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.