Polypeptide synthesis - transcription and splicing Flashcards
Briefly, describe how is protein manufactured?
- DNA provides the instructions in the form of a long sequence of bases.
- A complementary strand of this part of the DNA is made in the form of a molecule called pre-mRNA through a process called transcription.
- The pre-mRNA is spliced to form mRNA.
- mRNA is used as a template to which complementary tRNA molecule attaches and the amino acid which is carried on the tRNA can join to form a long chain of polypeptide in a process called translation.
How can you describe protein manufacturing in terms of like baking a cake?
When you bake a cake, basic equipment and ovens (cell organelles) are required and by using that they can make lots of different types of cake (protein) from a few basic ingredients (amino acids) and which particular cake is made depends on the recipe (genetic code).
By choosing different recipes at different times rather than making everything all at one time, the baker can meet seasonal demands and change according to demands.
DNA replication is like a published recipe book (genome), by making a photocopy of a recipe in the bakery (transcription). Making the cake by using the photocopied recipe is translation and if the original book is not removed from the library it can be used over and over again.
What is transcription?
Process of making pre-mRNA using parts of the DNA as a template.
How does transcription work?
- An enzyme acts on a specific region of the DNA causing the 2 strands to separate and expose the nucleotide bases in that region.
- One of the strands with the nucleotide bases is the template strand and it will pair up with their complementary nucleotide from the pool which is present in the nucleus. e.g. exposed G base will bind to C base of a free nucleotide.
- Enzyme RNA polymerase moves along the strand and joins the nucleotides together to form a pre-mRNA molecule.
- As the RNA polymerase adds nucleotides one at a time to build a strand of pre-mRNA, the DNA strands rejoin from behind. So only a short strand about 12 base pairs on the DNA is exposed at any time.
- When the RNA polymerase reaches the stop triplet code on the DNA strand, it detaches and the pre-mRNA is made.
What’s the difference of transcription in prokaryotic cells and eukaryotic cells?
Prokaryotic cells go through transcription where mRNA is made directly from DNA.
Eukaryotic cells go through transcription where pre-mRNA is made, then spliced into mRNA.
What would introns do to the synthesis of a polypeptide?
Exons codes for proteins in a gene but introns don’t and those introns would prevent the synthesis of a polypeptide.
What is splicing?
Where the base sequence that corresponds to the introns are removed and the functional exons are joined together by splicing.
Why is splicing not needed in prokaryotic cells?
They do not have introns so splicing of their DNA is not needed.
What happens to the mRNA after been spliced?
They are too big to diffuse out of the nucleus so they leave by the nuclear pore. Outside the nucleus, mRNA is attracted towards the ribosomes and the. become attached for the next stage - translation.
