3.10 : Protein Synthesis Flashcards
Describe transcription
The section of DNA that contains the gene unwinds and unzips under the control of a DNA helicase, beginning at a start codon. This involves the breaking of hydrogen bonds between the bases.
Free RNA nucleotides will base pair with complementary bases exposed on the antisense strand when the DNA unzips. ( remember uracil binds to adenine ).
Phosphodiester bonds are formed between the RNA nucleotides by RNA polymerase.
Transcription stops at the end of the gene and the completed short strand of RNA is called mRNA.
This then detaches from the DNA template and leaves the nucleus through a nuclear pore. The DNA double helix reforms.
The mRNA then travels to a ribosome in the cell cytoplasm for the next step.
What are the sense and antisense strands?
The strand that contains the code for the protein is called the sense strand and it runs from 5’ to 3’.
The other strand (3’ to 5’) is a complementary cope of the sense strand and does not code for a protein. This is the antisense strand and it acts as the template strand during transcription.
This means that the RNA strand formed carries the same base sequence as the sense strand.
Describe translation
After leaving the nucleus, the mRNA binds to a specific site on the small subunit of a ribosome.
The ribosome holds mRNA in position while it is decoded, or translated, into a sequence of amino acids.
Transfer (t)RNA is composed of a strand of RNA folded in such a way that three bases, called the anticodon, are at one end of the molecule.
This anticodon will bind to a complementary codon on mRNA. The tRNA molecules carry an amino acid corresponding to that codon.
When the tRNA anticodons bind to complementary codons along the mRNA, the amino acids are brought together one by one and the polypeptide chain grows as this happens. This eventually forms the primary structure of the protein.
Describe the structure of ribosomes and the role of rRNA
Ribosomes are made up of two subunits : one large and one small.
Ribosomes act as the binding site for mRNA and tRNA and catalyse the assembly of the protein.
These subunits are composed of almost equal amounts of protein and a form of RNA known as ribosomal (r)RNA.
rRNA is important in maintaining the structural stability of the protein synthesis sequence and plays a biochemical role in catalysing the reaction.
What happens after translation?
The amino acids fold into secondary and tertiary structures. This folding and the bonds that are formed are determined by the sequence of amino acids in the primary structure.
The protein may undergo further modifications at the Golgi apparatus before it is fully functional.
