Download Meiosis

Meiosis
Chapter 11-4
The Basics
Genes are located on the chromosomes.
Each organism must inherit one copy of every
gene from both parents.
Each organism has 2 complete sets of genes.
Those two sets must be separated so that each
gamete produced contains just one set of genes.
Chromosome Number
Humans have 46 chromosomes.
– 23 came from your mother
– 23 came from your father
These chromosomes are homologous (same
gene). This means that one complete set of genes
came from each parent.
All 46 chromosomes are present in every human
body cell (a.k.a. somatic cell).
This is the diploid chromosome number (2 sets).
Gametes
Gametes are sex cells—sperm or egg.
Gametes contain only 1 copy of each
chromosome.
These cells are haploid (1 set).
Humans have 23 chromosomes in every
gamete.
Meiosis
Meiosis is a process of reduction division.
The number of chromosomes per cell is cut in half
through the separation of homologous
chromosomes.
2 distinct stages:
– Meiosis I
– Meiosis II
By the end of Meiosis II, 4 haploid cells are
produced.
Meiosis I
Chromosomes are duplicated in interphase.
Prophase I
– Homologous chromosomes pair up
Metaphase I
– Spindle fibers attach to chromosomes to line them up
in the middle
– Crossing over occurs (genes are swapped between
homologous chromosomes to increase genetic
diversity)
Anaphase I
– Homologous chromosomes are separated
Meiosis I Results
Because of crossing over, the cells that are
produced have chromosomes that are different
from each other and from the original cell.
The cells produced are NOT identical copies like
in mitosis.
Meiosis I results in 2 haploid daughter cells that
are NOT genetically identical.
Meiosis II
The chromosomes are NOT replicated in between
Meiosis I and Meiosis II.
Prophase II—the 2 haploid cells from Meiosis I prepare
for another division
Metaphase II—chromosomes line up in the middle of the
spindle
Anaphase II—sister chromatids are pulled apart
Telophase—the nuclear membranes reform and the
cytoplasm is divided
Gamete Formation
Males produce 4 sperm cells each time
meiosis takes place.
Females produce 1 egg cell and 3 polar
bodies (non-functional cells) because
the cytoplasm is not divided equally.
Mitosis vs. Meiosis
Mitosis
Body cells
Meiosis
Sex cells (sperm/egg)
2 cells produced
4 cells produced
Diploid (2 copies of
each chromosome)
Haploid (1 copy of each
chromosome)
Genetically identical to
each other AND to the
original cell
Genetically different