Gene Expression: Transcription and Translation Notes

Gene Expression

• Gene expression is the process of using information in DNA to create protein.

• The flow of information in the cell: DNA --> RNA --> Polypeptide (protein).

Eukaryotic Gene Expression

1. DNA transcription (nucleus of the cell):

   • Gene’s DNA sequence is copied as a complementary RNA sequence (RNA transcript).

2. RNA processing (nucleus of the cell):

   • mRNA transcript is edited and modified for use later.

3. mRNA translation (cytoplasm of the cell):

   • Mature mRNA transcript is translated into a polypeptide in a ribosome.

Genes

• Genes are DNA sequences that encode information for proteins (and RNA).

• They are a specific sequence of A’s, C’s, G’s and T’s.

• Genes are found on specific places (loci) on chromosomes.

RNA Types

• rRNA (ribosomal RNA) - part of ribosome (structurally)

• mRNA (messenger RNA) - carries information for translation

• tRNA (transfer RNA) - brings correct amino acids to ribosomes

• snRNA (small nuclear RNA) - part of spliceosome (in nucleus)

• Only mRNA is translated; the other three are essential to the process.

• RNA molecules are complementary to the template strand of DNA.

Transcription

• Template DNA strand: directs synthesis of RNA strand.

• RNA Polymerase: joins RNA nucleotides together.

• Transcription factors: help initiate transcription.

• Ribonucleoside triphosphates (rNTPs): nucleotide building blocks.

Stages of Transcription

1. Initiation: RNA polymerase binds to DNA.

2. Elongation: RNA molecule is synthesized in the 5’ --> 3’ direction.

3. Termination: transcription ends; RNA polymerase falls off DNA.

Initiation of Transcription

• RNA polymerase binds to the promoter with the help of transcription factors.

• Separates DNA strands at the promoter and begins transcription.

• Transcription unit: region of DNA that is transcribed as a single mRNA molecule (includes promoter, gene, and terminator).

TATA Box

• Transcription factors bind to the TATA box within the promoter.

• TATA box is a non-coding DNA sequence that helps bind transcription factors.

• Bound transcription factors recruit RNA polymerase to begin transcription.

Elongation of Transcription

• RNA polymerase moves along the template strand.

• RNA polymerase adds complementary RNA nucleotides to the 3’ end of the growing RNA strand.

• RNA is synthesized in the 5’ --> 3’ direction.

Termination of Transcription

• RNA polymerase transcribes a termination sequence.

• The resulting RNA molecule forms a stem-loop that causes dissociation of RNA polymerase from DNA.

• RNA transcript dissociates from RNA polymerase.

RNA Processing

• Newly formed RNA strand is modified to become mRNA.

• Modifications:

  • Modification of the 3’ and 5’ ends.

  • Removal of non-coding regions (introns).

• Modification occurs in the nucleus before mRNA exits.

Modification of 5’ and 3’ ends

• 5’ cap (G cap): modified G nucleotide added to the 5’ end.

• poly-A tail: 50-250 A nucleotides added to the 3’ end.

• Functions:

  • Protect mRNA from degradation.

  • Regulate export out of nucleus.

  • Assist with ribosome attachment for translation.

Removal of Introns

• Exons: coding RNA sequences for translation

• Introns: noncoding RNA sequences (removed).

• mRNA processing (‘splicing’) involves removing introns and joining exons.

Intron Removal Process

• Protein/snRNA complex (spliceosome) cuts out introns and joins exons.

• Removed introns are broken down into individual nucleotides.

After Transcription

• Eukaryotes: mRNA must exit the nucleus for translation.

• Prokaryotes: mRNA is translated as soon as it is synthesized.

Transcription vs Translation

• Transcription

  • Nucleic acid --> Nucleic acid (same language)

  • Uses same “letters” (nucleotides)

• Translation

  • Nucleic acid --> Protein (different languages)

  • Uses different “letters” (nucleotides vs. amino acids)

mRNA

• mRNA codes for sequence of amino acids.

• mRNA contains 4 bases (A,U,C,G).

• Triplet code: mRNA is read as 3-nucleotide words (codons) that code for amino acids.

Genetic Code

• Genetic code is a set of rules used to link the codons to specific amino acids for translation.

• The code is redundant but not ambiguous.

  • Most amino acids have more than one codon.

  • A codon never codes for more than one amino acid.

  • A few special codons determine where mRNA translation will START and STOP

tRNA

• tRNA molecules match codons with amino acids.

• tRNA molecules ‘translate’ the genetic code.

• tRNA molecules are a folded-up strand of RNA.

• tRNA are “adapter” molecules that translate the code.

tRNA Translation

• A tRNA molecule can recognize an mRNA codon using its complementary 3-letter sequence (anticodon).

• Each tRNA molecule has:

  • a unique anticodon.

  • 3’ end for attachment to a specific amino acid.

• Aminoacyl-tRNA – tRNA with a bound amino acid.

mRNA Reading

• mRNA is read in words of three letters with no gaps or overlaps between words.

Translation

• Ribosome: the structure in which protein synthesis occurs.

• mRNA strand: contains the message (codons).

• aminoacyl-tRNA molecules: translate mRNA into amino acids.

Ribosomes

• Ribosomes hold the mRNA and tRNA molecules together during translation.

• There are three binding sites for tRNAs in a ribosome:

  • A site – next tRNA with amino acid

  • P site – tRNA with polypeptide

  • E site – exiting tRNA

Stages of Translation

• Initiation: mRNA, ribosome subunits, and the first tRNA bind together at the START codon (AUG).

• Elongation: ribosome travels down the mRNA strand; tRNA molecules add amino acids to make a polypeptide.

• Termination: translation ends when the ribosome reaches the STOP codon.

Termination of Translation

• A special protein called a release factor binds to the STOP codon.

• The polypeptide is cut free from the last tRNA.

• Ribosome subunits break apart.

Prokaryotes

• Since prokaryotes lack a nucleus translation can begin before transcription is fully completed.

• Transcription and translation are COUPLED.

Mutations

• Mutation: change in DNA nucleotide sequence.

• A point mutation is the change (or insertion/deletion) of a single nucleotide.

• Types:

  • Silent mutations

  • Missense mutations

  • Frameshift mutations

  • Nonsense mutations