Unit 4.4: Protein Synthesis SL

Proteins

Linear sequence of amino acids that are condensed together 

• We have a base 20 amino acids 

• Unique property of amino acids: 

  • The amino acids involved 

  • The sequences in which it condenses

RNA Polymerase functions

1. Binding to the promoter site on the DNA 

2. Has a helicase function that unwinds the DNA double helix 

3. Then it positions RNA nucleotides on the template strand with its complementary base pairing 

   1. It also performs the ligases role and links the sugar and phosphates with covalent bonds to form a continuous strand 

4. Detaches the RNA from the template strand → allows it to be single stranded

Importance of DNA stability

To prevent mutations that continue to pass to the RNA chains causing further issues

Transcription

The process of producing usable RNA strands 

• Occurs in the cytoplasm of prokaryotic cells

• Occurs in the nucleus in eukaryotic cells

  • Forms mature mRNA

• The cell can choose what part of the gene they need to transcribe and use 

  • Some genes are never expressed in certain cells 

Transcriptome

Full range of RNA types made in a cell

Housekeeping Genes

Genes that are always expressed and the proteins are always being coded for

Process of transcriotion

1. Initiation

2. Elongation

3. Termination

Transcription: Initiation

 RNA polymerase attached to the promoter DNA sequences 

• RNA will always be single stranded and shorted than DNA

Transcription: Elongation

RNA polymerase begins to synthesize the RNA transcript of the DNA

• The RNA polymerase contains a helicase function that separates the DNA to a template and coding strand → RNA codes along the template strand 

  • The strands are only temporarily/ momentarily separated → chemically vulnerable in this state 

• Uracil instead of Thymine in the complementary strand

Transcription: Termination

The RNA polymerase reaches a transcription terminator signal on the DNA

• It dislodges the growing RNA strand and releases the polymerase

Translation

Synthesis of polypeptides from mRNA

• The RNA produced through transcription is exported to the cytoplasm where it interacts with ribosomes for translation 

Messenger RNA (mRNA)

Carries genetic messages from the DNA to the ribosomes

• Contains codons for specific amino acids and start/stop codons to indicate the start and end of translation 

• Has a site for Ribosomes to bind to 

Transfer RNA (tRNA)

1. Functions in the cytoplasm to carry amino acids to the ribosomes 

• Translates the base sequence of mRNA into the amino acid sequence of polypeptides

• Contains a 3 base anticodon end and an attachment point for the amino acids 

  • Made of a distinct shape that will be recognized by enzymes which will then attach the right amino acid 

  • Transports amino acids to teh ribosme 

• Single strand that forms a clover-leaf shape 

  • 3’ end is where it attached to the amino acid

  • 5’ end is the anticodon loop that still needs to bond with mRNA

• 

Ribosomal RNA (rRNA

 Combines with the ribosomal proteins to create cytoplasmic ribosomes 

RIbosomes

• Structures that aid with the synthesis of proteins 

  • Contains Small subunit binds to the mRNA and the large subunit has binding sites for tRNA to catalyze peptide bonds between amino acids

  • mRNA has a site for small subunit of ribosomes to bind to 

Translation process

1. mRNA binds to the small subunit of the ribosome

2. Complementary tRNA binds to the first codon on the mRNA 

   • Enters to the A site

3. The nest tRNA which complements the next codon binds 

   • FIrst tRNA is then moved to along to the P Site

4. Ribosome will transfer the first amino acid to the second one forming a peptide bond 

   • 2nd tRNA has a dipeptide now

   • Occurs in the P site

5. Ribosome moves along the mRNA → releasing the tRNA’s as it takes its amino acid 

   • Released through the E site

6. This chain continues 

Gene Expression

Process of information being carried by a gene that has observable effects on an organism 

Function of Genes

• Specify the amino acid sequence which can then have an affect on our gene expression 

Genetic Code: 

1. DNA is great for data storage since it can hold long sequences of bases

2. Living organism contains 64 combinations of three base codons → form different amino acids or start/stop codons

Degeneracy

DIfferent Codes can be used for the same amino acid → duplicity 

• Incase something occurs to the other piece of DNA the amino acid can still be produced

Universality

Used by all living organisms and viruses - only minor changes

Mutations Changing Protein Structure

Change to the base sequence of a gene 

Point Mutation

Only one of the bases is changed 

Silent Mutation

Doesn’t affect the amino acid as it happened to be another code that produced the same amino acid

Missense Mutation

Creates a different amino acid 

Nonsense Mutation

Produces a stop codon instead

• The stop codon comes too early and it completely stops the whole polypeptide synthesis 

Sickle Cell Disease/Anemia

• Inherited condition 

  • Radical change due to a single base substitution that produce hemoglobin

    • Point mutation in the gene for beta-hemoglobin (Hb gene)

• Sickle cell anemia is a blood disorder in which red blood cells become sickle shaped and cannot carry oxygen properly

  • Most common in people with African ancestry

Mutation causing sickle cell disease

• The original code for the 6th amino acid is for glutamic acids (HbA = GAG) which changes to the 6th amino acid being valine (HbS = GUG)

• Ends up placing a non-polar amino acid in the part of the polypeptide that is exposed to water

  • Changes the shape by making it clump in low oxygen environments

  • Causes blockages and reduced blood flow