This document provides an overview of meiosis. Meiosis is the process by which germ cells are produced with half the normal number of chromosomes. It occurs in two divisions called Meiosis I and Meiosis II. In Meiosis I, homologous chromosomes pair and may exchange genetic material through crossing over. The homologous chromosomes then separate, reducing the chromosome number by half. Meiosis II then separates the sister chromatids, resulting in four haploid daughter cells each with a single set of chromosomes. These germ cells can then mature into egg or sperm cells.

1. MEIOSIS
GENETICS
CAMBELL & REECE
2012 CHAPTER 13

2. INTRODUCTION TO MEOISIS
 SOMATIC CELLS HAVE 23 HOMOLOGOUS CHROMOSOMES
 HALF FROM THE FATHER: PATERNAL
 HALF FROM THE MOTHER: MATERNAL
 SEX CHROMOSOMES ARE KNOWN AS GONOSOMES OR X
AND Y
 X AND Y DIFFER IN GENETIC COMPOSITION AND IN SIZE

4. HOMOLOGOUS CHROMOSOMES
 HOMOLOGOS CHROMOSOMES ARE SIMILAR IN LENGTH, CENTROMERE
POSITION AND GENETIC LOCATION (LOCUS)
 DIFFERENT VERSIONS OF A GENE ARE CALLED ALLELES AND CAN BE
FOUND IN THE SAME LOCUS ON PATERNAL AND MATERNAL
CHROMOSOMES

5. WHAT IS MEOISIS
 ALL HUMANS AND MOST PLANTS HAVE DIPLOID (2n)
CHROMOSOMES
 MEOISIS IS THE HALVING OF THE NUMBER OF
CHROMOSOMES TO HAPLOID (n)
 MEOISIS OCCURS IN THE SEX ORGANS TO PRODUCE
GAMETES THAT WILL MATURE INTO AN EGG CELL OR A
SPERM CELL
 FERTILIZATION IS THE FUSION OF GAMETES

7. INTERPHASE ONE
 IN THE CELL THERE IS A BUILD UP OF ENERGY
 DNA REPLICATION STARTS

8. PROPHASE 1
NUCLEOMEMBRANE AND NUCLEOLUS STARTS TO
DISAPPEAR
THE CENTRIOLI MOVE TO OPPOSITE POLES

9. CROSSING OVER
 CROSSING OVER IS THE PROCESS WHEREBY THE
HOMOLOGOUS CHROMOSOMES TOUCH AT POINTS
CALLED CHIASMATA
 THIS INCREASES THE GENETIC VARIATION OF THE
GAMETES

10. METAPHASE 1
 CENTRIOLI REACH POLES
 SPINDLE FIBRES FORM BETWEEN THEM
 HOMOLOGOUS CHROMOSOMES REACH POLES
EQUOTORIAL PLANES ATTACHED TO SPINDLE
FIBRES
 KINETOCHORE OF CENTROMERE ATTACH TO
SPINDLE FIBRES

11. ANAPHASE 1
 DUPLICATED CHROMOSOMES MOVE TO ONE PART OF
THE CELL
 SPINDLE FIBRES AND CENTRIOLI CONTRACT

12. TELOPHASE 1
 THE CYTOPLASM WILL FORM AN INVAGINATION
AND UNGERGO CYTOKENISIS
 THE NUCLEOMEMBRANE AND NUCLEOLUS
REAPPEAR
 TWO DAUGHTER CHROMOSOMES FORM THAT
ARE HAPLOID

13. PROPHASE 2
 NUCLEAR MEMBRANE AND NUCLEOLUS DISAPPEAR
 CENTRIOLI MOVES TOWARDS THE POLES

14. METAPHASE 2
 INDIVIDUAL DUPLICATED CHROMOSOMES ATTACH TO
SPINDLE FIBRES

15. ANAPHASE 2
 DAUGHTER CHROMATIDS MOVE TO THE POLES OF THE
CELLS

16. TELOPHASE 2
 FOUR NEW CELLS WITH HAPLOID NUMBER OF
CHROMOSOMES
 THESE CAN MATURE TO FORM SERM CELLS AND EGG
CELLS