8.4 Polypeptide synthesis: Transcription and splicing Flashcards
How does protein synthesis occur
. DNA provides instructions in the form of a long sequence of bases
. A complementary section of part of this sequence is made in the form of pre-mRNA
. The pre-mRNA is then spliced to make mRNA
.The mRNA is used as a template to which complimentary tRNA molecules attach and the amino acids they carry are linked to form a polypeptide
Cytoplasm contains biochemical machinery for each organism to make its unique proteins
what does this rely on
What does the type of protein made rely on
It relies on the instructions provided by the DNA in the cells nucleus
The machinery has the capacity to to make every protein from just 20 amino acids
The protein made relies on the genetic code
What is a good way to remember how proteins are made?
Link to bakery
How does it link to transcription and translation
. Bakery contains equipment like ovens (cell organelles) to make cakes (proteins) from ingredients (amino acids) by following a recipe (genetic code)
This recipe can be from a recipe book (Genome)
And taking a photocopy of this recipe to use in the bakery is transcription.
Actually making the cakes with this photocopied recipe is translation
What is transcription? What is made
The process of making pre-mRNA using part of the DNA as a template
Describe the process making pre-mRNA in transcription
1) An enzyme acts on a specific region of the DNA which causes the two strands to separate.
This exposes the nucleotide bases in that region
2) The nucleotide bases on one of the two DNA strands, known as the template strand, pair with their complimentary RNA nucleotides from the pool which is present in the nucleus
. The enzyme RNA polymerase moves along the strand to join these up to form a pre-mRNA molecule
Eg an exposed guanine base on template will pair to a cytosine free RNA nucleotide
Exposed thymine will pair to adenine
But exposed adenine will pair to uracil
3) As the RNA polymerase adds the nucleotides one at a time to build the strand of pre-mRNA, the DNA strands rejoin behind it,
so only about 12 base pairs on DNA are exposed at any one time
4) When the RNA polymerase reaches a particular sequence of bases on the DNA (the terminator sequence), it detaches and the production of pre-mRNA is complete
How many base pairs are exposed on DNA at any one time during transcription
As the RNA polymerase adds the nucleotides one at a time to build the strand of pre-mRNA, the DNA strands rejoin behind it,
so only about 12 base pairs on DNA are exposed at any one time
What happens to pre-mRNA after transcription and what is the process called
Splicing then occurs, in eukaryotic cells
pre-mRNA is spliced to form mRNA
. The introns in the chain are cut out as they do not code for proteins
This leaves behind the exons which do code for proteins
The introns are cut out because they would prevent the synthesis of a polypeptide.
So then the exons are joined together in splicing.
This means the mRNA is now small enough to leave nucleus through nuclear pores, where it then finds ribosomes to attach to
Why does splicing not occur in prokaryotic cells
They do not contain introns, only exons
so it is unnecessary to splice their DNA
