Protein synthesis

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