🦠 Gene Expression

3 RNA Structures involved in Gene Expression

Transfer RNA (tRNA)

• Made to carry out amino acids to the ribosome during translation, to the growing amino acid chain.

Messenger RNA (mRNA)

• Made to carry instructions for producing specific proteins to the ribosomes.

Ribosomal RNA (rRNA)

• Made to assist with translation of messenger RNA (mRNA).

Cell Signaling

1. Reception: Ligand binds to its receptor.

2. Transduction

3. Response: Where protein synthesis comes into the equation.

The Central Dogma of Biology

• DNA Transcription goes to…

• RNA Translation goes to…

• Protein.

Transcription (DNA)

Process in which RNA polymerase synthesizes on mRNA.

Step 1: Initiation

• Transcription factor(s) binds to the promoter sequence.

Step 2: Elongation

• RNA Polymerase synthesizes the mRNA by adding complementary nucleotides across from the template DNA strand.

  • Synthesizes mRNA in the 5’ to 3’ direction by reading from 3’ to 5’.

Step 3: Termination

• Transcription of a terminator sequence prevents further transcription of DNA by RNA Polymerase.

• Forms a hairpin loop on the mRNA sequence that stops further transcription.

What next? (Prokaryotes + Eukaryotes)

Prokaryotes

• Transcription occurs in the cytoplasm.

• Prokaryotes do not have introns (non-protein coding); immediately available for translation.

Eukaryotes

• Transcription occurs in the nucleus.

• Eukaryotic mRNA has to be processed to be translated.

Alternative Splicing

Introns are removed from pre-mRNA molecules by proteins called spliceosomes.

Translation (RNA)

Process in which ribosomes and tRNA synthesizes the amino acid sequence.

Step 1: Initiation

• Subunit of the ribosome bind to the start codon.

Step 2: Elongation

• Ribosome reads the mRNA sequence of 3 nucleotides called codons.

• The tRNA binds to the mRNA, transferring specific amino acid to the growing polypeptide (amino acid) chain.

Step 3: Termination

• Ribosome continues translating until it reaches the stop codon.

• It releases the mRNA and newly synthesized polypeptide (amino acid).

Prokaryotic vs. Eukaryotic Gene Expression

• mRNA is translated as it is transcribed.

• Occurs on ribosomes attached to the rough endoplasmic reticulum or found free-floating in the cytoplasm.

• Separated because eukaryotes have nuclei.

The Genetic Code is Universal

Like how books are written using the same language, but tell different stories because the words and letters are in different orders.

🔬 Point Mutations

Reading Frame

• Sequence of nucleotides that actually codes for protein and is “read” by the ribosome.

• Mutations to the reading frame sequence cause a change in protein structure; can prevent the protein from functioning.

The Central Dogma

• DNA —> RNA —> Protein

• Mutations can cause changes in the mRNA sequence, which can change the amino acid sequence and stop the protein from working correctly.

Types of Mutations

Point Mutations

• Change in 1 to 3 nucleotides; affect one gamete.

• Occurs during DNA Synthesis (in the cell cycle).

Chromosomal Mutations

• Mutations occur during meiosis.

• Affect big sections of chromosomes; affect multiple gametes.

Subtypes of Mutations

Substitution Mutation

• Original nucleotide is replaced.

• Three types:

  • Silent Substitutions

  • Missense Substitutions

  • Nonsense Substitutions

• Each type has a different effect on amino acids.

Silent Substitution Mutation

A change in a single nucleotide, does not result in a change in amino acid sequences.

• (No effect).

Missense Mutation

A change in a single nucleotide, results in a change in a single amino acid within a sequence.

Variable Effects of Missense Mutations

• Protein stops working entirely.

  • Mutation causes completely different chemical properties in amino acids; incorporated into the active site of a protein.

• Decreased functionality of the protein.

  • Amino acids with similar chemical properties incorporated into the active site of the protein.

• Increased or new function of the protein (very rare).

  • Mutation causes a change in amino acid sequence for protein to be more functional and/or gain a completely new function.

Nonsense Mutation

Substitutes a stop codon for an amino acid.

• A change in a single nucleotide; results in a premature stop-codon in mRNA sequence.

• Negative impact:

  • Codons after stop codon will not be translated.

  • Won’t fold or function properly.

Frameshift Mutation

• The addition or removal of a single nucleotide changes the reading frame.

• Two major types:

  • Insertion Frameshifts

  • Deletion Frameshifts

• Always has a negative effect.

Insertion Mutation

• Insertion of a single nucleotide.

• Changes the reading frame; also changes the entire amino acid sequence.

• Protein will not fold or function properly.

Frameshift: Deletion

• Deletion of a single nucleotide.

• Deletions change the reading frame; also changes the entire amino acid sequence after the deletion.

• Protein will not fold or function properly.

Mutations and Genetic Variation

• Mutations sometimes increase genetic variation.

• Only occurs if the mutation causes a change in the protein that alters the phenotype of the mutant organism.

  • E.g. Sickle Cell Anemia is caused by a missense mutation in the hemoglobin gene.

• Evolution is defined as a change in allele frequency in a population over time (mutation).