DNA, genes and protein synthesis

Question 1

What is a gene?

Answer 1

• A gene is a section of DNA that contains the coded information for making polypeptides and functional RNA.

Question 2

What is a locus?

Answer 2

• The particular position where a gene is located.

Question 3

What is a triplet?

Answer 3

• It is a combination of three bases which codes for one amino acid.

Question 4

Why must there be a minimum of three bases that code for each amino acid?

Answer 4

• There are only 20 amino acids
• Each amino acid must have its own code of bases on the DNA
• Only four different bases are present.
• If each base coded for a different amino acid, there would only be four amino acids.
• If pair of bases were used 16 codes would be possible, which isn’t enough for the 20 amino acids
• Using three bases, 64 different codes could be produced. This is enough to satisfy the 20 amino acids requirement.

Question 5

If there are 64 different triplets but only 20 amino acids, what do the rest of the triplets code for?

Answer 5

• Some amino acids are coded for by more than one triplet. The code is hence known as ‘degenerate code’ because most amino acids are coded by more than one triplet.
• Three triplets are known as ‘stop codes’ and mark the end of a polypeptide chain.

Question 6

What is meant by the code being non-overlapping?

Answer 6

• The base sequence is read only once.

Question 7

What is meant by the code being universal?

Answer 7

• Each triplet codes for the same amino acid in all organisms.

Question 8

What is a stop code?

Answer 8

• There are 3 triplets which code for the end of a polypeptide chain instead of for another amino acid

Question 9

What is meant by degenerate code?

Answer 9

Most amino acids are coded for by more than one triplet.

Question 10

What are exons and introns?

Answer 10

• Exons are coding sequences
• Introns are non-coding sequences

Question 11

What is the difference between prokaryotic DNA and Eukaryotic DNA?

Answer 11

• Euk is associated with proteins molecules called histones while pro are not associated with proteins
• Euk form chromosomes but pro does not
• Euk are longer molecules than Pro
• Euk are linear and form a line while Pro form a circle

Question 12

What is the structure of chromosomes?

Answer 12

• DNA is wrapped around proteins called histones
• Only visible as distinct structures when a cell is dividing and for the rest of the time chromatic are widely dispersed throughout the nucleus.
• Appear as two threads (chromatids) and they are exact copies of each other
• They are joined at a single point called the centromere

Question 13

What are homologous chromosomes?

Answer 13

• Homologous pair is always 2 chromosomes that carry the same genes but not necessarily the same alleles of the genes.

Question 14

What is an allele?

Answer 14

• Different form of the same gene

Question 15

Describe the structure of mRNA

Answer 15

• Pentose sugar and phosphate backbone
• 4 organic bases
• Long single helix
• Manufactured in the nucleus
• Acts as a template during protein synthesis
• Leaves nucleus to function in cytoplasm

Question 16

How is mRNA adapted for its job?

Answer 16

• mRNA codons code for amino acids to make polypeptides
• easily broken down so only exist when required
• single strand so able to leave through nuclear pore

Question 17

Describe the structure of tRNA

Answer 17

• Ribose sugar and phosphate backbones
• 4 organic bases
• Small single strand folded into clover shape
• Amino acid carried by extended chain
• Manufactured in the nucleus
• Contains the anticodon to mRNA’s codon and an attachment site there
• Has a point of attachment for the amino acid

Question 18

How is tRNA adapted for its job?

Answer 18

• Anticodon has complimentary base pairing to codon on mRNA
• Carries an amino acid to form the polypeptide chain

Question 19

What are the differences between mRNA and tRNA?

Answer 19

• Relative size -> tRNA is smaller than mRNA
• Shape -> tRNA has a clover shaped molecule and mRNA has a single helix molecule
• Chemical stability -> tRNA is more stable than mRNA
• Bonding -> tRNA has hydrogen bonding and mRNA bonding does not

Question 20

What are the steps to transcription?

Answer 20

• DNA helicase acts on the hydrogen bonds between the two strands in a specific gene to separate and expose the nucleotides bases in that region.
• The nucleotide bases on the template strand, pair with their complimentary ‘free’ nucleotides.
• The enzyme RNA polymerase joins the nucleotides along the strand with phosphodiester bonding in a condensation reaction to form a pre mRNA molecule
• As the RNA polymerase adds the nucleotides, the DNA strands re-join behind it so that only 12 base pairs on the DNA are exposed at one time.
• The RNA polymerase will recognise the ‘stop’ triplet code and detach.

Question 21

What are the two processes of protein synthesis?

Answer 21

• Transcription
• Translation
  (in that order)

Question 22

What is splicing ?

Answer 22

• Splicing turns pre mRNA to mRNA
• It only happens in eukaryotic cells
• It does this by removing the base sequences that code for the introns and joining together the functioning extrons.

Question 23

Why is splicing important?

Answer 23

• Pre mRNA is too large to diffuse out of the nuclear pores but mRNA is not.
• mRNA is attracted to the ribosomes to which it becomes attached

Question 24

What are the steps of translation?

Answer 24

• a ribosome becomes attached to the starting codon on mRNA
• the tRNA molecule with the complementary anticodon sequence moves to the ribosome.
• the tRNA molecule pairs up with its complementary codon on mRNA.
• A tRNA molecule with a complementary anticodon pairs up with the next codon on mRNA.
• The ribosome moves along the mRNA bringing together 2 tRNA molecules at any one time.
• In the presence of an enzyme and ATP the amino acids on the tRNA molecules are joined by a peptide bond.
• As the chain of amino acids become formed the tRNA at the start of the chain is released from its amino acid and is free to collect another amino acid from the amino acid pool in the cell.
• As each tRNA molecule with its complementary anticodon moves along the mRNA molecule further amino acids form further peptide bonds.
• This process continues until a ribosome reaches a stop codon
• At this point, the ribosome, mRNA and last remaining tRNA molecule all separate and the polypeptide chain is complete.

