Download Mitosis

Mitosis & Meiosis
What’s the difference?
Write down these
terms:
 Mitosis
 Karyotype
 Meiosis
 Allele
 Diploid
 Genotype
 Haploid
 Phenotype
 Sex chromosomes in a Male?
 Dominant
 Sex chromosomes in a Female?
 Recessive
 Number of chromosomes in a
 Homozygous
person?
 Heterozygous
 Pedigree
Mitosis
 Mitosis is the process where one cell divides into 2 identical cells
 This is how we go from 1 cell (the fertilized egg) to millions of cells!
 It is also HOW we grow & repair our bodies.
 What has to happen BEFORE mitosis can occur?
 The DNA has to
 replicate ( be copied!)
 All 46 chromosomes (yes, MOST human cells have 46
chromosomes) have to be copied so both DAUGHTER cells get the
same 46 chromosomes that were in the parent cell.
What is a Chromosome?
 The structure that the DNA forms in our cells
 The DNA is associated with proteins that compact it so it fits and is
organized inside the nucleus!
Parent cell
Chromosomes are copied
and double in number (sister
chromatids are attached at
by the centromere
Chromosomes
now split
2 daughter cells
identical to original
 Animation of mitosis:
 http://www.cellsalive.com/mitosis.htm
 So, we’ve got 46 chromosomes.
 How many different PAIRS of chromosomes do we
have?
 Are ALL the pairs the same?
 23 pairs of chromosomes
 Yes & No- 2 copies of chrom 1,
2 copies chrom
2……, except for the sex chromosomes (X & Y) in MEN
( XY) & Women (XX)
CHROMOSOME
NUMBER
 Number of chromosomes in body cells of a species is
diploid = 2n
(n is the number of different
chromosomes; 2n = 2 copies of each different
chromosome)
 The number of chromosomes in egg and sperm cells
(gametes) is haploid which is “n”, or 1 copy of each
different chromosome
What are diploid and
haploid for us?
 Diploid= 46chromosomes =2 copies of 23
different chromosomes (1-22 + XX or XY)
 Haploid = 1 copy of 23 different
chromosomes (1-22 + X or Y)

Where does each chromosome of the pair come
from? (Ex. You’ve got 2 copies of chr 1- where
did each copy come from?)


One chromosome comes from MOM, and one
chromosome comes from Dad
So, who determines the sex of the baby????

DAD- He is the ONLY parent that can contribute
the Y chromosome that makes the fetus male.
 How do the gametes (sex cells) end up with only 23
chromosomes? THINK MEIOSIS!!
 WHY do they need to be haploid (only 1 copy of each
chromosome or 23 total chromosomes)?
MEIOSIS
IT’S ALL ABOUT SEXUAL REPRODUCTION…
MEIOSIS
 A type of cell division where the number of
chromosomes is reduced by half
 ONLY occurs in gonads (ovaries or testes) during
formation of gametes (egg or sperm)
 Human body cells have 46 chromosomes; human
sperm and egg cells each have 23 chromosomes
Gametes are haploid for
SEXUAL REPRODUCTION!
 When egg and sperm combine during fertilization, each
brings half the total number of chromosomes for that
species
 Half + half = whole
 In humans, 23 chromosomes (from Dad) + 23
chromosomes (from Mom) = 46 chromosomes (child) !!!
FERTILIZATION
egg
+ sperm = zygote
23
+
n
Haploid
23
=
n
+
haploid
46
2n
=
diploid
Summary of Meiosis
 Type of cell division used to form the gametes
(egg & sperm) where chromosome number is
reduced to haploid (n).
 Meiosis is necessary for sexual reproduction.
 It involves 2 cell divisions, NOT 1 like in
mitosis.
Animation of Meiosis
 http://www.cellsalive.com/meiosis.htm
MORE ABOUT
CHROMOSOMES
 In humans, there are 22 different
“regular” chromosomes (numbered 1 to 22 by size &
shape) and 2 different sex chromosomes
 The sex chromosomes are X and Y
 Which sex chromosomes are in a girl?
 XX
 Which are in a boy?
 XY