MCBG Session 8 - Gene Expression Flashcards
Outline what is meant by the terms codon, start codon, stop codon
- By convention, the genetic code is written as it appears in an RNA copy of the DNA template
- Each group of 3 nucleotides is called a codon – triplets of nucleotides in mRNA that encode the information for a specific amino acid in a protein.
- Stop codons - A codon in mRNA that signals the end of translation – signals the end of protein synthesis. (UAA, UAG, and UGA)
- One codon (AUG) has two functions:
I. it encodes the information for the amino acid methionine
II. it is always the first codon in a gene, called the start codon – A codon present in mRNA that signals the location for translation to begin. The start codon marks the beginning of the coding sequence in a gene.
Outline the process of transcription in light of the following stages:
- Initiation
- Elongation
- Termination
Transcription begins when the DNA in a chromosome unwinds and one strand is used as template to make a pre-mRNA molecule. This process involves several steps:
- In a stage called initiation, RNA polymerase (an enzyme) and several regulatory proteins bind to a specific nucleotide sequence (called a promoter) that marks the beginning of a gene.
- In a stage called elongation, the strands of DNA unwind, and RNA polymerase reads the nucleotide sequence of the template strand. As it moves along, it inserts and pre-mRNA molecule. Remember that there is no T in RNA, so an A on the DNA template ends up as a U in the RNA transcript.
- This last stage of transcription is called termination. As RNA polymerase moves along the DNA template, it eventually reaches the end of the gene, marked by nucleotides called a 3’ termination sequence. When the RNA polymerase reaches the termination sequence, it stops adding nucleotides to the pre-mRNA and falls off the DNA template strand. In the process, the pre-mRNA molecule is released and the DNA strands re-form a double helix.
Outline the process of RNA splicing.
- Messenger RNA is processed and spliced.
- These pre-mRNAs are processed in the nucleus to remove introns, which are nucleotide sequences present in genes that are not translated into the amino acid sequence of a protein.
- Introns occur between exons, the nucleotide sequences that are transcribed, joined to other exons during mRNA processing, and translated into the amino acid sequence of a protein.
- As introns are removed, the exons are spliced together to form mature mRNA molecules.
- During processing, the ends of the mRNA are modified. A nucleotide cap – a modified base (guanine nucleotide) attached to the 5’ end of eukaryotic mRNA molecules – is added at the 5´ end of the mRNA, and a tail of 30 to 100 A nucleotides called a poly-A tail is added at the 3´ end.
- The poly-A tail – a series of A nucleotides added to the 3’ end of mRNA molecules – aids in export of the mRNA from the nucleus and plays a role in translation.
- The final product, a mature, processed mRNA, moves through a pore in the nuclear envelope into the cytoplasm, where translation takes place.
What are ribosomes?
- Ribosomes are cytoplasmic organelles with a large and a small subunit and are the site of polypeptide synthesis.
- Ribosomes can float in the cytoplasm or attach to the outer membrane of the rough endoplasmic reticulum (rER).
- Each ribosome subunit contains proteins and a type of RNA called ribosomal RNA (rRNA).
- During translation, the rRNA in the large subunit acts as an enzyme, linking amino acids together to form a polypeptide.
- Transfer RNA (tRNA) molecules bring amino acids to the mRNA ribosome complex during translation. Each tRNA molecule has two attachment sites:
I. A nucleotide sequence of three nucleotides called an anticodon that pairs with a complementary codon sequence in mRNA
II. A site for attachment of the amino acid specified by the mRNA codon.
- tRNA’s deliver their attached amino acids to the ribosome in the order specified by mRNA codons.
- Once at the ribosome, the amino acids are linked together by rRNA to form a polypeptide chain.
Outline the process of translation.
Translation produces polypeptides from information in mRNA.
- Initiation begins when 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. Because AUG is the start codon and also encodes methionine, this amino acid is inserted first in all human proteins. Initiation is complete when a large ribosomal subunit binds to the complex.
- During elongation, amino acids are added to the growing polypeptide chain. Recall that during initiation, the initiator tRNA carrying methionine is added to the initiation complex, so each polypeptide chain begins with methionine. As elongation begins, a tRNA carrying the second amino acid (in this case, valine) pairs with the second mRNA codon, and the rRNA of the large subunit acts as an enzyme and forms a peptide bond between the two amino acids. As the ribosome moves along the mRNA, other tRNAs carrying amino acids pair with mRNA codons, adding amino acids to the growing polypeptide chain.
- Termination occurs when the ribosome reaches a stop codon. Recall that three codons (UAA, UAG, and UGA) do not code for amino acids, and there are no tRNA molecules with anticodons for stop codons. Proteins called release factors bind to stop codons, then the 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.
- A stretch of DNA
- A chromosomal locus
- The recipe for a protein (code + regulation)
- 2 x 25 000 alleles per cell
- Unit of transcription
Distinguish between DNA replication, DNA transcripion and DNA Translation
- DNA Replication: making DNA
I. Needs an enzyme: DNA polymerase
II. Needs an activated substrate: dNTPs
III. Needs a template: DNA
IV. 3-stage process: initiation, elongation, termination
- DNA Transcription: making RNA
I. Needs an enzyme: RNA polymerase
II. Needs an activated substrate: NTPs
III. Needs a template: DNA
IV. 3-stage process: initiation, elongation, termination
- DNA Translation: making a polypeptide
I. Needs an enzyme: ribosome
II. Needs an activated substrate: amino acids
III. Needs a template: mRNA
IV. 3-stage process: initiation, elongation, termination
Compare and contrast the ribosomes of prokaryotes and eukaryotes.
- Prokaryotes
I. 3 rRNAs + 56 proteins
II. 30s + 50s subunits
III. 70s ribosome
- Eukaryotes
I. 4 rRNAs + 82 proteins
II. 40S + 60S subunits
III. 80S ribosome
Identify and outline the different types of rRNA.
- rRNA (ribosomal RNA)
I. RNA polymerase I (>80%)
II. Few kinds
III. Many copies of each
- mRNA (messenger RNA)
I. RNA polymerase II (~2%)
II. 100,000s kinds
III. Few copies of each
- tRNA (transfer RNA)
I. RNA polymerase III (~15%)
II. 100, 000s kinds
III. Very many copies
- miRNA (microRNA)
- Noncoding RNA
Identify the factors which characterise protein synthesis in bacteria.
- Simple promoters
- Different transcription factors
- Single RNA polymerase
- Coupled transcription-translation
- No post-transcriptional processing
- Short-lived mRNAs
- Simpler ribosomes
- Distinctive translation initiation mechanism
- Different translation factors
Identify and outline how antibiotics affect protein synthesis in bacteria.
- Tetracycline
I. Binds 30S ribosomal subunit
II. Blocks aminoacyl tRNA binding
- Chloramphenicol
I. Binds 50S ribosomal subunit
II. Affects peptidyl transferase action
- Erythromycin
I. Binds 50S ribosomal subunit
II. Prevents translocation (A-site remains unoccupied)
