Meiosis - whssbiozone

• 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)­mQS_FZ0
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
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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
one paternal
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It is diploid (2n) because it has two copies of each one
chromosome. maternal,
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
• when homologues join to form a tetrad
• the sticky region that holds two sister
chromatids together
• structures stored in the nucleus that code
the instructions to form each new cell
• two copies of each chromosome
• 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
• errors in mitosis or DNA replication
that can result in damaged cells
• only one copy of each chromosome
Getting it straight: Sister chromatids and homologous chromosomes
Consider chromosomes as they appear, short and thick, in metaphase
exists in two
forms; these
are homologous
(1 form is maternal,
1 form is paternal)
but you could have been....