Remember Homologous Chromosomes Paired chromosomes both chromosomes in
Remember! Homologous Chromosomes • Paired chromosomes; both chromosomes in a pair carry the same genes – Control the same inherited characteristics – Homologous – same information Diploid (2 n) 2 n=4 1 copy from mom & 1 copy from dad! Human cells are 2 n = 46! (23 from mom & 23 from dad)
Cell Division & Asexual Reproduction • Mitosis produces cells with the same information! – Identical daughter cells – Clones – Same amount of DNA
Asexual Reproduction • Binary Fission – Prokaryotes (bacteria) • Budding – Mostly single-celled eukaryotes (yeasts, amoebas, etc. )
What about us? • What if a complex, multicellular organism (like us) wants to reproduce? – Joining of egg & sperm Are egg & sperm cells made by mitosis? If they were, then… 46 46 Egg Sperm 92 Zygote It doesn’t work!
So, how do we make sperm & egg cells? • Must reduce the number of chromosomes in the cell from 46 chromosomes 23 chromosomes! – Must reduce the number of chromosomes by half Egg Zygote Sperm Fertilization
Gametogenesis – Gamete Formation • In male animals, the haploid gametes are called sperm. • In the female animals, the haploid gametes are called eggs. • The process in which male & female gametes join is called fertilization. – This produces a fertilized egg called a zygote!
Meiosis & Sexual Reproduction
Meiosis • The process that divides 1 diploid (2 n) cell to form 4 haploid (n) cells • How gametes (sex cells) are formed • Meiosis has 2 divisions: Meiosis I and Meiosis II
Meiosis – Production of Gametes (Sperm & Egg) • Chromosome number must be reduced – Diploid Haploid – 2 n n – In humans, 46 23 • Fertilization restores the chromosome number – Haploid Diploid – n 2 n
Sexual Reproduction Life Cycle Fertilization § 2 copies § Diploid (2 n) Meiosis § 1 copy § Haploid (n) Gametes
Meiosis • Before meiosis begins, cells go through interphase! • DNA Replication occurs in interphase, forming a duplicate copy of each chromosome.
Meiosis • After DNA Replication, each chromosome is made of 2 sister chromatids!
Meiosis – 2 Cell Divisions • Meiosis I – Reduces Diploid (2 n) Haploid (n) – Separates homologous pairs • Meiosis II – Sperm & egg formed – Separates sister chromatids Warning! Meiosis evolved from mitosis, so the stages & “machinery” are similar, but the processes are different! Do NOT confuse them!
Overview of Meiosis (I. P. M. A. T. )
2 n = 4 1 copy of each homologous pair Overview of Meiosis I 2 n = 4 Now sister chromatids Reduction 1 n = 2 Homologous pairs separated, sister chromatids remain
Meiosis I – similar to mitosis! Prophase I 1. Centrioles move to opposite sides of the nucleus 2. Nuclear envelope breaks down.
Prophase I 3. Homologous chromosomes pair up. This is different than mitosis! – Each pair of chromosomes align & a tetrad is formed.
Prophase I – Chromosomes exchange portions of their chromatids in a process called crossing over. – Crossing over produces new combinations of DNA!
Here is a tetrad formed from 2 sets of sister chromatids. Gene A – Height (“A” is tall & “a” is short) Gene B – Hair Color (“B” is brown hair & “b” is blonde) The gametes produced can carry several varieties of genes; they’re not identical to the parent cell! This person can pass on tall/brown traits, tall/blonde traits, short/brown traits, or short/blonde traits!
Metaphase I • Spindle fibers attach to the chromosomes at the centromere.
Anaphase I • Spindle fibers pull the homologous chromosomes toward opposite ends of the cell. • Chromosomes move & separate into 2 groups near the spindle. • Anaphase ends when the chromosomes stop moving.
Telophase I • Nuclear membranes form around chromosomes. Remember! Cytokinesis is when the cytoplasm divides!
Meiosis I End Results • Produces 2 haploid (n) daughter cells that have only 1 set of each chromosome • Each set is made of 2 sister chromatids.
Meiosis I
Meiosis II • After Meiosis I, cells enter Meiosis II: – No DNA Replication occurs between Meiosis I & Meiosis II. – Meiosis II separates the sister chromatids.
Prophase II • Spindle fibers appear.
Metaphase II • Chromosomes align like they do in mitosis & attach to the spindle at the centromere.
Anaphase II • Sister chromatids separate & move towards opposite ends of the cell.
Telophase II • Nuclear membranes form around chromosomes. • Then, cytokinesis!
Meiosis II End Results • Produces 4 haploid (n) daughter cells
What are the advantages to sexual reproduction? • Increased diversity – Independent assortment of chromosomes: random alignment of homologous chromosomes in Metaphase 1 – Crossing over: mixing of alleles across homologous chromosomes – Random fertilization – which sperm fertilizes which egg? • Driving evolution – Providing variation for natural selection
Variation from Independent Assortment of Chromosomes Random assortment in humans produces 223 (8, 388, 608) different combinations in gametes!
Variation from Crossing Over • Creates completely new combinations of traits on each chromosome
Variation from Random Fertilization • Sperm + Egg = ? – Any 2 parents will produce a zygote with over 70 trillion possible diploid combinations!
Sexual reproduction allows us to maintain both genetic similarity & differences. Jonas Brothers
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