1.1.2 From DNA to protein : a life story

Question 1

What is DNA ?

Answer 1

• 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 )

Question 2

What are the four bases of DNA?

Answer 2

• Adenine ( A )
• Thymine ( T )
• Guanine ( G )
• Cytosine ( C )

Question 3

How can a nucleotide be identified?

Answer 3

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

Question 4

What is the point of the DNA code?

Answer 4

It forms the instructions used to make proteins.

Question 5

What is a triplet?

Answer 5

3 nucleotides that correspond to an amino acid

Question 6

What nitrogenous bases pair with what?

Answer 6

A is the opposite of at
c is the opposite of G

Question 7

How do the DNA strands run and directionality ?

Answer 7

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’

Question 8

What is 3’ and 5’ ?

Answer 8

3 prime end
5 prime end

Question 9

What organelle makes proteins?

Answer 9

Ribosomes

Question 10

Where do ribosomes live?

Answer 10

The cytoplasm or the rough endoplasmic reticulum ( RER )

Question 11

Where is DNA located in the cell?

Answer 11

The nucleus

Question 12

What cannot enter / exit the nucleus ?

Answer 12

DNA cannot leave

Ribosomes cannot enter

Question 13

How can we get the code to the ribosome ?

Answer 13

Need to make a copy of DNA than CAN leave the nucleus .

Transcription

Question 14

Describe Messenger RNA ( mRNA )

Answer 14

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

Question 15

What is transcription ?

Answer 15

The process by which DNA is read to create a copy of mRNA

Question 16

Describe how transcription works

Answer 16

1. 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
2. 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
3. Once the RNA polymerase has finished reading the gene ( segment of DNA ) it falls off. The new RNA is released.
4. The RNA cannot leave yet though , it has to undergo processing

Question 17

What is the point of RNA processing ?

Answer 17

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

Question 18

What is RNA processing also called?

Answer 18

Post transcriptional modification

Question 19

What are the steps of processing ?

Answer 19

1. Introduction of a 5’ methyl cap
2. Addition of a poly A tail on 3 ‘ end
3. Splicing

Question 20

Describe the 5’ methyl cap

Answer 20

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

Question 21

Describe the poly A tail

Answer 21

Added at the 3 prime end. It consists of long string of adenine nucleotides

Question 22

Describe what splicing is

Answer 22

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.

Question 23

What are exons?

Answer 23

Exons are what is left behind after splicing. They are expressed, they code

Question 24

What is alternative splicing ?

Answer 24

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

Question 25

What happens after the mRNA code is done?

Answer 25

It can leave the nucleus!!!
It goes and finds a ribosome

ribosomes are where proteins are made in the process of translation

Question 26

What does it mean for the code to be universal?

Answer 26

It is exactly the same in every living thing

Question 27

What does it mean for a code to be degenerate?

Answer 27

Most amino acids correspond to multiple codons . More codons than amino acids

Question 28

What are start and stop codons?

Answer 28

Where the protein translation takes place and ends

Question 29

Describe translation

Answer 29

1. mRNA meets ribosome
2. ribosome latches onto it and finds the start codon
3. A transfer RNA ( tRNA ) that is complementary to the start codon brings an amino acid!
4. Each tRNA has a three nucleotide code that is complementary to each RNA codon. The three nucleotides are an anti codon
5. Once the first amino acid is in place, the second codon brings along an amino acid
6. 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.
7. At stop codon the ribosome detaches and the protein synthesis is done