6. protein synthesis (without...
TRANSCRIPT
Cell Biology
Protein Synthesis
Protein Synthesis 2 Stages: • Transcription
• Synthesis of mRNA from a DNA template • DNA segment is transcribed (“copied”) into a complementary
strand of mRNA • Why? Because DNA cannot leave the nucleus
• Translation • Ribosomes synthesize proteins using mature mRNA
transcript produced during transcription
Transcription (simplified…) 1. Initiation: DNA partially unzips
2. Elongation: Complementary base pairing of
mRNA nucleotides (H-bonds)
3. Termination: forms 1 mRNA strand and DNA heals
Initiation 1. RNA polymerase binds to promoter site
• Promoter identifies the start of a gene, which strand to be copies and the direction that it is to be copied
• DNA unwinds and unzips a section of DNA
2. RNA polymerase initiates mRNA synthesis on the template strand
Elongation 1. RNA polymerase pairs free RNA nucleotides to
exposed bases of template strand 2. Complementary Base are assembled
• Uracil replaces thymine • Free mRNA nucleotides are taken from within the
nucleoplasm
Termination 1. RNA polymerase reaches stop codon
2. DNA zips back • As polymerase passes by it heals the DNA strand
3. Newly synthesized mRNA separates from template DNA
mRNA Processing Before mRNA can leave the NUCLEUS, mRNA must be processed into a mature molecule:
• Cap added to 5’ and a poly-A-tail (150-200 Adenines) added to the
3’ end of the molecule • Introns (sections of DNA that do NOT contain a genetic message)
must be removed • Exons (the genes; sections of DNA) are spliced together
The Nucleus: the export of mRNA transcript • DNA is located in nucleus
• Transcription and mRNA processing occurs in the NUCLEUS
• Nucleus is surrounded by a DOUBLE MEMBRANE
• After mature mRNA transcript is produced, it moves out of the nucleus and into the cytoplasm through pores in the nuclear membrane
Next up: Translation…
Translation (simplified…)
1. Initiation • mRNA arrives at the ribosome
2. Elongation • tRNA brings a.a. to be assembled in sequence
3. Termination • Stop codon is reached and polypeptide (a chain of a.a.
joined by peptide bonds) is released
Vocabulary….
1. Codon • Three-nucleotide (bases) unit • Different codons code for different A.A.’s
• E.g (You need to use the table)
codon Amino acid
GGG Glycine
CCC Proline
AUG Methionine
Vocabulary…. 2. Anticodon
• Portion of a tRNA • 3 bases long • Complementary to a specific codon
Codon (mRNA)
Anticodon (tRNA)
Amino acid
CCC
GGG
GGG
Proline
CGA
GCU
Arginine
UCA
Initiation 1. mRNA arrives and attaches to small ribosomal subunit
at the 5’ region
2. Large ribosomal subunit arrives next, clamping over the start codon AUG • Large subunit has 2 binding sites P and A • First codon is aligned at the P binding site
3. First tRNA molecules carrying the amino acid methionine attaches to the AUG start codon on mRNA • This initiates ELONGATION
Elongation 1. A second tRNA molecule carrying another A.A. arrives at
ribosome’s A binding site
2. Peptide bond forms between the MET and the second A.A.
3. Ribosome moves in the 3’ direction down the mRNA by one codon at a time; First tRNA is released and moves off • Ribosome movement is called TRANSLOCATION
4. A third tRNA arrives with another A.A. and a second peptide bond is synthesized
5. The second tRNA is released while a fourth arrives
6. Process REPEATS to assemble a polypeptide strand
Termination 1. Elongation continues until a STOP codon is reached
2. Releasing factor (enzyme) arrives at a binding dite to separate the polypeptide chain (“protein”)
3. Assembly complex (ribosome, tRNA, mRNA) disassembles itself
4. mRNA strand can be translated again
Mutations • Permanent and inheritable genetic changes
• Change in the sequence of bases within a gene
Possible Causes: • Randomly occur
• Thus, can be hard to predict! • Environmental mutagens
• Can be physical or chemical • E.g. Radiation (x-rays, UV, gamma rays) • E.g. industrial chemicals, pesticides, air pollution, cigarette
smoke
Types of Mutations • Point Mutations (Base Substitution)
• Occurs at only ONE nucleotide in a DNA sequence • Involves substitution of a nucleotide base with another
different base
• Frameshift Mutations • May affect an entire polypeptide chain • Base pair is added or deleted from the DNA
Point Mutations • Three Types:
1. Silent Mutations 2. Missense Mutations 3. Nonsense mutations
1. Silent Mutations - Usually the result of a base substitution at the third
location of an mRNA codon - No effect on the protein coded for by the the mutated
DNA
- For example:
- Because UCA and UCU both code for a.a. Serine, the point mutation in DNA has no effect
Base in a DNA Strand
(Normal)
mRNA codon that results:
Codes for the amino acid:
Base in a DNA strand:
(With Mutation)
mRNA codon that results:
Codes for the amino acid:
AGT UCA Serine AGT UCU Serine
2. Missense Mutations - Base substitution changes an mRNA codon so that it
codes for a different amino acid - Changes the amino acid sequence
- For example:
Base in a DNA Strand
(Normal)
mRNA codon that results:
Codes for the amino acid:
Base in a DNA strand:
(With Mutation)
mRNA codon that results:
Codes for the amino acid:
AGT UCA Serine AAT UUA Leucine
3. Nonsense Mutations - Base substitution in DNA that changes an mRNA to
code for a stop codon - When translation occurs, this misplaced stop codon
causes the polypeptide formation to end prematurely
- For example:
Base in a DNA Strand
(Normal)
mRNA codon that results:
Codes for the amino acid:
Base in a DNA strand:
(With Mutation)
mRNA codon that results:
Codes for the amino acid:
AGT UCA Serine ACT UGA STOP
Frameshift Mutations • A base pair is ADDED to or DELETED from the
DNA • Changes composition of all the codons that are ’downstream’
from the added/deleted base pair
Every codon that is downstream of the added DNA base will now be different.