Gene Expression
How do genotypes become phenotypes?
• Chromosome:
– Structure found in the nucleus of all* cells
– Made of DNA coiled around histone proteins, compacted
• DNA:
– Double helix shape
– Stores genetic “gene” information
– Self-replicates to produce 2 exact copies of itself
• Gene:
– A segment of DNA that serves as a code for a specific phenotype
Gene for Type A Blood
Genotype
Phenotype
inherit
Propose how this occurs. What other factors might be
included “within” the blue arrow? How do you go from
alleles (A, a) to an actual phenotype that is noticeable?
DeoxyriboNucleic Acid
•
•
•
•
Antiparallel strands (gene is 5’ 3’)
Monomers = nucleotides
Adenine, Thymine, Cytosine, Guanine
1 nucleotide = 3 parts
–
–
–
Deoxyribose sugar
Phosphate
Base
• Uses complimentary base pairing rules
A  T and C  G
5’ CGTGGTTAAATCT 3’
The Central Dogma
of Biology
DNA
RNA
Protein
The Central Dogma
DNA
Transcription
RNA
Translation
Protein
DNA Replication
• DNA is copied accurately to preserve an
organism’s genotype
• Occurs before cell division so each
chromosome has 2 copies to give
– meiosis (cell division for gametes)
– mitosis (cell division for growth)
• Takes place in the nucleus
• Enzymes used in process:
– Helicase - Unwinds double helix
– DNA Polymerase - Creates new strand
DNA Replication
DNA Replication
“Gene”
5’  3’
5’ CGTGGTTAAATCT
3’ GCACCAATTTAGA
“Template
Strand”
3’  5’
Let’s model DNA Replication!
What’s next?
After DNA Replication, there is enough DNA make 2
new cells… and then again, and again until the
organism stops performing cell division (i.e., never,
really). Once a new cell is made, it can begin to use
the DNA to create phenotypes.
We call this next part Gene Expression, or the
production of a phenotype given information from the
genotype (“gene” = segments of DNA) and it can be
divided into 2 steps: Transcription and Translation.
Transcription
• Produces a RiboNucleic Acid (single) strand using a
DNA template
• DNA is “read” from 3’  5’ (“template” strand)
• Transcription enzyme = RNA Polymerase
• Takes place in the nucleus
• In gene expression, mRNA is made
• Base pair rules:
CG
GC
TA
AU
Let’s Transcribe!
Gene
Template
mRNA
Gene
Template
mRNA
5’ TCATGCAGAGGAGATTAA 3’
3’ AGTACGTCTCCTCTAATT 5’
5’
3’
5’ GATCAGGGAGACTTAGCA 3’
3’ CTAGTCCCTCTGAATCGT 5’
5’
3’
Translation
• Produces an amino acid sequence using mRNA as a
template
• mRNA is “read” from 5’  3’ in triplets (“codons”)
• Codons code for amino acids
• Start codon is AUG but stop codon varies
• Enzyme* that does this is called tRNA
• Takes place in cytoplasm at a ribosome
• After folding of polypeptide, a protein is formed!
The Genetic Code
Examples:
AUG is Met (start)
ACC is Thr
AGA is Arg!
UAA is Stop
UAG is Stop
UGA is Stop
Use the Genetic Code
to figure out what
amino acids are
assembled based on
mRNA strand
Alanine
Threonine
Glutamate
Leucine
Arginine
Serine
Stop!
Translation
Gene
Template
mRNA
Amino Acids
5’ GATCATGCGTCTCCTCTAATT 3’
Translation
Gene
Template
mRNA
Amino Acids
5’
3’
3’ CTCATACGGACAGTATTTTAC 5’
Once the amino acid sequence is complete, it folds
into a 3D “glob” = a protein! Proteins do all of our
cellular work, so they create phenotypes.
Discuss with a
neighbor:
How do you
get from
genotype to
phenotype?
Your Task
• Gene Expression Practice Worksheet
• Compare to key when finished to see if
you have the skills!
Homework due Friday
• Snorks
Discuss with your neighbors:
1.
2.
3.
4.
5.
Where does DNA Replication take place?
Where does Transcription take place?
What does Transcription produce?
Where does Translation take place?
What does Translation produce?
6. The Genetic Code is used to figure out what amino
acids are assembled based on the
strand
created from the
strand
What is the deal with 5’ and 3’?