S7) Gene Expression Flashcards
What is a codon?
Codons are triplets of nucleotides in mRNA that encode the information for a specific amino acid in a protein
What is a stop codon?
Stop codons are codons in mRNA that signal the end of translation (end of protein synthesis)
Identify 3 stop codons
- UAA
- UAG
- UGA
What is a start codon (AUG)?
A start codon is always the first codon in a gene, present in mRNA to signal the location for translation to begin.
AUG = methionine
How is genetic code written?
Genetic code is written as it appears in an RNA copy of the DNA template
What is transcription?
Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA (especially mRNA) by the enzyme RNA polymerase.
Identify the 3 stages in transcription
- Initiation
- Elongation
- Termination
Describe initiation in transcription
Initiation: RNA polymerase binds to a specific nucleotide sequence (called a promoter) that marks the beginning of a gene.
Describe elongation in transcription
- Elongation: DNA strands unwind, and RNA polymerase reads the nucleotide sequence of the template strand.
- As it moves along, it inserts and pre-mRNA molecule.
Describe termination in transcription
- Termination: RNA polymerase reaches the end of the gene, marked by nucleotides called a 3’ termination sequence
- RNA-polymerase stops adding nucleotides to pre-mRNA which falls off the DNA template strand and is released
- DNA strands re-form a double helix
What is an intron?
An intron is a sequence in the pre-mRNA molecule that is not transcribed
What is an exon?
An exon is a sequence in a pre-mRNA molecule that is transcribed
Outline the process of RNA splicing
- RNA splicing is when pre-mRNAs are processed in the nucleus to remove introns which occur between exons
- As introns are removed, the exons are spliced together to form mature mRNA molecules
How are mRNA molecules processed?
Ends of the mRNA are modified:
- A nucleotide cap is attached to the 5’ end of eukaryotic mRNA
- A poly-A tail is added at the 3´ end (aids in export of the mRNA from nucleus)
How is the mature mRNA molecule released?
The final product, mature mRNA is exported from the nucleus into the cytoplasm, where translation takes place
What is translation?
Translation is the process in which ribosomes in the cytoplasm/rER produce polypeptides from information in mRNA
What is the structure and function of ribosomes?
- Structure: cytoplasmic organelles with a large and a small subunit
- Function: site of polypeptide synthesis
What kind of ribosomes are there?
- Free ribosomes: float in the cytoplasm
- Bound ribosomes: attached to the outer membrane of the rough endoplasmic reticulum
Explain the role of ribosomes in translation
The large subunit contains ribosomal RNA (rRNA) which acts as an enzyme, linking amino acids together to form a polypeptide during translation
What is tRNA?
Transfer RNA (tRNA) molecules bring amino acids to the mRNA ribosome complex during translation.
Each tRNA molecule has two attachment sites. Identify and describe them
- An anticodon that pairs with a complementary codon sequence in mRNA
- A site for attachment of the amino acid specified by the mRNA codon
Identify the three steps in translation
- Initiation
- Elongation
- Termination
Describe initiation in translation
- Initiation: mRNA binds to a small ribosomal subunit and the anticodon of the initiator
- tRNA carrying the amino acid methionine pairs with the AUG codon of mRNA.
Describe elongation in translation
- Elongation: amino acids are added to the growing polypeptide chain
- The rRNA of the large subunit forms peptide bond between amino acids
Describe termination in translation
- Termination occurs when the ribosome reaches a stop codon (UAA, UAG, and UGA)
- Proteins called release factors bind to stop codons
- Polypeptide, mRNA, and tRNA are released from the ribosome
What is a gene?
Gene: a unit of inheritance which is transferred from a parent to offspring and is held to determine some characteristic of the offspring
Distinguish between DNA replication, transcripion and translation in terms of its overall function
- DNA Replication: making DNA
- DNA Transcription: making RNA
- DNA Translation: making a polypeptide
Distinguish between DNA replication, transcription and translation in terms of the enzymes involved
- DNA Replication: DNA polymerase
- DNA Transcription: RNA polymerase
- DNA Translation: rRNA
Distinguish between DNA replication, transcription and translation in terms of the activated substrates used
- DNA Replication: dNTPs
- DNA Transcription: NTPs
- DNA Translation: amino acids
Distinguish between DNA replication, transcription and translation in terms of the template needed
- DNA Replication: DNA
- DNA Transcription: DNA
- DNA Translation: mRNA
Compare and contrast the ribosomes of prokaryotes and eukaryotes in terms of:
- Size
- Subunits
- rRNAs
- Proteins
Identify 5 factors which characterise protein synthesis in bacteria.
- Simple promoters
- Single RNA polymerase
- No post-transcriptional processing
- Short-lived mRNAs
- Simpler ribosomes
