Transcription and Translation

Transcription and Translation Overview

• The processes of transcription and translation make a protein.

Protein Synthesis: Two-Step Process

• Protein synthesis is a two-step process involving transcription and translation.
• Transcription copies DNA into messenger RNA (mRNA).
• Translation happens when mRNA travels to a ribosome.
• In translation the mRNA sequence is translated into a protein using transfer RNA (tRNA).

RNA and DNA Nucleotides

• RNA contains the base uracil, while DNA contains thymine.
• RNA has the sugar ribose, DNA has deoxyribose.

Transcription Detailed

• The first step of gene expression, where a copy of DNA is made into an mRNA chain
• Firstly DNA is copied into an mRNA chain.
• The mRNA then leaves the nucleus and heads towards a ribosome.
• RNA polymerase binds to DNA and unwinds it, resulting in RNA.
• The start codon is AUG.
• The stop codon releases.
• The RNA is then edited, introns are removed, and a cap is added.
• Finally mRNA exits the nucleus to a ribosome

Translation Described

• Process by which mRNA is decoded and a protein is produced
• mRNA translates into an amino acid chain using tRNA.
• The amino acid chain folds into a protein.
• Transfer RNA (tRNA) brings together a base sequence and an amino acid.
• Anitcodons are 3 bases on tRNA.
• Codons are 3 bases on mRNA.
• tRNAs link to an amino acid, transfer it to a ribosome, and then link to the appropriate mRNA codon at the ribosome.
• A polypeptide chain is then produced.

Universal Genetic Code and mRNA Codons

• The universal genetic code is read in mRNA codons.

Ribosomes

• These are the "workbenches" of protein synthesis.
• They comprise proteins and ribosomal RNA (rRNA).
• A ribosome has large and small subunits that come together during translation.
• During translation tRNA molecules bind.

Genetic Regulation

• Protein production is regulated.
• Most genes do not stay "ON" all the time.
• More complex organisms have greater control of DNA regulation.
• Non-coding DNA makes up about 10% of the prokaryote genome.
• Non-coding DNA makes up over 98% of the human genome.
• Genetic regulation is affected by promoters, enhancers, transcription factors, and mRNA splicing.

Promoters and Enhancers

• Transcription requires RNA polymerase to align at a noncoding sequence called a promoter.
• The promoter lies "upstream" from a gene.
• Enhancers are noncoding segments that are some distance from the promoter.
• Transcription factors bind to both promoters and enhancers.
• Transcription factors facilitate the alignment of RNA polymerase at the promoter.
• These factors can alter the amount of gene product produced.
• Chicken: Lower transcription of the Hoxc8 gene results in 7 thoracic vertebrae.
• Mouse: Higher transcription of the Hoxc8 gene results in 13 thoracic vertebrae.

mRNA Splicing

• Exons are expressed, while introns are removed and are not expressed.

Alternative Splicing

• Primary transcripts are edited into different versions.
• Simple organisms have the same number of genes as humans (20,000-25,000).
• Humans are more complex due to genetic regulation.
• The same primary transcript can result in different mRNAs and different proteins.

Mutations Defined

• An error in DNA replication.
• This happens at a very low rate.
• Repair enzymes fix errors.
• If mistakes are made and not corrected, mutation occurs.
• A mutation is a permanent alteration in a cell's DNA base sequence.

Genetic Mutations

• Some known genetic mutations include human microcephaly, genetic mutation in flies.
• A genetic mutation can be a change in one or more bases.
• Mutations are very rare but play an important role in evolution.

Heritable vs. Non-Heritable Mutations

• Somatic cells are body cells that do not become eggs or sperm, and include acquired mutations like skin cancer or lung cancer.
• Germ-line cells are cells that become eggs or sperm, run in families, and include inherited mutations like Huntington's Disease or some cancers.

Mutagens Described

• Mutagens are substances that can mutate DNA.
• Examples include cigarette smoke and ultra violet light.
• Mutations are the only means of new genetic variation and accidental reorderings of DNA may produce new proteins.

Point Mutations

• Base-pair substitution replaces a base pair.
• Base-pair insertion adds a base pair.
• Base-pair deletion removes a base pair.
• Point Mutation: Original "The fat cat ate the wee rat." changed to "The fat hat ate the wee rat."
• Deletion: Original "The fat cat ate the wee rat." changed to "The fat ata tet hew eer at.".
• Frame Shift: Original "The fat cat ate the wee rat." changed to "The fat caa tet hew eer at.".

Chromosomal Mosaicism

• This is when there are two or more populations of cells with different genotypes in one individual.
• A mutation can become permanent if one base paired with another that is not complementary.

Consequences of Mutations

• Most have no effect, but when they do they are generally negative.
• Cancers can happen from a line of cells that have mutations.
• BRCA1 & BRCA2 are tumor suppressor proteins that repair damaged DNA.
• Breast and ovarian cancer can be increased in risk (5-10% of breast).
• The US Supreme Court has ruled that Myriad did not create or alter either the genetic information encoded in the BCRA1 and BCRA2 genes or the genetic structure of the DNA.
• Synthetically created Exon-ONLY strands can be patented.