Mitosis & Meiosis

Mitosis & Meiosis

What’s the difference?

Write down these terms:• Mitosis• Meiosis• Diploid• Haploid• Sex chromosomes

in a Male?• Sex chromosomes

in a Female?• Number of

chromosomes in a person?

• 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.

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!

2 daughter cells identical to original

Parent cell

Chromosomes are copied and double in number (sister chromatids are attached at by the centromere

Chromosomes now split

• 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.

• 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 + 23 = 46n n 2n

Haploid + haploid = 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

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.

What to look for in a Karyotype?

• When analyzing a human karyotype, scientists first look for these main features:

1. Are there 46 chromosomes?

2. Are there 2 identical pairs of each autosome and 2 sex chromosomes?

3. Are there any rearrangements between chromosomes or large deletions?

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

• THESE CHROMOSOME PAIRS (the 2 copies of the SAME chromosome) are ALLELES

Chromosome & Gene Pairs (AKA Alleles!!)

CHROMOSOME PAIR #1BLUE EYES

BROWN HAIR

TALL

BLUE EYES

BLOND HAIR

SHORT

= A GENE 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

• GRAY FUR IS DOMINANT OVER BLUE FUR

• 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!)

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 Sqare)

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)?

How do we track who has a trait in a family- a Pedigree!

• A chart that shows how a trait and the genes that control it are inherited within a family.

• A ‘family tree’ for a genetic trait or disease.

Pedigree symbols

Cystic Fibrosis Pedigree(recessive)

Cystic Fibrosis Pedigree

What if a trait is carried on the X chromosome?

• It is called a sex-linked trait.• Will it be inherited the SAME as trait on an

autosomal (numbered) chromosome?• NO- because the Male only has 1 copy,

and he gets it ONLY from MOM!!!!

Sex-linked recessive allele(haemophilia, red-green color blind)

Sex-linked recessive, cont.

Sex-linked, cont.

Pedigree

• Summary: Pedigrees help determine the inheritance of some alleles and predict them in offspring.

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!

L3 Quick Quiz:1. Most of the body’s cells are

___________ (diploid or haploid)?2. The gametes (sperm or egg) are

_______ (diploid or haploid)?3. How many TOTAL chromosomes do

people have?4. How many DIFFERENT PAIRS of

chromosomes do we have?

5. What is the result of Mitosis? 6. What is the result of meiosis?7. List 2 differences between mitosis

and meiosis?8. Where do the 2 different copies of

each chromosome come from in each of us?

9. Which are the sex chromosomes?10.Which sex chromosomes are found in

males? In females?

11.Ff = ? (homozygous OR heterozygous)

12. Ff = ? (genotype OR phenotype)13. Fuzzy seed= (genotype OR

phenotype)

14. Complete a Punnett square for height: T= tall t= short Mom= tt & Dad= Tt

15. A.List the ratios & possible genotypes for their child?

B. Phenotypes? 16.Complete a second Punnett

square where both parents are Tt and Tt.

17. a. List the ratios & possible genotypes for their child?

b. Phenotypes?

Example of Gray & Blue Bunnies

• Fur color (like MOST of our traits!) is determined by genes

•Gray is DOMINANT•Blue is RECESSIVE

F= Gene for bunny fur color

•One gene from Mom.•One gene from Dad.

Gray bunny- DOMINANT

Blue bunny- Recessive

Genotypes: combinations of alleles that cause trait

X

FF x FF FF

X

ff x ff ff

X ?

FF X ff

Ff

Heterozygous

Ff x Ff

F f F f

meiosis

Punnett Square

F

f

F f

F F fF

F f ff

Punnett Square

F

f

F f

F F fF

F f ff

Phenotypic ratio

F

f

F f

F F fF

F f ff

3:1

Genotypic ratio

F

f

F f

F F fF

F f ff

1:2:1