Question 25

What are the different phases of mitosis?

Answer 25

• Prophase
• Metaphase
• Anaphase
• Telephase

Question 26

What happens during prophase?

Answer 26

• Chromosomes become visible and shorten and thicken.
• Centrioles move to opposite poles of the cell
• Spindle fibres begin to develop from the centrioles
• The nucleolus disappears and the nuclear envelope breaks down.
• The chromosomes are drawn towards the equator of the cell by the spindle fibres.

Question 27

What is metaphase?

Answer 27

• Spindle fibres attach to centromere on each chromosomes
• Sister chromatids line up along the equator

Question 28

What is Anaphase?

Answer 28

• The centromeres divide into two and the spindle fibres pull the individual chromatids making up the chromosome apart.
• The chromatids move rapidly to their pole and are now referred to as chromosomes.
• The energy for the process is provided by mitochondria, which gather around the spindle fibres.

Question 29

What is telophase?

Answer 29

• Chromosomes reach their poles and become longer and thinner, later disappearing and leaving behind widely spread chromatin.
• The spindle fibres disintegrate and the nuclear envelope and nucleolus re-form.

Question 30

What is cytokinesis?

Answer 30

• The division of the cytoplasm

Question 31

What is meant by a genome?

Answer 31

• Complete set of genes in a cell

Question 32

What is meant by a proteome?

Answer 32

• The full range of proteins that a cell is able to produce?

Question 33

What are the three stages of the cell cycle?

Answer 33

• Interphase -> which occupies most of the cell cycle, and is sometimes known as the resting phase because no division takes place
• Nuclear division -> When the nucleus divides either into two (mitosis) or into four (meiosis)
• Division of the cytoplasm (cytokinesis) -> which follows nuclear division and is the process by which the cytoplasm divides to produce two new cells (mitosis) or four new cells (meiosis)

Question 34

What is cancer?

Answer 34

• It is a group of diseases caused by the damage to a gene that regulates mitosis and the cell cycle
• It leads to uncontrollable growth called a tumour.
• Benign tuours geow slowly, are more compact and less likely to be life threatening while malignant tumours grow rapidly, are less compact and are more likely to be life threatening.

Question 35

How can cancer be treated?

Answer 35

• drugs can block certain parts of the cell cycle. EG:
• It can prevent DNA from replicating
• inhibiting the metaphase stage of mitosis by interfering with spindle formation.
• These drugs are more effective against cancer cells because they are the ones which are dividing rapidly and are damaged to a greater degree. Normal cells that divide frequently also are damaged EG hair ect.

Question 36

Describe the cell division in prokaryotic cells and viruses

Answer 36

• This process is called binary fission
• The circular DNA molecule replicates and both copies attach to the cell membrane
• The plasmids also replicates
• The cell membrane begins to grow between the two DNA molecules and begins to pinch inwards, dividing the cytoplasm into two
• A new cell wall forms between the two molecules of DNA, dividing the original cell into two identical daughter cells, each with a single copy of the circular DNA and a variable number of copies of the plasmids

Question 37

What are the different phases within interphase?

Answer 37

First Growth Phase
Growth and normal metabolic roles
Synthesis Phase
DNA replication
Second Growth Phase
Growth and preparation for mitosis

Question 38

Describe an experiment to identify stages of mitosis from a plant?

Answer 38

1. Heat 1 mol dm-3 HCl at 60°C in a water bath.
2. Cut a small sample of the root tip using a scalpel.
3. Transfer root tip to HCl and incubate for 5 minutes.
4. Remove from HCl and wash sample in cold distilled water and remove the very
   tip using a scalpel.
5. Place tip on a microscope slide and add a few drops of stain (e.g. toluidine blue
   O). This makes the chromosomes visible and will therefore show which cells
   are undergoing mitosis.
6. Lower the cover slip down carefully onto the slide. Make sure there are no air
   bubbles in the slide which may distort the image, and that the coverslip doesn’t
   slide sideways which could damage the chromosomes.
7. Place under a microscope and set the objective lens on the lowest
   magnification.
8. Use the coarse adjustment knob to move the lens down to just above the slide.
9. Use the fine adjustment knob to carefully re-adjust the focus until the image is
   clear (you can use a higher magnification if needed).
10. To calculate mitotic index, cells undergoing mitosis must be counted (cells
    with chromosomes visible), as well as the total number of cells.

Question 39

What is the mitotic index and how is it calculated?

Answer 39

The mitotic index of a sample is the ratio of cells undergoing mitosis to the total number of
cells in a sample.
Mitotic index = number of cells with visible chromosomes / total number of cells in the sample

Question 40

How to find the actual size of a cell under a microscope?

Equation

Answer 40

Acutal size = size of image / magnification

Question 41

How do viruses replicate?

Answer 41

• Viruses attach to the surface of host cell
• Viruses inject their DNA into cell
• Replication takes place -> viral DNA / RNA replicates and new viral proteins are made (forming new capsids)
• New viral particles are assembled
• Host cell lyses (bursts) releasing the newly made viruses or particles leave individually through cell membrane via ‘budding’, often taking sections of membrane with them, which is the envelope surrounding some viruses.