Almost all human cells contain 46 chromosomes, and are diploid (2n). Q: If a sperm cell has 46 chromosomes (2n) & an egg cell has 46 chromosomes (2n), when they combine during fertilization, how many chromosomes will the baby have?
The Problem! No Good! If egg and sperm had same number of chromosomes as other body cells... baby would have too many chromosomes!
Meiosis The solution Meiosis produces cells that have only 23 chromosomes and are 1N haploid (half)
Drosophila (fruit flies) Diploid: 2N = 8 Haploid: 1N =? Lettuce Diploid: 2N = 8 Haploid: 1N =? Goldfish: Diploid: 2N =? Haploid: 1N = 48 You tell me!
The Meiosis Solution! Much Better! Egg and sperm with half the number of chromosomes than other body cells... baby would have just the right amount!
Chromosomes Most human cells have 46 (23 pairs) chromosomes One set of 23 from mom, and one of 23 from dad
Of the 46 (23 pairs) chromosomes: 44 chromosomes (22pairs): autosomes 2 chromosomes (1 pair): sex chromosomes (XX or XY)
All human cells have 46 chromosomes except for the egg & sperm cells Egg = 23 Sperm = 23 (1 set of chromosomes, 1N) All other cells = 46 (2 sets of chromosomes, 2N) (liver, cardiac, blood cells)
Mitosis vs. Meiosis Mitosis Asexual Reproduction Produces cells that are genetically identical of parent cell Meiosis Sexual Reproduction Combines genes from sperm & egg so offspring are genetically different from parents
Meiosis Makes 4 cells genetically different from parent cell & from each other halves the number of chromosomes (46 23) Used for sexual reproduction 46 chromosomes, diploid Gametes (egg or sperm cells with 23 chromosomes, haploid)
Fertilization restores the correct number of chromosomes for the offspring (baby)
Where does meiosis happen? In males- testes In femalesovaries
VOCABULARY Gametogenesis - production of gametes Sperm or egg Copyright Pearson Prentice Hall
VOCABULARY Spermatogenesis production of four equal sized gametes called sperm (1N) Copyright Pearson Prentice Hall
VOCABULARY Oogenesis: only one egg (1N), other three are called polar bodies and are not involved in reproduction.
Meiosis Occurs in 2 stages: Meiosis I Meiosis II
Meiosis I Interphase I Meiosis I Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Copyright Pearson Prentice Hall
Interphase I prior to Meiosis Cells undergo a round of DNA replication, duplicating their DNA (and other organelles). Copyright Pearson Prentice Hall
MEIOSIS I Prophase I Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. There are 4 chromatids in a tetrad. Copyright Pearson Prentice Hall
Homologous Chromosomes Pair of chromosomes - maternal & paternal - similar in shape & size. 1 tetrad carry genes controlling the same inherited traits. 4 chromatids
Homologous Chromosomes Homologous chromosomes sister chromatids paternal Tetrad sister chromatids maternal
Homologous Chromosomes eye color locus eye color locus hair color locus hair color locus Paternal Maternal
Crossing Over - Segments of non-sister chromatids break and reattach to the other chromatid causes VARIATION! nonsister chromatids Tetrad chiasmata: site of crossing over variation
Crossing-over: entirely new chromosomes are formed as new combinations of alleles are produced Copyright Pearson Prentice Hall
Check out the tetrads!
MEIOSIS I Metaphase I Spindle fibers attach to the centromeres on tetrads. Tetrads line up in the middle: metaphase plate. Copyright Pearson Prentice Hall
INDEPENDENT ASSORTMENT Tetrads line up randomly at along metaphase plate Leads to variation Dad s chromosome - blue Mom s chromosome - red
MEIOSIS I Anaphase I Spindle fibers pull the homologous chromosomes toward opposite ends of the cell. Tetrads split into single chromosomes (notice that chromosomes are different than parent due to crossing over). Copyright Pearson Prentice Hall
MEIOSIS I Telophase I Nuclear membranes form. Cell separates into two cells. The two cells produced have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I. Copyright Pearson Prentice Hall
Telophase I Each pole now has haploid (1n) set of chromosomes. Cytokinesis occurs and two haploid daughter cells are formed.
Chytokinesis
Meiosis I results in 2 haploid (1N) daughter cells, each with half the number of chromosomes as the original cell (23 for humans). Copyright Pearson Prentice Hall
Overview of Meiosis 1 4 duplicated chromosomes (2n) 2 duplicated chromosomes (1n) Copyright Pearson Prentice Hall
Phases of Meiosis Meiosis II Cells enter second phase of cell division Unlike meiosis I, neither cell goes through chromosome replication. Each of the cell s chromosomes has 2 chromatids (DNA is still duplicated in each cell) Copyright Pearson Prentice Hall
Phases of Meiosis Meiosis II similar to mitosis Telophase I and Cytokinesis I Meiosis II Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Copyright Pearson Prentice Hall
Meiosis II Prophase II Centrioles separate and move to opposite poles of cell new spindle fibers appear
Meiosis II Metaphase II Sister chromatids (chromosomes) line up at metaphase plate Spindle fibers attach at centromere
Meiosis II Anaphase II sister chromatids separate & move toward opposite ends of the cell
Meiosis II Telophase II and Cytokinesis Nuclei reform. Cytokinesis occurs. Four haploid daughter cells produced. gametes = sperm or egg
Telophase II
Fertilization Restores original chromosome number 23 46 N+N 2N
SIGNIFICANCE OF MEIOSIS Halving the chromosome number ensures that when gametes with the haploid number fuse to form a zygote the normal diploid number is restored. Meiosis leads to increased variation as it occurs to form both egg and sperm, when these combine during fertilization, even more variation of offspring results
Each gamete (egg/sperm) is not identical to its parent N When the two gametes combine, they form unique offspring not identical to either parent (zygote) Offspring may express some traits of the parent and/or entirely different traits from the parents depending on how genes interact with each other N 2N
Meiosis Animation meiosis square dance
Meiosis vs. Mitosis Comparison of Meiosis and Mitosis