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.

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

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.

• 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 Summary of Meiosis

• Type of cell division used to form the gametesgametes (egg & sperm) where chromosome number is (egg & sperm) where chromosome number is reduced to haploid (n).reduced to haploid (n).

• It involves 2 rounds of cell division,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 #1BLUE EYES

BROWN HAIR

TALL

BLUE EYES

BLOND HAIR

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)?