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 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