Biology Chapter 12: Protein Synthesis

Transcription

• Transcription is the process of copying a base sequence from DNA to RNA.
• RNA polymerase reads the DNA template in the 3' to 5' direction.
• The complementary RNA transcript is assembled in the 5' to 3' direction.
• Uracil (U) is paired with adenine (A) instead of thymine (T) in RNA.
• RNA synthesis begins at a promoter sequence on DNA.
• Elongation continues until RNA polymerase reaches a termination sequence.
• Termination releases mRNA by creating a hairpin loop.

Concurrent Transcription, Translation, and mRNA Degradation

• In prokaryotes, transcription, translation, and mRNA degradation happen concurrently.
• They occur in the same 5' to 3' direction.
• This is possible because there's no membrane separating the processes (nucleoid).
• Multiple RNA polymerases can transcribe a single bacterial gene.
• Multiple ribosomes can translate mRNA transcripts into polypeptides.
• Protein concentration can quickly rise in bacterial cells.

Overview of Pre-mRNA Processing

• Before pre-mRNA is translated into protein, it is modified by splicing.
• Splicing removes introns and reconnects exons to form mature mRNA.
• Introns are removed, and exons are spliced together.
• Some pre-mRNA molecules are spliced differently in different tissues.
• One gene can produce many different mRNA molecules.

Translation Overview

• After pre-mRNA is processed to mRNA, it is ready for translation into a polypeptide.
• Ribosomes consist of small and large subunits of protein and rRNA that bind mRNA.
• Many ribosomes can work on the same mRNA at a time.
• Translation begins at the start codon (AUG) which codes for methionine.
• Each amino acid is brought to the ribosome by a specific tRNA.
• tRNA has an anticodon sequence that interacts with the mRNA codon.
• Amino-acid charging enzymes ensure the correct amino acid is attached to the correct tRNA.
• Amino acids are linked by peptide bonds during elongation.
• Translation continues until a stop codon is reached.
• A release factor dissociates the components, releasing the polypeptide.
• Proteins fold during and after translation.

Protein Synthesis Machinery

• Protein synthesis consumes more energy than any other metabolic process.
• Proteins make up most of the cell mass.
• Translation involves decoding mRNA into amino acids that are covalently bound by peptide bonds to form proteins.
• Each amino acid has an amino (NH2) and a carboxyl (COOH) group.
• Ribosomes catalyze the reaction, forming a peptide bond and releasing 1 molecule of water (H₂O).

Translation Mechanisms

• Ribosomes attach to mRNA in the cytoplasm.
• Ribosomes move along the mRNA to form amino acid chains.
• Each tRNA brings a specific amino acid.
• tRNAs have anticodons that match mRNA codons.
• Ribosomes create peptide bonds between amino acids, forming a polypeptide.
• The process continues until a stop codon is reached.. The mRNA and polypeptide are released.

3 Types of RNA in Translation

• mRNA carries the coded message from the nucleus to the ribosomes.
• tRNA delivers amino acids to the ribosomes.
• Ribosomes are composed of proteins and rRNA, facilitating amino acid joining.

Regulation Intro

• Proper protein synthesis is crucial for cellular function.
• Gene expression regulation conserves energy by controlling when and how much a gene is expressed.
• Gene expression is the process of regulating a gene to create RNA, converted to proteins.
• Gene expression is controlled by factors such as timing, amount, and environmental conditions.

Prokaryotes & Operons

• In prokaryotes, DNA binding proteins control transcription.
• Every gene must be expressed, but not all simultaneously.
• An operon is a group of genes that are regulated together.
• The lac operon is a group of genes involved in lactose metabolism.
• It is inducible, meaning the expression is turned on by the presence of lactose.
• When lactose is present, a repressor protein changes shape and detaches from the operator. This allows RNA polymerase to transcribe lac genes for lactose utilization.
• When lactose is absent, the repressor binds to the operator, blocking transcription.

Lac Operon (Inducer)

• Inducible operons (like the lac operon) have proteins that activate or repress transcription based on cell needs.
• The lac operon has genes needed for lactose intake, but only if glucose is absent.
• Repressor binds to the operator to stop transcription if lactose is not present.
• If present, lactose alters the repressor shape preventing it from binding to the operator, which allows transcription of genes.

Prokaryotes vs. Eukaryotes

• Prokaryotes lack a nucleus; transcription and translation occur simultaneously.
• Eukaryotes have a nucleus; transcription happens within the nucleus, while translation occurs in the cytoplasm.
• Epigenetic factors at a DNA level alter gene expression (turning genes "on" and "off").

Transcription Factors Detailed

• TATA box, a short DNA sequence, and transcription factors help in positioning the RNA polymerase enzyme near the gene start.
• Transcription factors are DNA-binding proteins that influence gene expression.
• They can open tightly packed chromatin (heterochromatin) to allow RNA polymerase access to the DNA.
• Transcription factors can bind to enhancer regions, increasing RNA transcription rate (activating genes).
• Regulation can also occur by mRNA exit from the nucleus or gene protein degradation.

Development (Gene Regulation)

• Gene regulation is even more complex in eukaryotes, controlling the type and amount of protein produced in multi-cellular organisms.
• Differentiation and cell function are tied to specific gene expression..
• Homeotic genes control development by regulating the activation of other genes.
• Hox genes are a group of homeobox genes important for development.

Epigenetics

• Epigenetics alters gene expression without altering the gene's DNA sequence.
• Methylation (addition of methyl groups to DNA) and histone modification lead to either activating or repressing gene expression, making DNA more compact (heterochromatin) or less compact (euchromatin), respectively.

Cancer & Gene Expression

• Cancer is a disease of altered gene expression.
• Mutations can activate transcription factors, increase binding of the TF to the DNA leading to overexpression, or change how much protein is produced.
• Tumor suppressor genes normally prevent or regulate cell growth.
• Mutations in these genes can lead to uncontrolled cell division.
• Changes in epigenetic modifications (histone acetylation and DNA methylation) might lead to abnormal gene expression.
• Some genes are switched on or off inappropriately which negatively affect the cell cycle in cancer.

Targeted Therapies

• Targeted therapies exploit specific gene expression features of cancer cells.
• Strategies are being developed to treat specific mutations, and levels of genes expression in various types of cancers.
• These methods result in less invasive treatment for patients and allow for a more specific approach

Cancer & Translation

• Cancer can involve increased protein translation, changes in phosphorylation, or changes in different protein variants, including changes in cell death mechanisms.
• An aberrant expression of cell death mechanisms can contribute to cancer development.

Test your knowledge on the process of protein synthesis, including transcription and translation. This quiz covers key concepts such as RNA functions, the role of ribosomes, and the specifics of codons and amino acids. Perfect for students studying molecular biology!