Lecture 5: Protein Synthesis, Transcription & Translation Flashcards
Describe RNA polymerases.
Three types (I, II, III)
Requires a DNA template but not a primer for action (DNA dependent)
Forms a large transcription initiation complex with ~50 other proteins.
RNA polymerase I transcribes most rRNA genes
RNA polymerase II transcribes all protein coding genes
RNA polymerase III transcribes tRNA genes
Describe genes.
Genes are the basic physical and functional units of heredity. They are composed of DNA sequences, and carry the information needed for constructing proteins.
The nucleus of a human cell contains 30-40 thousand genes. A complete set of genes is called the genome (human genome has 46 chromosomes).
Genes contain exons (protein coding section of gene-expressed sequences), introns (non-protein coding - intervening sequences) and control elements for DNA transcription and translation.
Genes associated with a particular function can be located on completely different chromosomes.
What is transcription?
Transcription is the copying of DNA into a complimentary strand of RNA (mRNA, tRNA, rRNA)
Describe the process of mRNA transcription (3 steps).
- Initiation: RNA polymerase binds to the promoter region of the DNA and separates the strands. It innitiates mRNA synthesis at the start point of the template strand.
- Elongation: the RNA polymerase moves along the template strand and elongates the mRNA transcript one base at a time. At the same time, it unwinds the DNA double helix behind it.
- Termination: mRNA synthesis ends when the termination signal is reached. RNA polymerase and the completed mRNA transcript are released.
What is the DNA-RNA hybrid?
The DNA-RNA hybrid region consists of the unwound DNA, and the RNA nucleotides.
At any given moment, 16-18 base pairs of DNA are unwound and the most recently made RNA is still bound to DNA. This small region is the DNA-RNA hybrid.
Describe mRNA processing.
Occurs in the nucleus. Involves capping, splicing and polyadenylation.
CAPPING: addition of a modified nucleotide at the 5’ end. Protects against degradation by ribonucleases, and occurs while transcription is still in progress. The cap binds to specific proteins and is responsible for attaching the RNA to the ribosome and initiating translation. Also protects against nucleases.
SPLICING: removal of intervening sequences (introns) from the mRNA.
POLYADENYLATION: addition of a tail of adenine at the 3’ end. Important for mRNA stability.
Describe RNA splicing.
Alternative splicing results in 2 or more different products from the same gene. This increases the coding potential of the genome and adds complexity to the genome.
Describe the genetic code.
A sequence of 3 nitrogenous bases specifies a single amino acid and is called a codon. (encoded information must be read in groups of three).
More than one triplet may represent the dame amino acid (code is degenerate). There are 64 different codons, which code for a total of 20 amino acids.
Describe mRNA.
Codons of the mRNA specify the sequence of amino acids in the protein.
mRNA attaches to the ribosome for translation, and binds each tRNA via its complimentary anticodon.
What is rRNA?
Ribosomal RNA plus protein is known as a ribosome.
Describe ribosomes.
Site of translation.
Have large and small subunit (consisting of proteins and rRNA).
Has mRNA binding site and three tRNA binding sites (E-, P-, and A- sites.
Describe tRNA.
Has anticodon region, which contains 3 nitrogenous bases that are complimentary to a specific mRNA codon. It is a tight loop midway along the molecule, and specifies the amino acid that tRNA contributes to the protein.
tRNA tail binds to its specific amino acid.
Describe translation.
- Initiation: mRNA binds to 40S subunit of ribosome, and the ribosome locates start codon (AUG). Initiator tRNA binds carrying formyl-Met. 60S subunit binds to complete initiation complex. fMet is in P site.
- Elongation: next tRNA enters A site. fMet attaches to the next AA and is released from its tRNA. The two bound tRNA move along one position in the 5’ direction, and the first tRNA is released from the E site. Another tRNA binds to the A site codon.
- Termination of translation: the ribosome reaches a stop codon, and a release factor binds to the A site. The polypeptide chain is released with the help of the release factor. Ribosome-mRNA complex is dismantled.
