Chapter 6
Section 1 and Section 2
MEIOSIS
Gametes (sex cells) have half the number of
chromosomes that body cells (somatic cells) have.
Gametes = 23 chromosomes
Somatic Cells = 46 chromosomes
Egg (23) + Sperm (23) = human organism (46)
• Your body cells have 23 pairs of
chromosomes.
– Homologous pairs of
chromosomes have the same
structure.
– For each homologous pair, one
chromosome comes from each
parent.
• Chromosome pairs 1-22 are
autosomes (chromosomes that
have nothing to do with gender).
• Sex chromosomes, X and Y,
determine gender in mammals.
Karyotype shows chromosomes.
• Body cells are also called somatic cells.
• Germ cells (underdeveloped gametes)
develop into gametes through process of
Meiosis.
– Germ cells are located in the ovaries and testes.
– Gametes are mature sex cells: egg (23 chromosomes) and sperm
(23 chromosomes). Fertilized human egg = 46 chromosomes.
– Gametes have DNA that can be passed to offspring.
body cells
sex cells (sperm)
sex cells (egg)
Body cells are diploid.
• Diploid (2n) cells have two copies of every
chromosome. If n = 23, then 2n = 46.
– Somatic cells are diploid.
– 23 chromosomes come from dad and the other
23 chromosomes come from mom.
– Similar chromosomes (homologous
chromosomes) from parents will form pairs.
Gametes are haploid.
• Haploid (n) cells have one copy of every chromosome.
– Gametes are haploid. If n = 23, then haploid # is
23 because haploid = n.
– Gametes have 22 autosomes and 1 sex
chromosome.
Cells go through two rounds of division in meiosis.
Division 1 called Meiosis 1 and Division 2 called Meiosis 2.
• Meiosis reduces chromosome number and
creates genetic diversity through gametes.
Meiosis 1- Interphase, Prophase 1,Metaphase 1,
Anaphase 1, Cyokinesis 1. Everything is the same as Mitosis except for
crossing over during Prophase 1 and chromosomes line up in pairs along metaphase plate
during Metaphase 1. Keep in mind that after cytokinesis 1, the 2 daughter cells are not identical.
•Each chromosome is replicated (copied)
during S phase of Interphase. Just like
Mitosis.
•Similar to mitosis- but chromosomes line
up in pairs called Tetrads in prophase I
•Exchange of genetic information during
process called crossing-over to create
genetic diversity.
•Crossing over occurs once only during
Prophase 1.
Meiosis 1 Overview: Interphase (only occurs once at the very beginning),
Prophase 1, Metaphase 1, Anaphase 1, Telophase 1, Cytokinesis 1. Read caption
next to the pictures to get a detailed synopsis of events in the cell at each phase.
Cytokinesis I -
Meiosis 2 Overview: Prophase 2, Metaphase 2, Anaphase 2, Telophase 2, and
Cyotkinesis 2. Read caption next to the pictures to get a detailed synopsis of events
in the cell at each phase.
•No new
replication of
chromosomes
•Results in four
haploid
daughter cells
•Gamete
formation in
animals: egg
and sperm
Cytokinesis 2 -
Haploid cells develop into mature gametes.
Gametogenesis is the production of gametes.
Spermatogenesis: Formation of sperm.
– Sperm become streamlined and motile.
– Sperm primarily contribute DNA to an embryo.
Oogenesis: Formation of eggs.
– Eggs contribute DNA, cytoplasm, and
organelles to an embryo.
– During meiosis, the egg gets most of the
contents; the other cells form polar bodies.
Meiosis versus Mitosis
– Meiosis has two cell divisions while mitosis has one.
– In mitosis, homologous chromosomes never pair up and
cross over.
– Meiosis results in genetically different haploid cells; mitosis
results in genetically identical diploid cells.
Mitosis Review
• Mitosis makes more diploid cells.
• Mitosis occurs in every somatic cell; not germ cells.
• It is a means for growth and repair.
Meiosis Review
• Meiosis makes haploid gamete cells from diploid germ cells.
• Meiosis occurs in sex cells.
• Meiosis produces gametes.