Download 11- 4 Meiosis

Ch. 6-1, 6-2 Meiosis
Mendel’s principles of genetics require at least two things: First, each organism must inherit a
single copy of every gene from both its “parents.” Second, when an organism produces its
own gametes, those two sets of genes must be separated from each other so that each
gamete contains just one set of genes.
Homologous is the term used to refer to chromosomes that each have a corresponding
chromosome from the opposite sex parent.
Chromosome Number
(n = # of chromosomes in a gamete)
A cell that contains both sets of homologous chromosomes is said to be diploid (2n)
A single set of chromosomes is said to be haploid (n). Sex cells must be
haploid in order for humans to have a normal number of chromosomes.
Phases of Meiosis
Meiosis is a process of reduction division in which the number of chromosomes per
cell is cut in half through the separation of homologous chromosomes in a diploid cell.
Meiosis I – PMAT I (separates homologous chromosomes)
Prior to meiosis I, each chromosome is replicated.
In prophase of meiosis I each chromosome pairs with its corresponding homologous
chromosome to form a structure called a tetrad. There are 4 chromatids in a tetrad.
As homologous chromosomes pair up and form tetrads in meiosis I, they may exchange
portions of their chromatids in a process called crossing-over.
Meiosis II – PMAT II (separates sister chromatids)
The two cells produced by meiosis I now enter a second meiotic division.
Unlike the first division, neither cell goes through a round of chromosome replication
before entering meiosis II.
Each of the four daughter cells produced in meiosis II receives a single chromatid.
Those 4 daughter cells now contain the haploid number (N).
Gamete Formation - Gametogenesis
In male animals, the haploid gametes produced by meiosis are called sperm. Four usable
male gametes are formed by meiosis.
The female gamete is called an egg in animals and in some plants. One usable female
gamete is formed and 3 polar bodies that disintegrate in some organisms.
Comparing Mitosis and Meiosis
Mitosis results in the production of two genetically identical diploid cells, whereas meiosis
produces four genetically different haploid cells.
Fertilization – sex cell’s nuclei fuse to form one nucleus
Crossing over – What if homologous chromosomes fail to separate? ½ of gametes will have
an extra chromosomes, ½ of gametes will lack a chromosome.
Trisomy 21 – have an abnormal number of chromosomes (47 in humans)
In humans, an egg is fertilized by sperm, the zygote formed is a diploid cell. Humans have 1
pair of sex chromosomes and 22 pair of autosomes.
Sexual reproduction – 2 parents
Genes can recombine. *Meiosis is done in plants to produce gametes (pollen, egg)
Asexual – all the same genes