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Sexual Reproduction & Mendelian Genetics. Chapter 10 Pages 270-285. First, Some Vocabulary……. somatic cells – body cells, 46 chromosomes in humans gametes – sex cells, have half the number of chromosomes as compared to somatic cells, so 23 chromosomes in human gametes

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first some vocabulary
First, Some Vocabulary……

somatic cells – body cells, 46 chromosomes in humans

gametes – sex cells, have half the number of chromosomes as compared to somatic cells, so 23 chromosomes in human gametes

diploid = 2n number of chromosomes

haploid = n number of chromosomes


Fertilization – the process by which one haploid gamete combines with another haploid gamete.

oogenesis – the creation of an egg

spermatogenesis – the creation of sperm



Section 10.1

Pages 270-276

  • Is the type of cell division that makes gametes.
  • Occurs in reproductive cells in which one diploid cell (2n) produces four haploid cells (n) that are not genetically identical.
  • Is referred to as reduction division because it reduces the number of chromosomes from 2n to n.
  • Involves two consecutive cell divisions called Meiosis I and Meiosis II.
mitosis vs meiosis
Mitosis vs. Meiosis
  • Meiosis occurs in the cells of the reproductive organs to produce gametes (egg and sperm cells).
  • Mitosis occurs in body cells.
  • Each body cell has (2n) chromosomes after mitosis.
  • Each egg and sperm cell has (n) chromosomes after meiosis
haploid vs diploid
Haploid vs. Diploid
  • Egg and sperm cells are haploid (n) because they have HALF the # of chromosomes found in body cells.
  • Body cells are diploid (2n) because they have 2 sets of chromosomes.
  • When egg and sperm cells fuse = zygote has (2n) chromosomes.
meiosis animation
Meiosis Animation
meiosis i

Anaphase I

Interphase I

Prophase I

Metaphase I

Homologues separate and get pulled to opposite poles by spindle fibers.

DNA replicates. Chromatin condenses.

Synapsis – pairing ofhomologous chromosomes to form a tetrad (4 chromatids).

- Crossing over occurs

Homologous chromosomes line up along the equator of the cell.

Meiosis I

TelophaseI (not shown) – the cell divides into two haploid cells (see pg 273 of text).

crossing over in prophase i leads to genetic variation
Crossing Over in Prophase I leads to genetic variation:

Notice the grey and white chromosomes have exchanged segments. This change occurs during synapsis and is called crossing over. Crossing over is a process during which chromosomal segments are exchanged between a pair of homologous chromosomes.

meiosis ii

Prophase II

Metaphase II

Anaphase II

Telophase II

Meiosis I results in two haploid (n) daughter cells, each with half the number of chromosomes as the parent cell.

The chromosomes line up on the metaphase plate (or equatorial plate).

Sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (n) daughter cells.

Meiosis II
comparing mitosis to meiosis
Comparing Mitosis to Meiosis
  • Mitosis= results in 2 genetically identical, diploid (2N) cells.
  • Meiosis= results in 4 genetically different, haploid (N) cells.
comparing meiosis to mitosis

Prophase I

Homologous pairs form tetrads by synapsis.

Crossing over occurs.

Nuclear envelope breaks down, spindles form.

Metaphase I

Tetrads line up on the equator.

Anaphase I

Homologues separate.

Telophase I

New nuclei form and contain the n amount of DNA.

Meiosis II is just like mitosis.



Chromosomes condense

Nuclear envelope breaks down

Spindles form


Chromosomes line up on equator


Sister chromatids separate


New nuclei form with normal chromosomes

Comparing Meiosis to Mitosis
review questions for you
Review Questions For YOU!!

1. Why is it important for chromatin to condense?

  • Describe chromatin vs. chromosome.
  • Define the term “autosome”.
meiosis spermatogenesis
Meiosis – Spermatogenesis:
  • Male - sperm cells = Meiosis results in 4 viable sperm cells.




meiosis oogenesis
Meiosis - Oogenesis
  • Female - egg cells = one egg cell gets most of the cytoplasm and the other 3, called polar bodies, disintegrate.





A. Normal Meiosis

B. Failure of homologous chromosome pair to separate during Meiosis I

C. Failure of sister chromatids to separate during Meiosis II