Meiosis - whssbiozone

Meiosis
• The division of sex cells (gametes)
• Conserves the number of chromosomes
• Without meiosis, the number of chromosomes in a species would double with each generation
• Meoisis occurs in similar stages to mitosis, but has two distinct parts (2 divisions)
http://www.youtube.com/watch?v=D1_­mQS_FZ0
http://www.youtube.com/watch?v=kVMb4Js99tA
Meiosis I
all about homologues
Prophase I
• homologous pairs of chromosomes join to form a tetrad, and to CROSSING OVER
exchange parts of chromosomes occurs
Metaphase I
• tetrads line up along the middle of the cell
Anaphase I
• homologous pairs separate and move to opposite ends of the cell
• sister chromatids remain together
• this is called Independent Assortment
Telophase I
• two daughter cells are produced
• each daughter cell has 1/2 the number of chromatids in the original cell (from 4 chromatids to two chromatids in each new cell)
• this is the Reduction Division
Crossing­Over
Click to go back
The exchange of parts of non­sister chromatids from homologous chromosomes. The forming of the tetrad and process of crossing over is known as synapsis.
Independent Assortment
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• in meiosis I, the homologous pair separate, but with no pattern....
Daughter cells
like this,
This mother cell could divide to give....
...OR daughter cells
like this!
Meiosis I: the Reduction Division
Mother cell has already replicated DNA, so each chromosome has a sister chromatid. Homologous pairs make tetrads. Daughter cells are haploid (n) because they have only one copy of each chromosome.
one maternal
or
one paternal
Click to go back
It is diploid (2n) because it has two copies of each one
chromosome. maternal,
one
homologous paternal
chromosomes separate
sister chromatids are
still together
Meiosis II
Prophase II
• very brief
Metaphase II
• sister chromatids line up along the equator
Anaphase II
• sister chromatids separate
Telophase II
• four new cells are produced
• each new cell has half the number of chromosomes as a somatic cell
• each cell is haploid because it contains only one copy of each chromosome
all about sister chromatids
Terms to know:
drag the term to its definition
synapsis
• when homologues join to form a tetrad
centromere
• the sticky region that holds two sister
chromatids together
chromosomes
• structures stored in the nucleus that code
the instructions to form each new cell
diploid
• two copies of each chromosome
gametes
• reproductive cells
crossing over
• process during which non-sister chromatids
exchange genetic information
homologous chromosomes
• similar chromosomes that form pairs, one
from your mother and one from your father
sister chromatids
• two parts of a double-stranded chromosome
that have identical instructions
parent cell
• the original cell that will divide to form
two new cells
daughter cell
• a new cell produced through cell division
of a parent cell
mutations
• errors in mitosis or DNA replication
that can result in damaged cells
haploid
• only one copy of each chromosome
Getting it straight: Sister chromatids and homologous chromosomes
Consider chromosomes as they appear, short and thick, in metaphase
every
chromosome
exists in two
forms; these
are homologous
chromosomes
(1 form is maternal,
1 form is paternal)
but you could have been....