1.1.2 From DNA to protein : a life story Flashcards
What is DNA ?
- DNA is a molecule made of a string of nucleotides. It is wrapped around in a double helix.
- DNA stands for deoxyribonucleic acid
- DNA nucleotides can have four bases ( Adenine , Thymine, Guanine and Cytosine )
What are the four bases of DNA?
- Adenine ( A )
- Thymine ( T )
- Guanine ( G )
- Cytosine ( C )
How can a nucleotide be identified?
By it’s base and the order of the nucleotide bases. These constitute the DNA code.
E.g. if there is an A, then a C then a T nucleotide the code is ACT
What is the point of the DNA code?
It forms the instructions used to make proteins.
What is a triplet?
3 nucleotides that correspond to an amino acid
What nitrogenous bases pair with what?
A is the opposite of at
c is the opposite of G
How do the DNA strands run and directionality ?
The two strands sit opposite each other. The strands run anti parallel . This means that one strand points up and the other down.
One strand runs in a 3 ‘ -> 5’ and the opposite strand is 5’ -> 3’
What is 3’ and 5’ ?
3 prime end
5 prime end
What organelle makes proteins?
Ribosomes
Where do ribosomes live?
The cytoplasm or the rough endoplasmic reticulum ( RER )
Where is DNA located in the cell?
The nucleus
What cannot enter / exit the nucleus ?
DNA cannot leave
Ribosomes cannot enter
How can we get the code to the ribosome ?
Need to make a copy of DNA than CAN leave the nucleus .
Transcription
Describe Messenger RNA ( mRNA )
mRNA is a molecule that is a copy of DNA code . It CAN exit the nucleus and bring instructions for building a protein to the ribosome
What is transcription ?
The process by which DNA is read to create a copy of mRNA
Describe how transcription works
- DNA comes in a double helix. To be able to read the DNA code it needs to unwind . The enzyme helicase binds to the strand of DNA to do that
- RNA polymerase ( an enzyme responsible for copying DNA sequence into RNA ) attaches to the DNA strand and reads the DNA. While it is reading it builds an RNA molecule complementary to the DNA . in RNA thymine doesn’t exist so U is complementary to A
- Once the RNA polymerase has finished reading the gene ( segment of DNA ) it falls off. The new RNA is released.
- The RNA cannot leave yet though , it has to undergo processing
What is the point of RNA processing ?
Makes the RNA stronger less likely to break down
Increase diversity of RNAs that can be made from the one gene. Helps the cell interpret the same DNA in different ways
What is RNA processing also called?
Post transcriptional modification
What are the steps of processing ?
- Introduction of a 5’ methyl cap
- Addition of a poly A tail on 3 ‘ end
- Splicing
Describe the 5’ methyl cap
It is attached to the 5 prime end of the new RNA . It is a special nucleotide that has been modified to prevent the RNA from being eaten up
Describe the poly A tail
Added at the 3 prime end. It consists of long string of adenine nucleotides
Describe what splicing is
Splicing is removing chunks of RNA. The chunks are called introns . They are in the way, they are an interruption they do not code for anything.
What are exons?
Exons are what is left behind after splicing. They are expressed, they code
What is alternative splicing ?
Cells can change the splicing pattern of an mRNA so that some introns can become exons and vice versa. This allows the cell to produce multiple different proteins from the one DNA code
What happens after the mRNA code is done?
It can leave the nucleus!!!
It goes and finds a ribosome
ribosomes are where proteins are made in the process of translation
What does it mean for the code to be universal?
It is exactly the same in every living thing
What does it mean for a code to be degenerate?
Most amino acids correspond to multiple codons . More codons than amino acids
What are start and stop codons?
Where the protein translation takes place and ends
Describe translation
- mRNA meets ribosome
- ribosome latches onto it and finds the start codon
- A transfer RNA ( tRNA ) that is complementary to the start codon brings an amino acid!
- Each tRNA has a three nucleotide code that is complementary to each RNA codon. The three nucleotides are an anti codon
- Once the first amino acid is in place, the second codon brings along an amino acid
- The ribosomes join together the first two amino aids. The first tRNA leaves whilst a third latches onto the third codon. The cycle repeats until a stop codon.
- At stop codon the ribosome detaches and the protein synthesis is done
