Download MEIOSIS

Mitosis & Meiosis
What’s the difference?
Write down these terms:
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Mitosis
Meiosis
Diploid
Haploid
Sex chromosomes in a
Male?
• Sex chromosomes in a
Female?
• Number of chromosomes
in a person?
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Karyotype
Allele
Genotype
Phenotype
Dominant
Recessive
Homozygous
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?
• Answer: 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.
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
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!
• 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?
Answer:
• 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.
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• 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
=
46
2n
haploid
=
diploid
Summary of Meiosis
• Type of cell division used to form the gametes
(egg & sperm) where chromosome number is
reduced to haploid (n).
• It involves 2 rounds of cell division, NOT 1 like in
mitosis.
• Results in 4 HAPLOID daughter cells!
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
Karyotype
• A photograph of all of an
organisms
chromosomes.
• Scientists freeze cells at
the metaphase of
mitosis. At this stage,
chromosomes are easy
to isolate and stain.
Why perform a karyotype?
• Verify chromosome number (some
genetic diseases are caused by
MORE copies of a chrom.)
• Confirm chromosome shape,
structure and size.
Down’s Syndrome Karyotype (Trisomy
21)
ALL HUMANS HAVE THE SAME
GENES ON THE SAME
CHROMOSOMES!
• If mom’s chromosome #1 carries the genes for
eye color, hair color, and height, then dad’s
chromosome #1 also carries the genes for eye
color, hair color, and height
• Each chromosome of the pair contains an ALLELE
(or copy) of every gene.
Different versions of the same Gene
are known as Alleles!!)
CHROMOSOME PAIR #1
BLUE
EYES
BLUE
EYES
BROWN
HAIR
BLOND
HAIR
TALL
SHORT
= A GENE (allele) ON A
CHROMOSOME
• What is Phenotype?
– Outward expression of an allele
(how it looks- PHYSICAL)
– Ex.: Blue or Brown eyes, Tall or short, Artistic,
Athletic
• What is Genotype?
– Genetic makeup (the GENES) of an organism
– Ex. Ff, FF, ff
DOMINANCE
• Some genes are “stronger” than others;
they are called dominant
• The weaker gene is recessive
• EXAMPLES:
– BROWN EYES ARE DOMINANT OVER BLUE
EYES- A person may have BOTH genes, but we
ONLY see the Brown eyes
– Tall is dominant over short
• In many cases, one gene is
NOT stronger than the other.
• This is called Incomplete
Dominance
• This can cause a MIXED phenotype– For Ex.: Incomplete dominance of the gene for
red and white flower color will result in PINK
flowers.
Combinations of alleles
• FF and ff are Homozygous for the
genes and traits
– FF= homozygous dominant; phenotype is
DOMINANT trait
– ff = homozygous recessive; phenotype is
RECESSIVE trait
Combinations of alleles
• Ff is Heterozygous for the genes
(genotype).
– If F is dominant, a person who is Ff will look
the SAME as someone who is FF (different
genotype, same phenotype!)
How do we PREDICT which traits an
offspring will inherit?
• A Punnett square!!!
• Put MOM’s 2 genes (alleles) on the top;
Dad’s 2 genes (alleles) on the side
• Match up the possibilities in every square.
• For 1 trait, there are 4 possible offspring!
Example:
• F= dominant= fuzzy seed &
• f= recessive= smooth seed
• What are the genotype and phenotype for
homozygous dominant?
• What are the genotype and phenotype for
homozygous recessive?
• What are the genotype and phenotype for
heterozygous?
• Cross Ff x Ff (Punnett Square)- what are the
POSSIBLE genotypes and phenotypes of
offspring?
Answers:
• Homozygous dominant= FF= fuzzy
• Homozygous recessive= ff= smooth
• HETEROZYGOUS= Ff= fuzzy
• Have you ever noticed that certain traits run in
families? Like musical ability, athletic,
mechanical ability, more intellectual, etc.
• Our genes determine a LOT of who we are (but
NOT everything)
– What we are good at- sports, music, schoolwork
– Our height, eye color, hair color
– personality traits
Do Chromosomes ever change?
• YES! Changes can occur in the chromosome (a BIG
change- deletion, repeat, extra copy) or in the
sequence of the DNA (LITTLE change- switch the
nucleotide base (letter); delete 1 or more bases;
insert extra 1 or more bases).
• Both these types of changes are called MUTATIONS.
What MAY happen when there is a
mutation in the DNA or
chromosome?
• The offspring may develop a GENETIC
DISEASE.
• How is a GENETIC DISEASE (like cystic
fibrosis) different from an INFECTIOUS
DISEASE (strep throat)?