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Recipe/chef dna to protein
Unit 5A
Gene Expression
Review
• What is a gene?
– A segment of DNA that codes for a specific
protein
• What do you think it means to say
“express a gene”?
– Gene expression- The process by which
information in a gene is used to create the
protein that it codes for.
How does the cell “decode” the
sequence of DNA bases of a gene?
• In general:
DNA
The master
copy of
instructions
RNA
Protein
The
temporary
copy of the
DNA
The
product of
the DNA’s
instructions
Trait
Characteristic
of a
cell/organism
RNA
• A nucleic acid
• Consists of one long chain of nucleotides
• There are multiple types of RNA, each
with a different function
• One type of RNA can be used as a
temporary copy of DNA in order to make
protein
• The DNA is precious and stays safely in
the nucleus
DNA vs. RNA
• RNA is mostly single stranded
• RNA is made of nucleotides
that have a phosphate, ribose
sugar and a base
• Bases in RNA are A, U, C, G
– U = uracil
– What’s missing?
Compare and Contrast DNA and
RNA Structure
DNA
RNA
RNA is used in Protein Synthesis
Amino acids
• Proteins are made of __________.
• RNA molecules direct the assembly of
Polypeptides
amino acids into _____________.
• Which then fold into proteins.
Types of RNA
• Messenger RNA (mRNA)- a copy of the
instructions for making protein from the
DNA
Types of RNA
• Ribosomal RNA (rRNA)- RNA molecules
that bind with proteins to make the
ribosomes (where proteins are made)
Types of RNA
• Transfer RNA (tRNA)- transfers the correct
amino acid to the ribosome to make
proteins based on the mRNA sequence
The “Central Dogma” of Molecular
Biology
DNA
The master
copy of
instructions
RNA
The
temporary
copy of the
DNA
Protein
The
product of
the DNA’s
instructions
• First DNA is transcribed into mRNA
• Then, the mRNA is sent out of the nucleus
to a ribosome
• At the ribosome, tRNAs bring together
amino acids to translate the message in
the mRNA into protein.
DNA to RNA to Protein
2 processes are required for gene expression
1.Transcription- the process that uses base
pairing to make mRNA based on the
sequence of DNA
– Occurs in the nucleus
2. Translation decodes the mRNA message into
protein
– Occurs at the ribosome (in the cytoplasm)
Draw in diagram from DNA -> Secreted Protein
Making RNA
1. Transcription- DNA serves as the
template for making complementary RNA
molecules
• RNA is made in the nucleus, then moves
to the cytoplasm for protein synthesis
• RNA polymerase separates the DNA
strands and assembles RNA by matching
up base pairs
Making RNA
What pairs
with
adenine in
the DNA
when
making
the new
RNA
molecule?
DNA to mRNA practice
(transcription)
• Transcribe the following DNA sequence (gene)
• TACAATGGCGCTAGT
• The DNA is ultimately read three letters at a time
or we say in “triplets”
DNA Triplet code
Making a Polypeptide
2. Translation decodes the
mRNA message into
protein
– Occurs at the ribosomes
– Remember: the ribosome is
made of two subunits of
rRNA and proteins
The Genetic Code:
• The language spelled out by the sequence
of bases on the mRNA (A, U, C, G)
• Read 3 letters at a time
• Each 3 letter “word” is called a codon
• Each codon corresponds to 1 amino acid
DNA Triplet code
Transcription
mRNA codons
Translation
Amino acids
U- U- A - C - A - G - C - C - A
How to Read Codons
• There are 64 possible
codons
• The genetic code table
1. Start in the middle of the
circle with the first letter of
the mRNA codon
2. Move to the second ring to
find the second letter of the
codon
3. Find the third letter of the
codon in the smallest set of
letters
4. Read the amino acid in
that sector
Amino Acids
• There are 20 amino
acids commonly
found in proteins
• A string of amino
acids is a
polypeptide
• A completed and
folded polypeptide
is a protein
3 letter
abbreviation
1 letter
abbreviation
Start and Stop Codons
• Genetic punctuation marks
• AUG is the start codon
– codes for the amino acid methionine
• The mRNA is read starting at AUG
• After the mRNA is fully read, a stop codon
ends the production of the protein
• Three stop codons: UAA, UAG, UGA
– No amino acids are coded
mRNA to Protein Practice
• AUGUUACCGCGAUCA
How does the cell read the
codons?
1.
2.
3.
4.
The ribosome attaches to a mRNA molecule that has
entered the cytoplasm
As each codon on the mRNA passes through the
ribosome, tRNAs bring the proper amino acids into the
ribosome
The ribosome attaches these amino acids to each
other with peptide bonds creating a polypeptide
This occurs until the stop codon enters the ribosome
-at that point, the polypeptide is complete and is released from
the ribosome
5.
After the polypeptide is released, it finishes folding up
properly into a fully functioning protein
Video
Video
How does tRNA know which amino
acids to bring?
