DNA replication and protein synthesis Flashcards
Why is DNA replication known as semi conservative?
Each new DNA molecule formed contains one old strand of DNA and one new strand.
What is the benefit of SCR?
Ensures that there is genetic continuity (new cells inherit all the genes in the parent cell)
How is SCR carried out?
- DNA helicase causes the two strands of DNA to separate by catalysing the reaction that breaks the hydrogen bonds between the bases
- Free DNA nucleotides are attracted to their complementary bases, the DNA strand acts as a template for the formation of a new strand.
- DNA polymerase catalyses the formation of phosphodiester bonds between the adjacent nucleotides
- Two new, identical molecules of DNA are formed
What is a mutation and what is the effect of it?
Random and spontaneous errors leading to a change in the sequence of bases.
Change in triplet code- change in the sequence of amino acids- change in structure of the protein (eg shape of active site)- change in function (substrate cant bind)
What is a codon?
Three bases, coding for an amino acid
What is a gene?
A section of DNA that contains the complete sequence of bases to code for a protein.
What are the characteristics of the genetic code?
It is universal- all organisms use it
It is degenerate- different combinations of bases can code for the same amino acid (64 base triplets available, 20 amino acids)
It is non overlapping- no base is read more than once, a start codon ensures that the codons are read “in frame” (from base 1)
What is the problem with transcription and how is it overcome?
DNA is contained within a nuclear envelope inside the nucleus to ensure that it doesn’t get damaged. DNA is too large to leave the nucleus, but protein synthesis takes part in the ribosomes in the cytoplasm. To get around this problem, the base sequence of genes must be transcribed and transported to the ribosomes, via mRNA.
Which is the template strand?
Only the sense strand has the code for the protein to be synthesised, the antisense strand is a complementary copy and does not code for a protein.
The antisense strand acts as a template during transcription so that the mRNA has the same base sequence as the sense strand.
Describe the process of transcription
- DNA helicase unwinds the double helix by breaking the hydrogen bonds between the bases
- Free RNA nucleotides base pair complementarily with the antisense strand (U instead of T)
- RNA polymerase catalyses the formation of phosphodiester bonds between the nucleotides
- RNA polymerase detaches from the DNA and it goes back into its double helix structure
- mRNA detaches from the DNA template and leaves the nucleus through the nuclear pore to go to the ribosomes
Describe the process of translation
- mRNA binds to the small subunit on a ribosome at its start codon
- tRNA brings a complementary anticodon to bind to the start codon, and the amino acid corresponding to that codon
- a maximum of two tRNAs can be bound at the same time so the ribosome moves along the mRNA, releasing one tRNA at a time
- peptide bonds are formed between the amino acids (catalysed by peptidyl transferase- RNA component of the ribosome)
- the ribosome reaches the stop codon and the polypeptide is released
What are ribosomes made up of?
One large and one small subunit. Have rRNA (important in catalysing)
What is the structure of tRNA?
A strand of RNA folded in a way that on one end, there is an anticodon and on the other end, there is a point of attachment for an amino acid.
