Transcript Meiosis Notes - Brookwood High School

Meiosis Notes
CP Biology
Ms. Morrison
Chromosome Number
 Every organism gets half of
chromosomes from one parent and
half from the other parent
 Two sets of chromosomes =
homologous chromosomes
Chromosome Number, pg 2
 Cells with homologous chromosomes
= diploid (2N)
 Gametes (egg, sperm) – contain only
one set of chromosomes = haploid
(N)
 Humans – 23 pairs (46 chromsomes)
Meiosis
 Form of cell division to reduce the
number of chromosomes in cell to
half (2N  N)
 Two stages to process: Meiosis I and
Meiosis II
 At end of Meiosis II – one diploid cell
has become four haploid cells
 Used to produce gametes (does not
occur in regular body cells)
Meiosis I
 Picture overview:
Meiosis I, pg 2
 Interphase I – all chromosomes are
replicated
 Prophase I – each chromosome pairs
with its homologous chromosome to
form a tetrad (4 chromatids)
Meiosis I, pg 3
 Prophase I – during this phase,
chromosomes in tetrads may
exchange portions of their chromatids
in process = crossing over (gives new
gene combinations
Meiosis I, pg 4
 Metaphase I – spindle fibers attach to
chromosomes along center of cell
 Anaphase I – spindle fibers pull the
homologous chromosomes toward
opposite ends of the cell
Meiosis I, pg 5
 Telophase I/Cytokinesis – nuclear
membranes form and the cell
separates into two cells
 At end of Meiosis I – two daughter
cells that are haploid because they
contain one set of chromosomes
(duplicated, though)
Meiosis II
 Two haploid daugther cells go through another
division similar to mitosis but does NOT have
replication prior to prophase II
Meiosis II, pg 2
 Prophase II – nuclear membranes
disappear and spindle forms
 Metaphase II – chromosomes line up
along center of cell like those in
mitosis
Meiosis II, pg 3
 Anaphase II – sister chromatids
separate and move towards opposite
sides of the cell
 Telophase II/Cytokinesis – nuclear
membranes reform and the two cells
separate into four cells – all are
haploid
Gamete Formation
 Males – haploid gametes = sperm,
always produce four sperm through
meiosis
 Females – haploid gamete = egg,
uneven divisions through Meiosis I &
II so egg receives most of cytoplasm,
remaining three cells = polar bodies
(not used in reproduction)