4A: Genetic information

Question 1

What are the base pairing rules (in DNA and RNA) ?

Answer 1

DNA:

• A joins to T (2 hydrogen bonds)
• C joins to G (3 hydrogen bonds)

RNA:

• A joins to U (U replaces T)
• G joins to C

Therefore, when discussing the proportions of a particular base, you will always find A=T and C=G

Question 2

Which end of DNA is DNA polymerase complementary to?

Answer 2

It is complementary to the 3’ end

= so it moves in one direction from 3’ to 5’

Question 3

Draw a nucleotide

Answer 3

Phosphate
|
Pentose —- Nitrogenous base
Sugar

Question 4

What are the functions of the antiparallel strands in DNA? (4)

Answer 4

• Maintains 2 copies of DNA in case of mutation
• Protects bases
• Required for semi-conservative replication
• Improves molecular stability

Question 5

Describe what a gene is

Answer 5

A gene is a base sequence of DNA that codes for:
• the amino acid sequence of a polypeptide
• a functional RNA (including ribosomal RNA and tRNAs).

Question 6

What do genes do?

Answer 6

Genes carry the information to produce the amino acid sequence of a polypeptide

Question 7

When discovering how DNA bases coded for amino acids, why did scientists suggest that there must be a minimum of 3 bases that coded for each amino acid?

Answer 7

Because:

• Only 20 different amino acids regularly occur in proteins
• Each amino acid must must have its own code of bases on the DNA
• Only 4 different bases (A,G,C,T) are present in DNA
• If each base coded for a different amino acid, only 4 different amino acids could be coded for.
• Using a pair of bases, 16 different codes are possible, which is still inadequate
• 3 bases produce 64 different codes, more than enough for 20 amino acids

Question 8

What is the code (3 bases for each amino acids) called?

Answer 8

Each one is called a triplet

Question 9

What are 2 features of genetic code relating to amino acids coding?

Answer 9

• A few amino acids are coded for by only a single triplet

- The remaining amino acids are coded for by between 2 and 6 triplets each

Question 10

What is a genome?

Answer 10

Complete set of genes in a cell

Question 11

What is a proteome?

Answer 11

Full range of proteins produced by the genome

Question 12

What is the function of RNA?

Answer 12

Transfers coded info from the DNA into the cytoplasm where it can be translated into proteins

Question 13

What is a codon?

Answer 13

A sequence of 3 bases that codes for an amino acid

Question 14

What are the 3 types of RNA?

Answer 14

Transfer - tRNA
Messenger - mRNA
Ribosomal - rRNA

Question 15

Describe mRNA

Answer 15

• The code for the gene in the DNA molecule is ‘transcribed’ (copied) into an mRNA molecule
• So mRNA carries the code for a particular polypeptide from the nucleus to the cytoplasm, it can travel through nuclear pores
• Triplets of bases on the mRNA = codons
• Single stranded helix
• Has a short lifetime (is broken down)

Question 16

Describe tRNA

Answer 16

• Each amino acid is attached to a tRNA with the appropriate anticodon
• Matches the triplet code in an mRNA molecule to their respective amino acid
• Short (only 80 nucleotides long)
• Looped clover-leaf structure
• On the middle loop there is a triplet base sequence called the anticodon –> it’s complementary to one of the 64 possible codons on the mRNA

Question 17

Describe rRNA & ribosomes

Answer 17

• Together with proteins forms ribosomes

- Ribosomes have 2 subunits, small and large, and are assembled in the nucleolus of the nucleus

Question 18

What is transcription?

Answer 18

The process of making pre-mRNA using part of the DNA as a template

Question 19

What is the DNA template strand also known as?

Answer 19

The coding strand

Question 20

If the coding DNA sequence is CGA, what will the mRNA and tRNA sequences be?

Answer 20

Coding DNA sequence: CGA
mRNA sequence: GCU
tRNA sequence: CGA

Question 21

Describe the (6) stages of transcription

Answer 21

1. RNA polymerase binds to a section of the DNA
2. The hydrogen bonds on the DNA molecule are broken (by DNA helicase which is attached to RNA polymerase) = exposing the bases
3. The nucleotides on one of the 2 DNA strands, known as the template strand, bind to complementary mRNA nucleotides present in the nucleus (C to G, A to U)
4. RNA polymerase moves along the strand, joining the nucleotides together to make a pre mRNA molecule
5. The DNA strands rejoin behind the newly forming strand of pre-mRNA
6. When the RNA polymerase reaches the stop codon (triplet code), it detaches and the pre-mRNA is complete

Question 22

DNA is made from sections called \_\_\_\_\_ (protein-coding)
and \_\_\_\_\_ (non- coding)

Answer 22

DNA is made from:
Exons = protein-coding
Introns = non-coding

Question 23

During transcription, the code from both _____ and _____ is found in the pre-mRNA

Answer 23

Introns and exons

Question 24

Why is splicing not required in prokaryotes?

