Fourth Six Weeks
DNA – Deoxyribose Nucleic Acid – our genetic information molecule
 Code or “Instructions” for making proteins
 Proteins do the work of the cell and also are structural (what you look like
or “inherit”)
 DNA controls the cells activities; tells the cell what to do by making
specific proteins that do the work! Get it? 
A NUCLEOTIDE
Structure of DNA – it is a polymer or “chain” of nucleotides
A nucleotide contains 3 parts: a phosphate, deoxyribose sugar,
and one of the 4 bases Adenine (A), Thymine (T), Guanine (G),
and Cytosine
A pairs with T; C pairs with G
DNA is TWO CHAINS OF
NUCLEOTIDES twisted like a
ladder.
We call it a DOUBLE HELIX
DOUBLE = 2 CHAINS or STRANDS
HELIX = SPIRAL OR TWISTED
DNA Replication – Making copies of the DNA.
Takes place in the NUCLEUS during the “S” PHASE of the cell cycle.
“S” for synthesis or “to make or copy”
DNA REPLICATION
1st – DNA is “unzipped” and the two strands are separated.
IN THE NUCLEUS
2nd – DNA polymerase (a protein) adds the complimentary
bases to both of the unzipped strands. (The unzipped strands act as a template.)
3rd – The original DNA winds back up and the new DNA is completed!
Fourth Six Weeks
PROTEIN SYNTHESIS – OR IN OTHER WORDS…MAKING PROTEINS!!!
Two Steps:
Step 1: DNA is used to make messenger RNA (occurs in the nucleus)
Step 2: messenger RNA is used to make proteins (occurs in the cytoplasm)
Step 1:
DNA
Double stranded
Bases: T, A, G, C
Deoxyribose sugar
RNA
Single stranded
Bases: U, A, G, C
Ribose Sugar
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In the nucleus, DNA is used as a template to make
single stranded RNA. However, “T” is replaced with
“U” which stands for Uracil. RNA has no “T” bases.
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RNA leaves the nucleus thru a nuclear pore and enters the cytoplasm. This is why it is called MESSENGER
because it carries a message out of the nucleus!
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If we transcribe the DNA code into mRNA (messenger RNA) code, it would look like this:
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Ribosomal RNA (rRNA) is floating around in the cytoplasm (free floating ribosomes… think back to the parts
of a cell!)
rRNA attaches to mRNA and reads it 3 letters at a time. “R” for ribosomal, “R” for Read.
Step 2:
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3 letters = a codon = an amino acid
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Amino Acids are the “building blocks” to proteins (think back to macromolecules!) There are 20 essential
amino acids. Each one has a specific 3 letter code.
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Every time rRNA reads 3 letters, an amino acid is assigned.
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The molecule that brings the amino acids is called Transfer RNA (tRNA). tRNA brings the amino acids like a
truck. “T” for transfer, “T” for truck.
Type of RNA
mRNA (messenger RNA)
LOCATION
Nucleus
NICKNAME
“messenger”
rRNA (ribosomal RNA)
tRNA (transfer RNA)
Cytoplasm
Cytoplasm
“Reader”
“Truck”
JOB
Disposable copy of DNA – carries the
message of the type of protein to be made
Reads mRNA 3 letters at a time
Brings the amino acids to the rRNA
Peptide Bonds – bonds between amino acids – links amino acids together
In the cytoplasm…
Fourth Six Weeks
Genes – locations on a chromosome that codes for a particular
trait (eye color, hair color, height, etc.)
Alleles – variations of a gene (blue eyes, brown eyes, hazel eyes, etc.)
Every person has two sets of chromosomes: 23 from mother, 23 from father
Number 1 from mother MATCHES number 1 from father.
They have the same genes located on the same spots on the chromosomes.
Matching chromosomes are called HOMOLOGOUS CHROMOSOMES.
Homologous
Chromosomes
Alleles can be dominant or recessive:
A dominant allele is the one that will show. It gets a capital letter.
A recessive allele is the one that is hidden. It gets a lower case letter.
For example… Brown eyes – dominant – “B”
Blue eyes – recessive – “b”
A genotype is your allele or letter combination. You can only have TWO letters for each gene since you only inherit
two…one from mom and one from dad.
For example, the person above has the genotype “Bb”. The dominant letter comes first, the recessive letter second.
You can have three possible genotypes. If you have one or both dominant alleles, you will show that trait. If you have
both recessive alleles, you will show that trait. For example:
“BB” – Brown eyes
“Bb” – Brown eyes
“bb” – blue eyes
The PHENOTYPE is the appearance. For example, brown eyes, blue eyes, green eyes.
