Transcript HL#1 Meiosis - hutchhighIBbiology

IB HL #10
•Meiosis reduces chromosome
number by copying the
chromosomes once, but
dividing twice.
•The first division, meiosis I,
separates homologous
chromosomes.
•The second, meiosis II,
separates sister chromatids.
•In prophase I, the chromosomes condense and
homologous chromosomes pair up to form
tetrads
• Centrioles move to the poles in animal cells
•Spindle microtubules forms from each
pole to the equater
• Nucleoli break down
• Nuclear membrane starts
to break down
Crossing-over occurs
•At metaphase I, the bivalents line up at the equater.
• Spindle microtubules from the two poles attach to the
centromeres on different chromosomes in each pair.
• At the end of metaphase I the chromosomes start to move
• In anaphase I, the homologous chromosomes
separate and are pulled toward opposite poles.
• This halves the chromosome
number.
• Each chromosome still consists
of two chromatids.
• Because of crossing-over the
two chromatids are not identical
• At the end of anaphase I the
chromosomes reach the poles.
•In telophase I, movement of homologous chromosomes
continues until there is a haploid set at each pole.
• Each chromosome consists of linked sister chromatids.
• Nuclear membranes form around the groups of
chromosomes at each pole.
•Cytokinesis by the same mechanisms as mitosis
usually occurs simultaneously.
• The chromosomes partially uncoil.
• The cell divides to form two haploid
cells
• The two cells may enter a brief period of
Interphase or immediately proceed to the
next division. DNA is not duplicated.
• Meiosis II is very similar to mitosis.
• During prophase II chromosomes become shorter
and thicker again by uncoiling.
• Centrioles move to the poles in animal cells and
spindle microtubles form.
• At the end of Prophase II the
nuclear membranes break down.
•At metaphase II, spindle fibers attach to the
centomeres and the sister chromatids line up at
the equater
• At the end of metaphase II the centromeres
divide
•At anaphase II, the
centomeres of sister
chromatids separate
and sisters chromatids
travel toward opposite poles.
•In telophase II, separated sister
chromatids arrive at opposite
poles and uncoil.
•Nuclear membrane forms around the
chromatids.
•Cytokinesis separates
the cytoplasm.
•Nucleoli appear
•At the end of meiosis,
there are four haploid
daughter cells.
SUMMARY OF MEIOSIS
1. Meiosis involves two divisions. One cell or nucleus divides
to form four cells or nuclei.
2. The chromosome number is halved, from diploid to haploid.
3. An almost infinite amount of genetic variety is produced,
as a result of crossing-over in Prophase I and the random
orientation of bivalents in Metaphase I.
Mendel’s law of segregation
Mendel’s law of segregation states that alleles of a
gene on homologous chromosomes separate when
gametes are formed.
Mendel’s law applies to two (or more) alleles
(alternate forms of a gene). The allele pairs separate
in Meiosis I, when a diploid cell divides into two
haploid cells, before the formation of gametes.
So, for example each sperm or egg will get just one
big R, or one little r, or one big T, or one little t.
Mendel’s law of independent assortment
• The law of independent assortment says that alleles
of genes (that are not linked) located on different
chromosomes separate independently.
• In other words alleles for different traits are
transmitted independently.
• During Meiosis I, the homologous chromosomes line
up randomly on the equator during metaphase
ensuring random distribution of alleles.