Session 1: Transcription and Translation Flashcards
What are the three steps of protein synthesis in eukaryotic cells?
- Transcription of mRNA
- mRNA processing
- Translation
What is a gene, and what are it’s three main parts?
A sequence of DNA that is transcribed into RNA. It’s main parts are promoter, coding sequence, and terminator. A promoter is the non-coding sequence responsible for the initiation of transcription (binding site for RNA polymerase). A coding sequence is The region of DNA that is transcribed by RNA polymerase. And the terminator is the sequence that is responsible for terminating transcription.
Is the anti-sense or sense strand transcribed?
The antisense strand is transcribed and its sequence is complementary to RNA transcript.
Describe the transcription of mRNA.
RNA polymerase attaches to the promoter on the Anti-Sense or Template strand and separates the DNA strands. It then moves along the DNA in the 3’ to 5’ direction, pairing up RNA nucleotides with their DNA complements and adding them to the 3’ end of the growing RNA molecule. Once RNA polymerase has gone past the terminator, the enzyme releases the completed mRNA and detaches from the DNA.
Describe mRNA Processing.
The ends of the RNA do not code for a protein. RNA processing begins with alteration of these ends. A 5’ cap is added to the beginning of the RNA transcript, and a 3’ poly-A tail is added to the end. The next step is the removal of portions of the coding segment that do not actually code for protein. These are called introns. The remaining sections of the mRNA are spliced together (in a variety of ways) and are called exons. The processed mRNA then travels to the ribosomes in the cell (rough ER or cytoplasm).
Describe ribosomes (the site of translation).
They are made of protein (stability) and rRNA (catalytic). Ribosomes contain two distinct subunits: Small subunit contains an mRNA binding site, Large subunit contains three tRNA binding sites (A, P, E). Ribosomes exist freely in the cytosol or are bound to rough ER. They can differ in size.
Describe Transfer RNA molecules used for translation.
Transfer RNA carries specific amino acids to the ribosome. They have four key regions:
Acceptor stem (carries the amino acid)
Anticodon (complementary to an mRNA codon)
T arm (associates with the ribosome)
D arm (associates with a tRNA-activating enzyme)
How do you get a “charged” tRNA?
tRNA-activating enzymes join ATP to an amino acid which creates a ‘charged’ amino acid–AMP complex. The phosphorylated amino acid is then linked to a specific tRNA molecule and the AMP is released. The energy in the ‘charged’ amino acid is then used for peptide bond formation
What are the four main processes of translation? Define them.
Initiation: ribosomal complex surrounds mRNA (small subunit and large subunit of ribosomes with mRNA in the middle). Elongation: A new amino acid is added to a developing peptide chain based on the mRNA codon and tRNA anticodon match. Translocation: The ribosome moves to the next codon position.
Termination: Ribosomal complex and polypeptide dissociate from mRNA. The processes of elongation and translocation are repeated as the ribosome moves along the transcribed mRNA sequence in a 5’ → 3’ direction
Describe genetic code.
Peptides (proteins) are formed from 20 different amino acids in different orders and combinations. The mRNA codons code for particular amino acids. AUG codes for methionine, the starting amino acid for all peptide chains. Three special base triplets – UAA, UAG, and UGA – do not code for amino acids, but instead act as stop codons. Many amino acids have more than one codon.
Describe how polypeptides fold into unique shapes and the affect of this.
Polypeptides fold into unique shapes which may be essential to their function and role (E.g. the active site on an enzyme). These shapes are based on the relative position and the chemical properties of each amino acid. Their final structure is therefore determined by the sequence of amino acids that was coded for in the gene that was transcribed and translated.
Describe non-coding DNA.
The vast majority of the human genome is comprised of non-coding DNA (genes only account for ~ 1.5% of the total sequence). Historically referred to as ‘junk DNA’, these non-coding regions are now recognised to serve other important functions. Examples include satellite DNA, telomeres, introns, ncRNA genes and gene regulatory sequences
Overall what is transcription and translation?
Transcription is when DNA makes RNA (mRNA). And translation is when mRNA matches up with tRNA and makes proteins.
