Transcript BioH From DNA to proteins

From DNA to proteins
BioH Chapter 13
The BIG picture –
Gametes to genes
DNA - RNA comparison
Characteristic
• Monomer
•Sugar
•Bases used
•Usual
location
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DNA
Nucleotide (P-S-B)
Deoxyribose
A,T,C,G
Nucleus only
•Function
• Carries/transfers
genetic info
•Structure
• Double strand
RNA
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Nucleotide (P-S-B)
Ribose
A,U,C,G
Nucleus AND
cytoplasm
• Carries/transfers
genetic info AND
PROTEIN SYNTHESIS
• Single strand
Transcription
The process of using a DNA template to make RNA
(Click for video)
Transcription details
• Enzymes used
 Helicase – unwind & start strand separation
 RNA polymerase – brings complementary
base-matching nucleotides
 Ligase – corrections and gap corrections
• Promoter sequence on mRNA - signals “start” for
transcribing DNA sequence into RNA sequence
• ONE strand only – forming juvenile RNA
• Uracil used instead of Thymine
• Use Cytosine, Guanine, Adenine (same as DNA)
Juvenile RNA
 Immature RNA formed by transcription in nucleus
(juvenile RNA, pre-mRNA)
 Forms mature mRNA at nuclear envelope through
use of specific enzymes
 Most eukaryotic genes contain base sequences
that must be removed before translation can occur
(introns). They will never be used to form proteins.
 The RNA genes that remain (exons) form specific
proteins that determine traits
Starting Translation
NEED solve three issues:
1. Instructions on what specific proteins to build
2. Capture and provide raw material (amino acids)
with which to build proteins
3. Place at which to build proteins
mRNA
tRNA
rRNA
Types of RNA
mRNA – messenger RNA –
carries protein building
instructions (very long)
tRNA – transfer RNA – picks
up amino acid components
and delivers them to a
ribosome to be assembled
into proteins (3 bases long)
rRNA – ribosomal RNA –
attracts proteins to form a
ribosome site for protein
synthesis (medium length)
Translation
Process of translating mRNA base sequence into proteins
Providing the
message - mRNA
• Contains the coded
instructions to make specific
proteins (based on the
nucleotides’ base sequence)
• 3 bases as a group (triplet)
are called a codon
• Use the Genetic Code
charts to decipher which
amino acids are coded by
each codon
Genetic Code charts
(video)
Use the mRNA codon (transcribed from DNA) to read the charts
64 codons code for only 20 different amino acid building blocks
Supplying raw material - tRNA
• tRNA attracts amino acids in
cytoplasm based “lock and key”
structure
• Matches the codon on the mRNA
with its own 3- base anti-codon
• Delivers specific amino acids to
functional ribosome to build
polypeptide chain (protein)
(Video)
Ribosome Formation - rRNA
 rRNA combines with other proteins to form the
basis of a functional ribosome
 Small subunit
 Large subunit
(Video)
Finally – TRANSLATION !
Three stages: Initiation, Elongation & Termination
 Initiation
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Mature mRNA leaves nucleus into cytoplasm through nuclear
pores
Many free amino acids, tRNA and ribosomal subunits floating
in cytoplasm
Initiator tRNA (attached to the amino acid methionine)
attaches to small ribosomal subunit, which then attaches to
end of mRNA
mRNA moves through ribosome until reaches “start” codon
(AUG) on mRNA
Large subunit attaches, forming
functional ribosome
Starts next phase = elongation
More translation
 Elongation
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The initiator tRNA anticodon-mRNA codon bond causes the
next bonding site in the ribosome to attract the specific
anticodon of another tRNA
A 2nd tRNA (with its amino acid) bonds with the 2nd m RNA
codon
Once this happens, enzymes help form a peptide bond between
the two nearby amino acids
While other enzymes break the initiator amino acid bond and
the first codon-anticodon bond, releasing the intiator tRNA and
leaving Methionine to form a peptide bond with the second
amino acid
The 2nd tRNA moves into the 1st ribosomal bonding site
A 3rd tRNA is attracted to the open
ribosomal bonding site, allowing the
process to continue
This process continues until a “STOP”
codon is read, initiating the last phase
= termination
Translation is terminated
Termination
•The mRNA codon UAA, UAG or AGA (“STOP”) occupies the
second ribosomal bonding site
•No tRNA anticodon bonds with these codons
•This signals the synthesis process to stop
•The polypeptide chain (protein) is released from the ribosome
•The two ribosomal subunits separate
We have
PROTEINS!
NEXT?
• Free-floating proteins
• Golgi apparatus
packaging
• Enter ER for transport
Problems – Gene Mutations
Point Mutations – single base-pair change
Substitution – one base substituted for the correct one
May result in only one amino acid mistake – protein may still
function correctly
Frameshift Mutations
 Insertion – extra base inserted
Deletion – base is omitted
Results in all codons after mutation
to be incorrect and may cause protein
to malfunction
Problems – Gene Mutations
Point mutations – single base changes
Deletion – base omitted
Insertion – extra base inserted
Substitution – one base substituted for the correct one
Transposition – DNA segments transposed
(exchanged) with another
Mutation causes
Spontaneous
Exposure to mutagens (UV, gamma & X-rays)
Natural & synthetic chemicals
Significance and causes of gene mutations
Importance
Many, if not most, mutations are neutral – causing little or no effect
on protein function
Can be harmful, causing genetic disorders – cystic fibrosis, sickle
cell disease, cancers, HIV tolerance
Can be beneficial, evolutionary changes have come about due to
positive mutations that allow organisms to better survive their
environment
Causes
Spontaneous
Exposure to mutagens
(UV, gamma & X-rays)
Natural & synthetic carcinogenic chemicals
(video)
The whole process