DNA, CHROMOSOMES,

MITOSIS, AND MEIOSIS

DNA DNA stands for

“Deoxyribonucleic acid”

DNA is the genetic material of an organism; it is passed down from parents to offspring

DNA’s shape is called a “double-helix” (like a twisted ladder)

DNA DNA strands are made up

of a few different substances: Deoxyribose Sugar, Phosphate, and Nitrogen Base

The “sides” of DNA are made up of Sugar and Phosphate; the “rungs” of DNA are made up of Nitrogen Bases (4) AdenineThymine CytosineGunaine

DNA AND GENES DNA strands make up something called

“genes”

Genes are found on “chromosomes” and they call for a person to have a specific trait (like brown hair or blonde hair etc.)

A chromosome is a doubled rod of chromatin; chromosomes are inside the cell’s nucleus

A chromosome is made up of 2 identical rods called “chromatids”

CHROMOSOMES 2 chromatids are held together by a

“centromere”

CHROMOSOMES Humans have 23 pairs of chromosomes, 46 in all 22 pairs of chromosomes determine an

individual’s traits; 1 pair of chromosomes determines if a person is male or female

Females have 2 “X” chromosomes; Males have an “X” and a “Y” chromosome

Chromosome #23 determines the gender of a human

MALE

FEMALE

HOW DO YOU GROW? All the cells in your body come from a

single fertilized egg; your body grows through a process of continuous cell divisions called “Mitosis”

Each cell in your body goes through the Cell Cycle which has 3 stages : Interphase, Mitosis, and Cytokinesis

During Mitosis, a body cell duplicates the chromosomes and then divides into 2 identical “daughter cells” each with 1 complete set of chromosomes

INTERPHASE Interphase is the period before cell

division occurs

During interphase, the cell grows to its mature size, makes a copy of its DNA (DNA REPLICATION), and prepares to divide into 2 cells

MITOSIS Mitosis is the stage in which the nucleus

divides into 2 new nuclei During Mitosis, one copy of the DNA is

distributed into each of the 2 daughter cells

Mitosis occurs in “somatic” cells (body cells)

Interphase

CYTOKINESIS The cytoplasm divides and organelles

are distributed into each of the 2 identical daughter cells

Same number of chromosomes as parent cell (23 pairs=46 chromosomes

Interphase

MEIOSIS Process where the # of chromosomes is

reduced by half to produce sex cells a.k.a. “gametes” (Sperm and Eggs)

Meiosis occurs in sex cells As in Mitosis, the chromosomes are

copied in Meiosis and the parent cell divides into 2 “daughter cells” with the same # of chromosomes as the parent

Then, those 2 cells divide again and form 4 “daughter cells” with HALF the # of chromosomes as the original parent cell

MEIOSIS 1 parent cell with

4 chromosomes produces 4 daughter cells with 2 chromosomes each

Chromosome # reduced in HALF!!

MEIOSIS So… Humans have 46 chromosomes but

how many chromosomes do their sex cells have???

A sperm has 23 chromosomes from Dad An egg has 23 chromosomes from Mom Therefore, how many chromosomes does a

zygote have? (a zygote is the initial cell formed when a sperm meets an egg)

The 23 chromosomes from the sperm combine with 23 chromosomes from the egg to create a new human with 46 chromosomes46 46

2323

46

DAD DETERMINES… SORT OF

When a zygote is made it receives 23 chromosomes from each parent

Mom’s 23rd pair of chromosomes are “XX”

Dad’s 23rd pair of chromosomes are “XY”

So… Mom can only pass on an “X” chromosome to the zygote

But… Dad can pass on either an “X” or a “Y”

If Dad passes an “X” to the zygote, the baby will be a girl (XX)

If dad passes a “Y” to the zygote, the baby will be a boy (XY)

So… the 23rd chromosome determines an individual’s gender, the other 22 determine the physical traits of a person

GENES AND ALLELES Alleles are the different forms of a gene;

there are 2 alleles per gene

There are dominant alleles represented with a capital letter i.e. “B”

There are recessive alleles represented with a lower-case letter i.e. “b”

A dominant allele will “mask” a recessive allele meaning that the dominant allele will be physically visible on the person

ALLELES EXAMPLE: Brown eyes are dominant to blue eyes “B” is the allele for Brown eyes and “b”

is the allele for blue eyes Lil’ Archie gets a “B” allele from his dad

and a “b” allele from his mom. What color are his eyes????

GENOTYPE VS. PHENOTYPE In our example, Lil’ Archie had a “B”

allele and a “b” allele Lil’ Archie’s Genotype (combination of

alleles) is “Bb” Lil’ Archie’s Phenotype (physical

appearance) is brown eyes because “B” for brown is dominant over “b” for blue

HOMOZYGOUS VS. HETEROZYGOUS Lil’ Archie’s genotype is called

“heterozygous” because he has 2 DIFFERENT alleles (Bb) for the eye color trait

Some people’s genotype is called “homozygous dominant” because they have 2 of the SAME dominant alleles for a trait (BB)

Some people’s genotype is called “homozygous recessive” because they have 2 of the SAME recessive alleles for a trait (bb)

PUNNETT SQUARES A punnett square is a chart that shows

all of the possible combinations of alleles in a genetic cross

For example, Lil’ Archie (Bb) gets married to Lil’ Susie (BB) and we want to do a punnett square for their future children’s eye color

BB(brown Eyes)

Bb(brown Eyes)

BB(brown Eyes)

Bb(brown Eyes)

Lil’ Archie’s Genotype

Lil’ Susie’s genotype

B b

B

B