BIOL 217 Topic 11: Gene Expression

BIOL 217 Topic 11: Gene Expression

• Gene expression is the process by which DNA directs protein synthesis, encompassing transcription and translation.
• DNA is inherited, leading to the expression of specific traits through protein synthesis.
• The information content of genes resides within the specific nucleotide sequences.
• Proteins link genotype to phenotype.
• The central dogma describes the flow of genetic information: DNA → RNA → Protein.
• RNA acts as a bridge between genes and the proteins they encode.

Learning Objectives

• Understand the central dogma, genetic code, and how genetic information flows from genes to proteins.
• Compare and contrast transcription and translation.
• Explain the steps in transcription (initiation, elongation, termination) and the key molecules involved.
• Understand different types of mutations and their impact on the translated protein.

Transcription

• Transcription is the first stage of gene expression, and it's DNA-directed RNA synthesis.
• RNA synthesis is catalyzed by RNA polymerase.
• Transcription comprises three stages: initiation, elongation, and termination.
• In prokaryotes, translation can begin before transcription is complete.
• In eukaryotes, the nuclear envelope separates transcription from translation.
• Eukaryotic RNA transcripts undergo RNA processing to form mature mRNA from a primary transcript.

Transcription and RNA Polymerase

• RNA polymerase synthesizes RNA strands by joining RNA nucleotides together in a 5' to 3' direction.
• RNA polymerase follows base-pairing rules, but uracil replaces thymine.
• RNA produced is complementary to the DNA template strand.
• Promoters are DNA sequences where RNA polymerase attaches.
• In bacteria, the sequence signaling the end of transcription is called a terminator.
• A transcription unit is the stretch of DNA that is transcribed.

Synthesizing an RNA transcript - Initiation

• Promoters are transcriptional start points, extending several dozen nucleotides upstream of the start point.
• The TATA box is crucial in forming the initiation complex in eukaryotes.
• Transcription factors mediate the binding of RNA polymerase and initiate transcription.
• The transcription initiation complex is the completed assembly of transcription factors and RNA polymerase II bound to a promoter.

Synthesizing an RNA transcript - Elongation

• As RNA polymerase moves along DNA, it unwinds the double helix, exposing 10-20 bases at a time.
• Transcription progresses at a rate of 40 nucleotides per second in eukaryotes.
• A gene can be transcribed simultaneously by several RNA polymerases.
• Nucleotides are added to the 3' end of the growing RNA molecule.

Synthesizing an RNA transcript - Termination

• The mechanisms of termination differ in bacteria and eukaryotes.
• In bacteria, RNA polymerase stops transcription at the end of the terminator sequence.
• In eukaryotes, RNA polymerase transcribes the polyadenylation signal sequence, releasing the RNA transcript 10–35 nucleotides past the sequence.

Translation

• Translation is the process where genetic information flows from mRNA to protein.
• Translation creates a polypeptide based on the mRNA information.

Molecular Components involved in translation

• Cells translate mRNA messages into protein using transfer RNA (tRNA).
• tRNAs carry specific amino acids to the ribosome, where they are attached to the growing polypeptide chain.

Transfer RNA (tRNA)

• Each tRNA molecule facilitates the translation of a specific mRNA codon into a particular amino acid.
• Each tRNA carries a specific amino acid and has an anticodon at the other end that base-pairs with a complementary codon on the mRNA.
• tRNA has a characteristic L-shape.

Accurate Translation Requires Two Major Steps

• A correct match between tRNA and amino acid is catalyzed by aminoacyl-tRNA synthetase.
• A correct match between tRNA anticodon and mRNA codon is essential.

Ribosomes

• Ribosomes facilitate mRNA codon-tRNA anticodon coupling, essential for protein synthesis.
• Ribosomes are composed of large and small ribosomal subunits, made of proteins and rRNA.

Ribosomal binding sites for tRNA

• Ribosomes possess three binding sites for tRNA: A site, P site, and E site.
• The A site accepts tRNA carrying the next amino acid.
• The P site holds tRNA carrying the growing polypeptide chain.
• The E site releases discharged tRNAs.

Building a polypeptide

• Translation involves three stages: initiation, elongation, and termination.
• Energy is required for some steps in translation, and factors aid each stage.

Mutations

• Mutations are changes in the genetic information of a cell.
• Mutations of one or a few nucleotides can affect protein structure and function, potentially leading to genetic disorders or hereditary diseases.

New Mutations and Mutagens

• Spontaneous mutations occur during DNA replication, recombination, or repair.
• Mutagens are physical or chemical agents that cause mutations.

DNA Mutations

• Point mutations involve changes in just one nucleotide pair in a gene.
• Point mutations can be nucleotide-pair substitutions or insertions/deletions, affecting the abnormal protein production.
• Nucleotide-pair substitutions can include silent mutations (no change in amino acid), missense mutations (change to a different amino acid), and nonsense mutations (change to a stop codon), potentially leading to nonfunctional proteins.