Presentation on theme: "Sexual Reproduction: Meiosis. What is Sexual Reproduction? Combines genetic material from two parents to produce a new individual Combines genetic material."— Presentation transcript:
Sexual Reproduction: Meiosis
What is Sexual Reproduction? Combines genetic material from two parents to produce a new individual Combines genetic material from two parents to produce a new individual Creates an increase in variation among the offspring Creates an increase in variation among the offspring Populations can adapt to changing environments Populations can adapt to changing environments The world is more enjoyable with variation The world is more enjoyable with variation Costly – it takes twice as many individuals to produce an offspring Costly – it takes twice as many individuals to produce an offspring
Genetic material (DNA) is found in chromosomes Each chromosome has a “twin” that: contains the same genes codes for the same traits has the same shape is the same size Has the same location of the centromere Has the same banding pattern when stained One chromosome came from the father, the other came from the mother These twins are called homologous. Homologous Chromosomes
Cells that have only one of each chromosome are called Haploid (n). This is half of the usual amount of DNA. Cells that have two of each chromosome are called Diploid (2n). This is the normal condition for most cells.
N = 1 –
Homologous Chromosomes Humans have 23 pairs of chromosomes – 22 pairs of autosomes and one pair of sex chromosomes In females they are all homologous In males, the autosomes are homologous, but the sex chromosomes are not Karyotype – arranges chromosomes in homologous pairs
Special Considerations: 1.If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? 1.If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? 2.In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have? 2.In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?
Meiosis Purpose: Divide the nucleus to reduce the number of chromosomes in half Divide the nucleus to reduce the number of chromosomes in half Ensure that each new cell has a complete set of chromosomes Ensure that each new cell has a complete set of chromosomes Create gametes Create gametes Also called reduction division Also called reduction division
Human Life Cycle Most of our cells are diploid (2n) Reproductive cells called gametes are haploid (n) They are produced in gonads (ovary and testes) from diploid gametocytes Gametes
Meiosis I and Meiosis II Two Stages of Meiosis In meiosis I the homologous chromosomes are separated In meiosis II the sister chromatids are separated Four cells are produced, each with one set of chromosomes (n). A cell normally has two sets of chromosomes (2n)
Meiosis I The phases of meiosis I are similar to mitosis: Prophase I, Metaphase I, Anaphase I, Telophase I The formation of tetrads is called synapsis Dyads are sister chromatids attached by the centromere
Prophase 1 chromosomes condense homologs pair up forming tetrads (four chromatids are lined up) Nuclear envelope fragments Crossing-over can occur in this stage.
What is Crossing Over? During synapsis of prophase I: Homologous chromosomes tangle and trade parts of their DNA Variation increases Genes that are usually linked (on the same chromosome) can become separated
Homologous chromosomes line up at the equatorial plate. Meiosis 1 Metaphase 1
Nondisjunction Disjunction is the separation of tetrads or sister chromatids Nondisjunction- no separation occurs
Turner Syndrome XO
Klinefelter Syndrome XXY
Down Syndrome (trisomy 21)
Four cells not identical to parent Two cells genetically identical to parent cell 2n n n Meiosis vs. Mitosis
MitosisMeiosis One divisionTwo divisions Two daughter cells per cycle Four daughter cells per cycle Daughter cells genetically identical Daughter cells genetically different Same chromosome number as parents Chromosome number half that of parents Occurs in somatic cellsOccurs in gametocytes Throughout life cycle Completed after sexual maturity Used in growth, repair, asexual reproduction Sexual reproduction, new gene combinations