Protein Synthesis and Genetics Overview

Protein Synthesis Overview

• Protein synthesis is the process of making proteins using the genetic code in DNA.
• This process involves two key stages: transcription and translation.

Har Gobind Khorana

• Indian American Biochemist
• Won Nobel Prize for demonstrating how genetic information is translated into proteins.
• Confirmed the genetic code consisted of 64 distinct three-letter words.
• Created an artificial gene in 1972.

One Gene – One Polypeptide

• Beadle and Tatum's experiment proposed that one gene codes for one polypeptide.
• This concept has been refined to better reflect the complexity of how genes code for proteins.

Sickle Cell Anemia

• An example illustrating how a single mutation in a gene for hemoglobin can alter protein structure.
• In sickle cell anemia, one mutation results in valine replacing glutamic acid in the hemoglobin ß-chain.

Central Dogma of Molecular Biology

• Information flows from DNA to RNA to protein.
• Transcription converts DNA to mRNA.
• Translation converts mRNA to a protein.

Comparing DNA and RNA

• RNA sugar is ribose, not deoxyribose like DNA.
• RNA is single-stranded, whereas DNA is double-stranded.
• RNA contains uracil instead of thymine.

Types of RNA

• Messenger RNA (mRNA): Carries instructions from the nucleus to the cytoplasm for polypeptide synthesis.
• Transfer RNA (tRNA): Carries amino acids to the ribosome, matching them to the mRNA code.
• Ribosomal RNA (rRNA): Forms part of the ribosome, the site of protein synthesis.

What is Protein Synthesis?

• Protein synthesis is the process of creating proteins using the genetic code in DNA.
• Transcription produces mRNA from a DNA template.
• Translation creates a protein using mRNA instructions.

Transcription Overview

• Initiation: RNA polymerase binds to DNA at a promoter site.
• Elongation: RNA polymerase adds complementary RNA nucleotides to the growing mRNA strand.
• Termination: RNA polymerase recognizes a stop signal and releases the mRNA transcript.

Transcription: Initiation

• Transcription factors help RNA polymerase recognize and bind to the promoter region (often including a TATA box).
• RNA polymerase unwinds the DNA to expose the template strand.

Transcription: Elongation

• RNA polymerase synthesizes mRNA in the 5' to 3' direction using the DNA template strand.
• The promoter region is not transcribed.
• No primer is needed.

Transcription: Termination

• RNA polymerase recognizes a specific termination sequence.
• In prokaryotes and eukaryotes, termination sequences differ.
• Termination causes mRNA to disassociate from the DNA template.

Post-Transcriptional Modifications

• Capping: Addition of a 7-methylguanosine cap to the 5' end of pre-mRNA to protect it from degradation.
• Polyadenylation: Addition of a poly-A tail (adenine nucleotides) to the 3' end of the pre-mRNA to enhance stability and export from the nucleus.
• Splicing: Removal of introns (non-coding regions) and joining of exons (coding regions) to produce mature mRNA.

Translation Overview

• Initiation: Ribosome recognizes the start codon and binds to mRNA.
• Elongation: Ribosome moves along the mRNA, adding amino acids to the growing polypeptide chain.
• Termination: Ribosome encounters a stop codon and releases the complete polypeptide.

The Genetic Code: Codons

• The genetic code is read in codons, which are groups of three nucleotide bases on mRNA.
• Each codon specifies a particular amino acid.

The Genetic Code

• There are 20 different amino acids in proteins.
• There are 64 possible codons.
• Multiple codons can code for a single amino acid (redundancy).
• The start codon is AUG.

Ribosomal RNA (rRNA)

• rRNA is a component of ribosomes.
• Ribosomes have 2 subunits (small and large).
• Ribosomes move along mRNA in the 5' to 3' direction.

Transfer RNA (tRNA)

• tRNA carries amino acids to the ribosome.
• tRNA is single-stranded and shaped like a cloverleaf.
• The anticodon on tRNA recognizes the codon on mRNA.

Translation: Initiation

• Small ribosomal subunit binds to the mRNA at the start codon (AUG).
• Initiator tRNA with anticodon UAC binds to the start codon.
• Large ribosomal subunit joins the complex.

Translation: Elongation

• tRNA carrying the next amino acid enters the A site.
• Peptide bond forms between amino acids in the P and A sites.
• tRNA in the P site moves to the E site (exit site).

Translation: Termination

• When a stop codon is reached, a release factor enters the A site.
• The polypeptide chain is released.
• The ribosome subunits dissociate.

Protein Synthesis: Summary

• Transcription produces mRNA from DNA.
• Translation uses mRNA to build a polypeptide chain.
• Post-transcription modifications involve capping, polyadenylation, and splicing.

Practice Questions (Page 15)

• The provided sequence is a simplified example of a gene segment.
• Questions on identifying promoters, transcribing, and translating the sequence require this fragment.
• They address scenarios where various steps in protein synthesis are disrupted.
• The exercises highlight the consequences of disruption.
• Difference between DNA replication and protein synthesis is asked in the same section.

Practice Questions (Labeling Diagrams)

• (Page 28, 29) Exercises involve labeling the components of the translation process and a diagram of protein synthesis.
• These exercises help solidify understanding of the components and their roles.

Topics not fully covered

• Some of the listed topics are not thoroughly explained in the slides and need further exploration for a comprehensive study.

Explore the essential processes of protein synthesis including transcription and translation. Learn about the contributions of Har Gobind Khorana to genetics, the implications of the one gene-one polypeptide hypothesis, and the role of mutations in diseases like sickle cell anemia. This quiz provides insights into the central dogma of molecular biology.