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Sexual Reproduction
Meiosis
Sexual Reproduction (Production of sperm cells and egg cells)
 Genes (a segment of DNA coding for specific traits) are located
on chromosomes are carried from 2 parents (male and female) to
offspring
 1 chromosome from male,1 chromosome from female
 1 complete set of information from each parent: (diploid-2n)
 1 from male
 1 from female
 A pair of chromosomes (carrying
similar genetic information)
Each colored
=homologous chromosomes band is a
gene
(diploid-2n)
 Karyotyping
 are used to examine an
individual’s chromosomes.
 Karyotypes are made from a sample of
a person’s blood.
 Arranged from largest to smallest
 Last pair of chromosomes are
The exception, they are
sex chromosomes which determine the
gender of the offspring
XX = female,
XY = male
Meiosis:
 Cell division that halves the total number of chromosomes
within the specie
 Ex. Frog has 8 half = 4, human has 46, half =23
 Involves 2divisions of the nucleus
 # of chromosomes cut in half (haploid- n)
 homologous chromosomes are separated
 cell division similar to mitosis BUT 2x’s
 cells produced through meiosis are called gametes or sex cells, which
contain only half,1/2, the hereditary information needed.
Spermatogenesis
oogenesis
 Sperm cells from the male contains ½ the hereditary info
or 23 chromosomes
 Egg cells from the female contains ½ the hereditary info
or 23 chromosomes
 Sperm + egg =fertilization
23 + 23 = 46 chromosomes
 A newly formed cell from 1 sperm cell and 1 egg cell
is called a zygote
 Together, the organism has the correct number of chromosomes
 The resulting offspring is genetically different (variation) from
the parents.
Stages of Meiosis:
Prophase I
Interphase
 Chromosomes (DNA) replicate
Prophase I
 Chromosomes pair up with other like
chromosomes called homogulous pairs
Homogulous pair
 pairing of homogulous chromosomes =
synapsis
 This group of 4 chromatids (2 whole chromosomes) is called
atetrad
 Chromosomes can exchange genetic
informationcrossing over
Metaphase I
 Tetrads line up in the middle
Metaphase I
Anaphase I
 Tetrad separate & move to opposite
ends
 Chromosomes are still dbl. stranded
Anaphase I
Telophase I/ cytokinesis
 chromosomes reach the opposite ends of
the cell, and cytokinesis begins
 2 cells  diploid (2n)
Telophase I
Prophase II
 centrioles separate
 centromeres attach to spindle fibers
Prophase II
Metaphase II
Metaphase II
 chromosomes move to the middle
Anaphase II
 centromere divides
 chromatids move to opposite ends
 chromosomes are haploid (n)
Anaphase II
Telophase II/cytokinesis
 4 cells are formed
 ½ the genetic information
 nuclear membrane returns
 cytoplasm divides
Nondisjunction, a common genetic disorder occurs when chromosomes
do not separate properly during Anaphase I or Anaphase II.
Meiotic Nondisjunction at Meiosis I Animation