Download Notes on Mitosis and Meiosis

Mitosis v Meiosis
Types of Cell Divisions
Replication of chromosomes
Replication is the process of
duplicating a chromosome
Occurs prior to division
Replicated copies are called sister
chromatids
Held together at centromere
Meiosis – A Source of Distinction
Why do you share some but not all characters of each parent?
What are the rules of this sharing game?
At one level, the answers
lie in meiosis.
Meiosis
–cell division - each cell divides twice
- occurs rapidly
- produces haploid cells (n)
- (start - 46 double stranded chromosomes (2n)
After 1 division - 23 double stranded chromosomes (n)
After 2nd division - 23 single stranded chromosomes (n))
- occurs in our germ cells (remember latin? )
– cells that produce our gametes
- egg and sperm
Meiosis – A Source of Distinction
Meiosis does two things -
1) Halves the number of chromosomes
This is a good idea if you’re going to combine two cells to make a new
organism.
In meiosis, one diploid cells produces four haploid cells.
2) Meiosis mixes the forms of each gene that each sex cell
(egg or sperm) receives.
This makes for a lot of genetic diversity. This trick is accomplished
through independent assortment and crossing-over.
Genetic diversity is important for the evolution of populations and
species.
Meiosis
a cell division forming gametes
Goal: reduce genetic material by half
Why?
from mom
from dad
child
too
much!
meiosis reduces
genetic content
“Putting It All Together” - Fertilization
Meiosis: cell division in two parts
Sister
chromatids
separate
Homologs
separate
Meiosis I
(reduction
division)
Meiosis II
(equational
division)
Diploid
Haploid
Haploid
Result: one copy of each chromosome in a gamete.
One Way Meiosis Makes Lots of
Different Sex Cells (Gametes) –
Independent Assortment
Independent assortment is the random
separation of homologous chromosomes.
This lead to VARIETY!
Crossing-Over
Crossing-Over: when 2 duplicated chromosomes line up very
close to one another during Prophase I and share genetic info.
More Variety!!!
Recombination (crossing over)
Occurs in prophase of
meiosis I
A
A
B
B
C
a
a
b
C
b
c
c
d
d
Generates diversity
D D
E
F
E
F
e
f
•Creates chromosomes with new combinations
of alleles for genes A to F.
e
f
Recombination (crossing over)
Occurs in prophase of
meiosis I
A
a
B
b
C
C
c
c
Generates diversity
D D
Letters denote genes
Case denotes alleles
E
F
d
E
F
d
e
f
•Creates chromosomes with new combinations
of alleles for genes A to F.
e
f
Recombination (crossing over)
Occurs in prophase of meiosis I
Generates diversity
a
A
B
b
C
D
Letters denote genes
Case denotes alleles
E
F
A
a
B
b
C
D
E
F
c
c
d
d
e
f
•Creates chromosomes with new
combinations of alleles for genes A to F.
e
f
Meiosis Following a Single
Chromosome Pair
Crossing over
Oogenesis – A Path of Meiosis in
Humans
Woman are less busy in meiosis than
men – meiosis produces only a few
hundred mature eggs over a lifetime.
Spermatogenesis – Another Path
of Meiosis in Humans
Men are busy – meiosis produces
roughly 250,000,000 sperm per day.
Boy or Girl? The Y Chromosome “Decides”
Y chromosome
X chromosome
Boy or Girl? The Y Chromosome “Decides”
A. Sexual Reproduction versus Asexual reproduction
great genetic variety versus no genetic variety
1. Asexual repro. needs only one parent
2. Sexual repro. needs two sex cells
a. Sperm--male sex cell
b. Egg--female sex cell
Fertilization
Fertilization is the joining of sperm and ovum.
• Meiosis II in the ovum is completed at the time of
fertilization forming one ovum and one polar body.
• Following fertilization, chemical reactions occur
preventing additional sperm from entering the ovum.
Mitosis
Meiosis
2
Number of divisions
1
Number of
daughter cells
2
4
Yes
No
Same as parent
Half of parent
Where
Somatic cells
Germ cells
When
Throughout life
At sexual maturity
Growth and repair
Sexual reproduction
Genetically
identical?
Chromosome #
Role
What Meiosis is About
Meiosis allows the creation of unique individuals through sexual reproduction.