46 CHROMOSOMES

1
Bell Ringer


46 CHROMOSOMES
46 CHROMOSOMES


92 CHROMOSOMES

1. What is this process called?
2. Is this scenario reasonable? Why or why not?

2
Bell Ringer



23 CHROMOSOMES
46 CHROMOSOMES
23 CHROMOSOMES


3
MEIOSIS
SEXUAL REPRODUCTION
4
Meiosis Overview

• The purpose of meiosis is to create gametes
  (sperm and eggs) with half the original number of
  chromosomes.
• Meiosis begins with one diploid cell (2 of each
  type of chromosome) and ends with four haploid
  cells (one of each chromosome).
• 4 sperm cells in males
• 1 egg cell with 3 polar bodies in females

5
Chromosomes

• Chromosome A structure that carries genes.
• Each human cell has 46 chromosomes (23 pairs)
• Each human gamete has 23 chromosomes (0 pairs)

6
Meiosis

• Meiosis
• occurs in testes of males
• occurs in ovaries of females
• Involves two divisions with one DNA replication.

7
Interphase

• Before meiosis begins, DNA must replicate itself
  so enough copies will be available for the four
  new cells being created. This happens in
  Interphase.
• Chromosomes in the replicated state are made of
  two SISTER CHROMATIDS that are connected by a
  CENTROMERE.

8
Meiosis I

• Occurs in Four Phases
• Prophase I- Homologous chromosomes pair up.
  Crossing over may occur in this phase.

9
Metaphase I

• Homologous Chromosomes line up at the spindle
  equator.
• The random arrangement of pairs on either side of
  the spindle equator results in variation in the
  genotypes found in the gametes.

10
Anaphase I

• Homologous chromosomes are pulled to opposite
  ends of the cell by the spindle fibers.

11
Telophase I

• The cell begins to pinch together and form two
  distinct haploid cells.
• How many chromosomes were in the original cell?
• A short Interphase occurs but no DNA replication

12
Meiosis II

• Prophase II- No pairing of homologues (they are
  in separate cells).
• Metaphase II- Sister chromatids line up at the
  spindle equator.
• Anaphase II- Sister chromatids are pulled to
  opposite ends of the two cells.
• Telophase II- Cytokinesis occurs in both cells
  resulting in 4 haploid cells.

13
Gametogenesis

• The purpose of meiosis is to create gametes.
• The production of male gametes is called
  spermatogenesis

14
Spermatogenesis
15
Oogenesis

• The production of female gametes is called
  oogenesis.

16
Oogenesis
17
Oogenesis
18
SEXUAL REPRODUCTION

• Definition the union of two gametes
  (fertilization), thereby combining the genes from
  two different organisms.
• It is responsible for the abundant variation that
  can be seen in organisms that reproduce in this
  way.

19
Fertilization

• The gametes that are formed by meiosis may
  randomly combine to form offspring that contain a
  variety of traits.

20
Summary

• Meiosis results in the production of gametes
  which contain a variety of combinations of the
  parental genes.
• Variation in gametes results from meiosis I when
  
• 1) In prophase I when crossing over may occur
  between homologues.
• 2) During metaphase as homologues randomly line
  up on either side of the spindle equator.

21

• Fertilization contributes to variation when the
  winner of the sperm marathon hits the target egg
  and claims its prize!