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MEIOSIS
A REDUCTION DIVISION
Background Info:
• Chromosomes are composed of genes
– Each c-some has hundreds of genes
– Body cells have 46 (23 pairs of) c-somes
• Diploid: 2 of each pair: 46 total (body cells)
• Haploid: 1 of each pair: 23 total (gametes)
• Homologous Chromosomes: a pair of
chromosomes that are the same length
with the same centromere position
– One from each parent
Overview:
• Occurs in the reproductive organisms of
organisms that reproduce sexually.
• The c-some number is cut in half through
two consecutive divisions: a reduction
division.
• Each gamete formed will provide half of
the information for the offspring
Interphase I:
• Interphase: same as mitosis
– G1:Cell functions normally
– S Phase: chromosomes replicate and pair up
with their sister chromatid (identical)
– G2: spindle apparatus is made, chromatin
condenses
Prophase I:
• Nucleus breaks down
• Chromosomes condense
• Synapsis occurs:
– 2 sister chromatids pair up forming a tetrad
made up of 2 pairs.
• Crossing over occurs:
– The tetrads are tightly held together thus the
sisters exchange information.
• Spindle forms and moves to the opposite
poles
Prophase I:
Metaphase I:
• Chromosomes centromeres attach to the
spindle fibers
• Homologous chromosomes line up at the
equator of the cell
Metaphase I:
Anaphase I:
• Homologous chromosomes separate and
move to opposite poles of the cell
Anaphase I:
Telophase I:
• The spindle fiber breaks down
• Sister chromosomes uncoil in some
species, however they are still attached at
the centromere
– The sisters may not be identical due to
crossing over
• The cell divides= 2 Diploid (2n) cells
– In some species the nucleus reforms
Meiosis II:
• Prophase II: No new replication!!
– Spindle fibers form in each of the new cells
and attach to the centromeres
• Metaphase II:
– Chromosomes line up at equator
• Anaphase II:
– Centromeres split, sister chromatids separate
and move to opposite poles
• Telophase II:
– Four nuclei form around the chromosomes,
spindle breaks down, cells divide
Products of Meiosis:
• Four new gametes are formed
• Each nucleus contains a haploid number
of the chromosomes
• Each of the four gametes contains half of
the genetic information used to form a
zygote.