Storing and using genetic information Flashcards
Describe the double helical structure of DNA.
DNA is a double helix composed of a sugar phosphate backbone that is flexible which allows the base pairs to connect.
The bases complementary pair as follows:
Adenine to Thymine
Guanine to Cytosine.
Describe the process by which DNA is replicated
DNA replication is semi-conservative meaning that one half of each new molecule of DNA is old: one half new.
Special protein (helicase) unzips the double helix by breaking the H bonds. It does this by working as a “corkscrew”.
New nucleotide molecules are then paired with the DNA strands.
They are replicated in 5’ to 3’ direction.
Describe how a base sequence in DNA is transcribed into a base sequence in RNA.
For transcription to occur, the 2 DNA strands need to temporarily separate, which allows the enzymes to access the DNA template.
RNA polymerase enzyme binds to the gene and tells it to unwind so it can read the bases in the DNA strand.
= The enzyme is now ready to make a strand of DNA with a complementary sequence of bases.
During the process of building the mRNA molecule, there is a brief time where the newly formed RNA is bound to the unwound DNA.
Following this, there is termination of transcription as the RNA polymerase crosses a stop sequence.
The mRNA strand is then complete and detaches from DNA.
It can then leave the nucleus.
Describe, in outline, the post-transcriptional processing of RNA
Following the creation of the mRNA molecule, and the removal of introns (that don’t code for protein) by alternative splicing, the tRNA translates the mRNA sequence into an amino acid sequence.
The tRNA acts as an adaptor molecule between the coded amino acid and the mRNA.
(rRNA (which is a component of ribosome and produced in the nucleus) is transported to the cytoplasm where it combines with proteins to form a ribosome)
Describe the initiation part of the translation of RNA
Translation occurs in 3 phases.
1) Initiation - a tRNA molecule with the amino acid Methionine attached at the top and a codon at the bottom interacts with and binds to the start codon on mRNA. This process happens by a large ribosomal unit on which the tRNA are help in place when needed and are discarded when not.
Overall outcome of this phase: There is an amino acid ready to start a polypeptide.
Describe the elongation part of translation of RNA.
2) Elongation - the ribosome moves along the mRNA, one codon at a time, this is called a translocation event. (During each translocation event the chargeds tRNAs move from the A site to the P site and then the E site for removal. This is happening in a direction towards 3’.
Peptide bonds form between the N terminus of the amino acid at the A site and the C terminus of the amino acid at the P site. The formation of these bonds is catalysed by peptidyl transferase.
Describe the process of termination in translation of RNA.
3) Termination - the termination of translation occurs when a stop codon (nonsense codon) is encountered. The process stops because there is no complementary tRNA for these stop codons. This encounter of the stop codons causes the amino acid at the P site to detach from its tRNA, releasing the polypeptide.
The ribosomal subunits dissociate.
