MEIOSIS
CREATING GAMETES (SEX
CELLS)
Page 97
EQ: Why is meiosis important for sexually reproducing organisms?
CHROMOSOMES
CHROMOSOMES
• A human body cell (called a somatic cell) has 23 pairs of homologous
chromosomes for a total of 46 chromosomes.
• One chromosome of each homologous pair comes from the mother (called a
maternal chromosome) and one comes from the father (paternal
chromosome).
• Homologous chromosomes are
similar but not identical.
• They carry the same genes in
the same order, but the alleles
for each trait may not be the
same
DIPLOID AND HAPLOID
• A cell that contains both sets of homologous
chromosomes is said to be diploid
• (2 sets; 2N; 46 chromosomes)
• Your somatic cells are diploid
• A cell that only contains 1 set of
chromosomes is said to be haploid.
• (1 set; N; 23 chromosomes)
• Your eggs/sperm cells are haploid
MITOSIS VS. MEIOSIS
Mitosis
• Results in 2 identical diploid
cells
• Involves asexual reproduction
• 1 division
• Function: growth & repair
•
•
•
•
Meiosis
Results in 4 genetically
different haploid cells
Involves sexual reproduction
2 divisions
Function: create sex cells
MITOSIS VS. MEIOSIS
WHAT IS MEIOSIS?
• Meiosis is a two-part cell division
process occuring in organisms who
sexually reproduce
• The point of meiosis is to create gametes
(egg & sperm cells) so that babies can be
made
• The random formation of gametes
during meiosis creates genetic diversity
in a population.
Chromosomes
from parents
Meiosis I
Chromosomes
from both parents
are copied and
paired so that
crossing over can
occur
MEIOSIS 1
Chromosomes
replicate
Homologous
chromosomes pair up
Crossing over occurs
Daughter nuclei
have single
chromosomes and
a new mix of
genetic material
Nucleus divides into 2 nuclei
Meiosis II
Daughter nuclei
divide again
PHASES OF MEIOSIS
• Meiosis I: (P,M,A,T) 1
cell divides to create 2
diploid daughter cells
• Meiosis II (P,M,A,T) 2
cells divide to create 4
haploid cells; this is
when the gametes form
Meiosis I
PHASES OF MEIOSIS
• Your initial cell has 46 chromosomes
– 23 from mom, and 23 from dad
• These 46 chromosomes will duplicate into double stranded chromosomes
– all 46 were initially single chromatids however they have replicated and become
double stranded
– There are still only 46 chromosomes!
• In Meosis 1 the chromosomes will align in pairs
– These are homologous pairs (ie code for same traits, one is from mom, the other
from dad)
– This is when crossing over occurs
• The cell divides and you get two cells. Each cell now has 23 double stranded
chromosomes. This is the end of Meiosis I
PHASES OF MEIOSIS
Meiosis II
• In both daughter cells, the chromosomes are aligned at the middle of the
cell (NOT replicated) & then pulled apart
A
B
CROSSING OVER
Occurs in Meiosis I
1. Homologous
chromosomes form a
tetrad
2. Chromatids cross over
one another
3. The crossed sections of
chromatids are
exchanged
***This produces a new combination of
genes which contributes to diversity
within a population. ***