DNA, genes and protein synthesis Flashcards
1
Q
What is a gene?
A
- A gene is a section of DNA that contains the coded information for making polypeptides and functional RNA.
2
Q
What is a locus?
A
- The particular position where a gene is located.
3
Q
What is a triplet?
A
- It is a combination of three bases which codes for one amino acid.
4
Q
Why must there be a minimum of three bases that code for each amino acid?
A
- 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.
5
Q
If there are 64 different triplets but only 20 amino acids, what do the rest of the triplets code for?
A
- 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.
6
Q
What is meant by the code being non-overlapping?
A
- The base sequence is read only once.
7
Q
What is meant by the code being universal?
A
- Each triplet codes for the same amino acid in all organisms.
8
Q
What is a stop code?
A
- There are 3 triplets which code for the end of a polypeptide chain instead of for another amino acid
9
Q
What is meant by degenerate code?
A
Most amino acids are coded for by more than one triplet.
10
Q
What are exons and introns?
A
- Exons are coding sequences
- Introns are non-coding sequences
11
Q
What is the difference between prokaryotic DNA and Eukaryotic DNA?
A
- 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
12
Q
What is the structure of chromosomes?
A
- 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
13
Q
What are homologous chromosomes?
A
- Homologous pair is always 2 chromosomes that carry the same genes but not necessarily the same alleles of the genes.
14
Q
What is an allele?
A
- Different form of the same gene
15
Q
Describe the structure of mRNA
A
- 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
16
Q
How is mRNA adapted for its job?
A
- 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
17
Q
Describe the structure of tRNA
A
- 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
18
Q
How is tRNA adapted for its job?
A
- Anticodon has complimentary base pairing to codon on mRNA
- Carries an amino acid to form the polypeptide chain
19
Q
What are the differences between mRNA and tRNA?
A
- 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
20
Q
What are the steps to transcription?
A
- 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.
21
Q
What are the two processes of protein synthesis?
A
- Transcription
- Translation
(in that order)
22
Q
What is splicing ?
A
- 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.
23
Q
Why is splicing important?
A
- 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
24
Q
What are the steps of translation?
A
- 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.
25
What are the different phases of mitosis?
- Prophase
- Metaphase
- Anaphase
- Telephase
26
What happens during prophase?
- 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.
27
What is metaphase?
- Spindle fibres attach to centromere on each chromosomes
- Sister chromatids line up along the equator
28
What is Anaphase?
- 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.
29
What is telophase?
- 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.
30
What is cytokinesis?
- The division of the cytoplasm
31
What is meant by a genome?
- Complete set of genes in a cell
32
What is meant by a proteome?
- The full range of proteins that a cell is able to produce?
33
What are the three stages of the cell cycle?
- 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)
34
What is cancer?
- 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.
35
How can cancer be treated?
- 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.
36
Describe the cell division in prokaryotic cells and viruses
- 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
37
What are the different phases within interphase?
**First Growth Phase**
Growth and normal metabolic roles
**Synthesis Phase**
DNA replication
**Second Growth Phase**
Growth and preparation for mitosis
38
Describe an experiment to identify stages of mitosis from a plant?
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.
39
What is the mitotic index and how is it calculated?
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
40
How to find the actual size of a cell under a microscope?
| Equation
Acutal size = size of image / magnification
41
How do viruses replicate?
- 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.