Answer 24

Because prokaryotes DO NOT have introns so splicing is not required.

Question 25

Describe the splicing of pre-mRNA.

Answer 25

- The non-functioning introns are later removed from the sequence of the pre-mRNA - The remaining exons are joined together by splicing to form mature mRNA

Question 26

Describe how the same sequence of DNA can code for several different proteins. Hint: Exon combinations

Answer 26

The exon sections can be joined together in a variety of combinations, so the same sequence of DNA can code for several different proteins.

Question 27

What can affect the splicing of pre-mRNA?

Answer 27

Mutations

Question 28

Give an example of a disease that is caused by splicing failures.

Answer 28

Alzheimer’s disease is caused by splicing failures, so non-functional polypeptides are made

Question 29

What is translation?

Answer 29

The process of converting the codon sequence in the mRNA molecule into an amino acid sequence

Question 30

Describe how mRNA leaves the nucleus

Answer 30

mRNA leaves the nucleus via nuclear pore, then synthesises a polypeptide

Question 31

What are the (7) stages of translation? Hint: 1. mRNA leaves nucleus via nuclear pore after splicing

Answer 31

1. mRNA leaves nucleus via nuclear pore after splicing 2. A ribosome attaches to the start codon of the mRNA 3. A tRNA (with the complementary anticodon sequence to the first mRNA codon) pairs with the codon on the mRNA 4. The ribosome (which has 2 subunits, smaller one on the bottom of the mRNA) has 2 codon binding sites. So 2 tRNA molecules at a time are brought together 5. A peptide bond forms between the 2 amino acids on the tRNAs with the use of ATP (which is hydrolysed to provide energy) 6. The ribosome moves along the mRNA to the next codon so the first tRNA is released and a new tRNA arrives at the next codon 7. This continues with up to 15 amino acids being added each second until a polypeptide chain is built up + a stop codon is reached

Question 32

Describe DNA in prokaryotes

Answer 32

In prokaryotic cells, DNA molecules are short, circular and not associated with proteins

Question 33

Describe DNA in eukaryotes

Answer 33

- In the nucleus of eukaryotic cells, DNA molecules are very long, linear and associated with proteins, called histones. - Together a DNA molecule and its associated proteins form a chromosome.

Question 34

In eukaryotes, the mitochondria and chloroplasts contain what?

Answer 34

The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short, circular and not associated with protein.

Question 35

What is the fixed position that a gene occupies called?

Answer 35

A gene occupies a fixed position, called a locus, on a particular DNA molecule.

Question 36

Describe the genetic code in DNA

Answer 36

Universal, non-overlapping and degenerate.

Question 37

What is the main difference in translation in prokaryotes vs eukaryotes

Answer 37

* In prokaryotes, transcription results directly in the production of mRNA from DNA. * In eukaryotes, transcription results in the production of pre-mRNA; this is then spliced to form mRNA.

Question 38

Why do not all gene mutations result in a change in the sequence of amino acids

Answer 38

Due to the degenerate nature of the genetic code, not all base substitutions cause a change in the sequence of encoded amino acids

Question 39

What can mutations produce that meiosis cannot?

Answer 39

- Mutations can produce genetic novelty - Independent assortment and crossing over during meiosis and the random nature of fertilisation mixes up allelic combination possibilities but does not produce genetic novelty (no new alleles or genes are produced )

Question 40

Describe why DNA is 'degenerate'

Answer 40

Code is known as 'degenerate' because most amino acids are coded for by more than 1 triplet

Question 41

Describe why DNA is 'universal'

Answer 41

The same specific base triplets code for the same amino acids in all living things

Question 42

Which end of DNA is DNA polymerase complementary to?

Answer 42

3' end

Question 43

# Translate this DNA code into mRNA and tRNA code: ACGTGA

Answer 43

mRNA: UGCACU tRNA: ACGUGA