The GENOTPYE is the allele combination. BB, Bb, or bb
If you have two capital or two lower case letters (identical alleles) you are called HOMOZYGOUS. (BB or bb)
If you have one capital and one lower case letter you are called HETEROZYGOUS (Bb)
Homozygous genotypes are called PUREBRED. You can be Purebred Dominant (DD) or Purebred Recessive (bb)
Heterozygous genotypes are called HYBRID.
If you wanted to know what your offspring would look like or what genes they would inherit,
you could do a PUNNET SQUARE = SHOWS THE PROBABILITY OF OFFSPRING
The parents genes are placed along the top and the side of the square as shown.
However, you have to separate the genes. This is because you can only give half of your genes
Since the other parent gives half.
For example, “G” is dominant for green eyes. “g” is recessive for blue eyes. Below is a cross
between two parents that are heterozygous for eye color. They have green eyes.
Notice that the parent’s genes are separated because you can only give one allele. The
other parent gives the other allele so the offspring has two just like the parents.
Inside the Punnett Square are the possible offspring.
4 boxes total, each box represents 25% chance.
In this cross, your child could have a:
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25% chance of being homozygous dominant (GG) with green eyes
50% chance of being heterozygous (Gg) with green eyes
25% chance of being homozygous recessive (gg) with blue eyes
Fourth Six Weeks
A TEST CROSS is a cross between a homozygous dominant and a homozygous recessive parent. It is called a TEST
because 100% of the offspring are heterozygous and the trait that shows tells you which is dominant. For example:
 Purebred tall person is crossed with a purebred short person.
 100% of the offspring are tall.
 The dominant trait is then tall. The recessive trait is short.
 “T” = tall allele; “t” = short allele
In a test cross:
Parents = are called “P” for parent generation
Offspring = are called the F1 generation (100% heterozygous)
If we cross two offspring from the F1 generation we get the F2 generation:
The F2 generation
always produces a
3:1 ratio where 3 are
dominant and 1 is recessive.
It also produces a 1:2:1 ratio where:
o 1 Homozygous Dominant (TT)
o 2 Heterozygous (Tt)
o 1 Homozygous recessive (tt)
RULE #1 – Cross a homozygous dominant with a homozygous recessive = 100% heterozygous
RULE #2 – Cross two heterozygous parents = 3:1 phenotype and 1:2:1 genotype
A MONOHYBRID CROSS is a cross involving ONE trait as in the examples above.
What about TWO traits? For example…height and eye color?
What are the chances of having a brown eyed, tall child?
A DIHYBRID CROSS is a cross involving TWO TRAITS.
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Dihybrid Cross-a cross that shows the probable outcome of a genetic cross with two traits.
Law of Independent Assortment-that alleles for traits assort independently of each other.
Sex-linked trait-a trait that can be located on the sex chromosomes. EX. Colorblindness
Types of Inheritance
o Incomplete (red and white flowers make pink) A blending of the two phenotypes (the heterozygous
condition).
o Codominance (roan color in cows)-equal sharing of each phenotype. The heterozygous condition.
o Multiple Alleles (blood type) A and B are “dominant” and O is recessive. Controlled by multiple different
alleles. There are several different blood types: AA, AO, AB, BB, BO, and OO
o Sex-linked trait-a trait that can be located on the
sex chromosomes. EX. Colorblindness
Pedigree-shows the relationships within a family. Can be
used to track human heredity and the inheritance of genetic
diseases.
Gene-sequence of DNA that codes for a protein.
Mutation-Mistakes in the genetic code.
o Point Mutations-involve one point in the DNA
sequence
 Substitutions-changing out one nucleotide
for another.
 Deletions-the erasing of one nucleotide
Fourth Six Weeks
 Insertion-Adding in an extra nucleotide to the sequence.
Chromosomal Mutation
 Involve changes in the number or structure of a chromosome
 Deletion-delete part of the chromosome
 Duplication-part of the chromosome is copied twice
 Inverstion-part of the chromosome is placed in the wrong spot
 Translocation-the swapping of information in-between two chromosomes.
Somatic Mutation-mutations in pairs one 1-22. (everything but the sex chromosomes)
Germ Mutation-mutations in the sex chromosomes
Recombinant DNA-The joining of synthetic sequences to the original DNA. Can be used to insert trackers into the
DNA. Used extensively with bacteria.
Plasmid-small circular DNA found in bacteria that are useful in DNA transfer.
Cloning-producing animals that are genetically identical to the original members of the population.
Fused Cell-Denucleate one cell and fuse it with the nucleus of a fertilized cell. This is the best method to create a
cloned animal.
Stem Cell-a cell that is undifferentiated and can be utilized for any number of purposes. They are utilized for
research in several different fields.
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