyte? Which sex chromosomes can a sperm have? How many functioning eggs are produced from one oocyte? Which sex chromosomes do eggs have? Definitions Gametes, somatic cells, diploid, haploid, crossing over, independent assortment, cytokinesis, centromere, centrioles, centrosome, chromatid, polar bodies, spermatogenesis, oogenesis 30