Download Lecture 2 DNA to Protein

Organization of DNA
DNA ð RNA ð PROTEIN
Housekeeping information
Tom Hartman
Figure 3–11
DNA as chemistry
Organization of DNA
Figure 3–11
DNA as data storage
DNA as a molecular model
Note:
A-T
C-G
A – adenine
T – thymine
C – cytosine
G – guanine
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DNA as heredity
Organization of DNA
• Double helix
– and associated proteins
• Nucleosomes:
– DNA coiled around histones
• Chromatin:
– loosely coiled DNA (cells not dividing)
• Chromosomes:
– tightly coiled DNA (cells dividing)
DNA and Genes
• DNA:
– instructions for every protein in the body
What is genetic code?
• Gene:
– DNA instructions for 1 protein
– DNA instructions for RNA (new)
Genetic Code
• The chemical language of DNA
instructions:
– sequence of bases (A, T, C, G)
– triplet code:
• 3 bases = 1 amino acid
A – adenine
T – thymine
C – cytosine
G – guanine
KEY CONCEPT
•
•
•
•
The nucleus contains chromosomes
Chromosomes contain DNA
DNA stores genetic instructions for proteins
Proteins determine cell structure and
function
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Protein Synthesis
• Transcription:
How do DNA instructions
become proteins?
Protein Synthesis
• Processing:
– copies instructions from DNA to mRNA (in
nucleus)
• Translation:
– ribosome reads code from mRNA (in
cytoplasm)
– assembles amino acids into polypeptide chain
mRNA Transcription
• A gene is transcribed to mRNA in 3 steps:
– by RER and Golgi apparatus produces protein
– gene activation
– DNA to mRNA
– RNA processing
• Physical chemical configurations code for
data: start ð copy ð stop
DNA polarity
• DNA is a double stranded molecule.
• Each strand is a polynucleotide and, at the chemical level,
the strands start with a free 5’-hydroxyl group and end with a
3’-hydroxyl.
• The strands run antiparallel 5’-3’ vs 3’-5’ with the
appropriate nucleotides pairing A-T, C-G.
• The two stranded, antiparallel, complementary DNA
molecule forms the double helix.
• One strand, the sense or coding strand, contains the
information (the genes).
• The other strand is the template or antisense strand.
Codons
• A codon is a three letter ‘word’.
• The start is defined by an initiation ‘word’.
• Each three letter ‘word’ is a reading frame
and if the frame shifts by one letter then the
result could be very different and may be
lethal!
• RNA uses uracil not thymine
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Codons
• 5’-3’ ATCGTACCG
• 3’-5’ TAGCATGGC
• 5’-3’ AUCGUACCG
Step 1: Gene Activation
sense strand
antisense strand
mRNA
• Uncoils DNA, removes histones
• Start (promoter) and stop codes on DNA
mark location of gene:
– coding strand is code for protein
– template strand used by RNA polymerase
molecule
Step 2: DNA to mRNA
Step 3: RNA Processing
• Enzyme RNA polymerase transcribes
DNA:
• At stop signal, mRNA detaches from DNA
molecule:
– binds to promoter (start) sequence
– reads DNA code for gene
– binds nucleotides to form messenger RNA
(mRNA)
– mRNA duplicates DNA coding strand, uracil
replaces thymine
–
–
–
–
code is edited (RNA processing)
unnecessary codes (introns) removed
good codes (exons) spliced together
triplet of 3 nucleotides (codon) represents one
amino acid
Codons
Anatomy of a gene
Stop
Start
Intron
•
•
•
•
•
Genes are located on the sense strand
mRNA is coded off the antisense strand
Gene has a start codon whichis often ATG (methionine elsewhere).
Eukarytoic genes have introns (edited off the mRNA in the nucleus
and spliced)
Genes have stop signals (termination)
– UAG (amber)
– UGA (opal/umber)
– UAA (ochre)
Table 3–2
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An amino acid wheel
Degeneracy of the genetic code
• There are 20 aas and a stop code.
• There are 4 bases and a need for 21 codes.
• Two letter codes would give 42=16 codes
whereas 43=64 codes.
• Thus there may be several ways to code for
an amino acid. (e.g. valine could be GUA,
GUU, GUC, GUG)
• This may protect against point mutations.
Sickle cell anaemia
Translation (1 of 6)
• One amino acid difference
• mRNA moves:
– Valine replaces Glutamic acid
– from the nucleus
– through a nuclear pore
• One nucleotide change
– GTA replaces GAA (GUA replaces GAA in mRNA)
• Huge change in protein morphology at low [O2] as
haemoglobin crystallizes causing RBC to change
shape.
• But it does give some measure of protection
against malaria.
Figure 3–13
Translation (2 of 6)
Translation (3 of 6)
• mRNA moves:
– to a ribosome in cytoplasm
– surrounded by amino acids
• mRNA binds to ribosomal
subunits
• tRNA delivers amino acids
to mRNA
Figure 3–13 (Step 1)
Figure 3–13 (Step 2)
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Translation (4 of 6)
Translation (5 of 6)
• tRNA anticodon binds to
mRNA codon
• 1 mRNA codon translates to
1 amino acid
• Enzymes join amino acids
with peptide bonds
• Polypeptide chain has
specific sequence of amino
acids
Figure 3–13 (Step 3)
Translation (6 of 6)
Figure 3–13 (Step 4)
Note the preponderance of RNA
• rRNA in the production and activity of
ribosomes.
• mRNA transcribing (editing and splicing)
genes.
• tRNA transporting and assembling
polypeptides.
• This has been taken as a hint of the primacy
of RNA in the history of life.
• At stop codon,
components separate
Figure 3–13 (Step 5)
Nucleus Controls Cell
Structure and Function
• Direct control through synthesis of:
– structural proteins
– secretions (environmental response)
• Indirect control over metabolism through
enzymes
KEY CONCEPT
• Genes:
– are functional units of DNA
– contain instructions for 1 or more proteins
– Occasionally just RNA
• Protein synthesis requires:
– several enzymes
– ribosomes
– 3 types of RNA
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KEY CONCEPT
• Mutation is a change in the nucleotide
sequence of a gene:
– can change gene function
• Causes:
– exposure to chemicals
– exposure to radiation
– mistakes during DNA replication
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