MOLECULAR BIOLOGY
DNA is used as the blueprint to direct the production of certain proteins
Gene
segment of DNA coding for a RNA segment.
These RNA segments will be used to produce a polypeptide (structural or enzymatic protein)
Each gene has a precise beginning and an end
Gene Expression
The DNA nucleotide sequence codes for the order in which amino acids are put together to form proteins
Every three nucleotides on the mRNA (codon) codes for a specific amino acid
Genetic Code
20 different amino acids but 64 possible codons
Some redundancy (repetition)
Transcription
Information is transferred from DNA to RNA
Occurs in the nucleus
Types of RNA
All three types of RNA are transcribed from DNA
mRNA
rRNA
tRNA
Messenger RNA
carries the coded message from the DNA to the ribosome
Ribosomal RNA
reads the mRNA
Transfer RNA
transfers the correct amino acid to the ribosome
Overview of Transcription
The segment of DNA that contains the gene for a specific protein will unwind and the complementary RNA strand will be made by incorporating the RNA nucleotides
Each gene has a precise…
beginning known as the promotor region and an end known as the termination sequence
Stages in transcription of RNA
Initiation
Elongation
Termination
Initiation
(Transcription)
Transcription factors bind to the promoter region (TATA box) of the DNA to turn gene on
RNA polymerase then initiates transcription by binding to the transcription factor
Unwinds the DNA
Elongates the RNA segment
Elongation
(Transcription)
Nucleotides are added in the 5’ to 3’ direction by RNA polymerase
They form temporary hydrogen bonds with the DNA template
As the DNA helix reforms the RNA peels away
Termination
(Transcription)
the termination sequence causes transcription to end
Pre-RNA segment dissociates from the DNA and then becomes either mRNA, rRNA, or tRNA
Post-transcriptional Modifications for mRNA
5’ cap – a guanine triphosphate (GTP) is added signal for ribosomal attachment in the cytoplasm
3’ poly A tail – polyA polymerase adds ~250 “A” nucleotides to the end protects RNA from being degraded by nucleous
Splicing
Exons
Introns
Exons
coding region
Introns
noncoding region
Cleaved out by snRNPs, and exons are spliced together
Post-transcriptional Modifications for rRNA
rRNA associates with proteins to form two subunits (40s and 60s)
Leaves the nucleus and enters the cytoplasm
Post-transcriptional Modifications for tRNA
Folds into a three dimensional structure (clover shaped)
Translation
Going from the mRNA nucleotide code to amino acid code
mRNA is read by a ribosome (rRNA) to determine the sequence of amino acids
Occurs in the cytoplasm
Players in Translation
mRNA strand
Ribosomes (rRNA)
tRNAs carrying amino acids
enzymes
rRNA
(translation)
has a mRNA binding site and three tRNA binding sites
A site (amino-acyl binding site)
P site (peptidyl binding site)
E site (Exit site)
tRNA
(translation)
Has an anticodon three base sequence that is complementary to a codon on the mRNA
3’ end of the tRNA contains a binding site for a specific amino acid
Stages of Translation
Initiation
Elongation
Termination
Initiation
(translation)
mRNA binds to the 40s ribosome subunit
The initiator tRNA binds to the mRNA start codon (AUG) at the P site on the ribosome
The arrival of the 60s subunit completes the initiator complex
Elongation
(translation)
The next tRNA enters at the A site
The enzyme peptidyl transferase forms a peptide bond between the amino acid on the P site and the new amino acid on the A site.
The ribosome then moves down the mRNA (translocation)
The tRNA that was at the A site is now at the P site and the tRNA that was at the P site is now at the E site and exits
Termination
(translation)
Elongation continues until a stop codon on the mRNA is reached (UAA, UAG, UGA)
The polypeptide is then released from the ribosome by a release factor
Charging of tRNA
Amino acids are floating freely in the cytoplasm
The enzyme amino-acyl tRNA synthetase attaches the amino acids to the 3’end of the tRNA
Requires ATP
Polysomes
Several ribosomes can simultaneously translate the same mRNA strand to make multiple copies of the same polypeptide
Post-translational modifications to the Polypeptide
The start methionine is removed by the enzyme aminopeptidase
Protein will under go folding or modifications
Cleavage into smaller fragments or joined with other polypeptides
Chemical modifications: addition of carbohydrates or lipids
Transport to its destination
Mutation
a change in the sequence of bases within a gene
Caused by a mistake during DNA replication (rare as DNA polymerase proofreads)
Or due to environmental factors called mutagens
Mutations can be somatic or germinal
Germinal mutation
Gametes (all cells in fertilized egg would have mutation)
Can be silent then show up later in life
Ex: Family history of cancer
Can be passed down by hereditary
Somatic mutation
Skin cells (all skin cells would be different but would not affect other tissues)
Can occur due to age
Types of Mutations
Point mutations
Frame-shift mutation
Point mutations
(substitutions) – change in a single nucleotide
Due to redundancy of the genetic code it may or may not change the amino acid
Silent mutations do not change the protein
Frame-shift mutation
caused by insertion or deletion of a nucleotide
Changes the reading frame of the codons, usually results in a non-functional protein
Are all mutations bad?
They are also the source of the rich diversity of genes in the world
They contribute to the process of evolution by natural selection