• Each tRNA carries
only 1 kind of amino
acid
• Each tRNA has 3
unpaired bases that
match up with the
codons on the mRNA
– These are called
anticodons
anticodon
The Genetic Code is Nearly
Universal!!
• Almost all organisms have the same three
letter codes in DNA leading to the same
amino acids brought in to make the
proteins.
• From bacteria to humans, the triplet code
is the same.
DNA (gene):
TGA CGA TTT CTC ACT ACA CGC GCC CTT
Transcription↓
mRNA
Translation↓
Polypeptide
An different way to model
protein synthesis
Stanford’s take on Protein Synthesis
Mutations
Mutations
• Mistakes in copied DNA that cause
variations in genetic material
• These mistakes are passed down to
offspring if they occur in sex cells
Overactive feather blocking
gene creates a bald neck
Mistake in the “bithorax” gene
causes 2 sets of wings
2 Basic Categories
1.Gene mutations- changes in a single gene
2.Chromosomal mutations- changes in a
whole chromosome
1. Gene Mutations
• Are point mutations- changes at a single
point in the sequence
• Occur during replication
• Are passed down to daughter cells
• 3 types:
– Substitutions
– Insertions
– Deletions
Substitutions
• One base is changed to a
different base
• Affects a single amino
acid
• Sometimes have no effect
at all
– How could this happen?
– If the substitution creates a
codon that leads to the
same amino acid as before
Insertions
• One extra base is inserted into the
DNA sequence
• Every codon after the insertion will
change!
• Considered a “frameshift” mutation
because they shift the reading
frame of the genetic message
• The amino acids chosen will
change
• Will the resulting protein work?
– Very unlikely!
Deletions
• One base is deleted
from the DNA
sequence
• Every codon after will
change
• Also a “frameshift”
mutation
• Changes the amino
acid sequence and
resulting protein
Analogy
Read the following sentence (remember, it’s read in triplets):
The fat cat ate the rat.
Insertion:
• Add a b in front of the f in fat. Shift everything to the right.
• _____ _____ _____ _____ _____ _____
Deletion:
• Delete the f in fat. Shift everything to the left.
• _____ _____ _____ _____ _____ _____
Substitution:
• Substitute the letter b for the f in the word fat and read the sentence
again.
• _____ _____ _____ _____ _____ _____
• Which change does the least damage to the sentence
structure?____________________
• What does this tell you about the mutation types?
2. Chromosomal Mutations
• Changes in the number of chromosomes
or the structure of chromosomes
• Can change the location of genes on a
chromosomes
• Can change the number of copies of a
gene
4 Types of Chromosomal Mutations
• Deletion- loss of part of
chromosome
• Duplication- extra copy
made of part of
chromosome
• Inversion- reverses direction
of part of chromosome
• Translocation- part of one
attaches to another
chromosome
Causes of Mutations
• Mistakes in replication
• Mutagens- chemical or physical
environmental agents
– Pesticides
– Tobacco smoke
– X-rays
– UV rays
Mutations are Harmful and Helpful
• Harmful- if the
change results in a
nonfunctional
protein
• Ex: a point mutation
in the hemoglobin
gene results in
sickle cell anemia
Sickle Cell Anemia
• A nucleotide substitution causes the
hemoglobin protein to change shape and
no longer carry oxygen efficiently
• This causes the rigid, sickle shape in the
red blood cells
• These cells get caught in capillaries and
cause pain and other health problems
Sickle Cell Anemia
Mutations are Harmful and Helpful
• Helpful- produces a better
functioning protein for a
certain environment
• Mutations can lead to
evolution
• Example: There is a human
mutation that makes people
have increased resistance to
HIV
Cellular Specialization
• When replication takes place, is all of the DNA in
a cell copied?
• Is the DNA in different cells of your body exactly
the same?
• If the DNA in every cell is the same, how do cells
become different from each other?
• The light switch analogy:
– Cells “turn on,” or transcribe and translate only
those genes that code for the proteins they need.
Other genes are “turned off” because the proteins
they code for are not needed by the cell.
Cellular Specialization Example
The following is a short sequence of DNA. Four different imaginary genes
are shown. The protein for each gene is listed under the code.
Given the following cells, list the genes above that would be “switched on” or
expressed (active) in the cell and list those that would be “switched off” or
not expressed (inactive)?
WBC
Skin Cell
Muscle Cell
Nerve Cell
ON__________
ON___________
ON___________
ON_____________
OFF_________
OFF___________
OFF___________
OFF_________
Coding vs. Non-coding DNA
• Coding DNA: areas of DNA that code for
genes
proteins, also known as ___________.
– ~2% of the genome
• Non-coding DNA: stretches of DNA that do not
code for protein
– What does the other ~98% of the DNA do then?
– Some areas regulate/control the genes, other areas
are genes that no longer function (i.e. the genes for a
tail), etc.
– The function of much of the non-coding DNA is still
unknown
Is Complexity Dictated by the NonCoding DNA?