Storage and retrieval of genetic information

Question 1

DNA

Answer 1

The genetic material of an organism, forming a sequence that can be translated into all the proteins expressed.

Question 2

Genotype

Answer 2

The complete set of genetic material of an organism, transferred from parents to offspring.

Question 3

Phenotype

Answer 3

The observable characteristics or traits of an organism, produced by both its genotype and environment.

Question 4

Structure of nucleic acids

Answer 4

Nucleic acids are comprised of a sugar-phosphate backbone linked by phosphodiester bonds, with each nucleotide containing a nitrogenous base.

Question 5

Structure of DNA

Answer 5

• double stranded, helical structure
• uses thymine (T)
• no hydroxyl (OH) group
• stable
• high molecular weight

Question 6

Structure of RNA

Answer 6

• single stranded
• uses uracil
• ribose has a hydroxyl (OH) group at C2
• unstable
• low molecular weight

Question 7

Complementary base pairing

Answer 7

The process by which genetic information is encoded, allowing DNA to be replicated and RNA to be synthesised. 1 purine and 1 pyramidine are joined by hydrogen bonding (adenine+thymine= 2 H bonds, guanine+cytosine= 3 H bonds).

Question 8

Non-coding DNA

Answer 8

Less than 5% of the human genome is made up of genes. Non-coding DNA includes gene regulatory sequences, introns and repeating elements.

Question 9

Gene regulatory sequences

Answer 9

• ‘promoter’ and ‘enhancer’ elements play an important role in gene expression
• can be upstream (5’) or downstream (3’) of a gene
• typically several per gene
• can be ~450kb from a gene

Question 10

Introns

Answer 10

• intervening segments found within genes, between coding DNA sequences (exons)
• removed from primary RNA by splicing machinery

Question 11

Repeating elements

Answer 11

• represent ~25% of our DNA
• vary in size from 1.4-6kb
• most common repeats are LINE and SINE families
• likely derived from virus
• can be used to detect polymorphisms in DNA fingerprinting

Question 12

Semi-conservative replication

Answer 12

One strand of replicated DNA is newly synthesised, while the other is an old copy.

Question 13

Polarity of DNA

Answer 13

DNA has a 5’ end (carbon attached to triphosphate) and a 3’ end (carbon attached to hydroxyl).

Question 14

Deoxynucleoside triphosphates

Answer 14

dNTPs are the building blocks for DNA synthesis, and are composed of a ribose sugar, phosphate group and nitrogenous base.

Question 15

Joining dNTPs

Answer 15

1) Complementary dNTP hydrogen bonds to template strand.
2) Second dNTP lines up next to 3’ end of first dNTP.
3) DNA polymerase joins dNTPs via a covalent bond.

Question 16

DNA polymerase

Answer 16

• proceeds in a 5’ to 3’ direction
• requires dNTPs
• must have a template strand and an RNA primer
• fast; adds 1000 bases per second
• has ‘proof reading’/editing activity; can undo incorrect pairing

Question 17

RNA primer

Answer 17

Acts as a starting block for DNA polymerase by providing a free 3’ end to receive incoming dNTPs. Deposited by RNA primase.

Question 18

Leading strand

Answer 18

DNA polymerase synthesises DNA continuously in a 5’ to 3’ direction, towards the replication fork.

Question 19

Lagging strand

Answer 19

DNA polymerase binds at multiple sites, synthesising Okazaki fragments of DNA in a 5’ to 3’ direction, away from the replication fork. These are then joined by DNA ligase.

Question 20

Replication bubbles

Answer 20

Replication of the whole chromosome occurs outwards from multiple origins of replication, forming replication bubbles.

Question 21

DNA proof reading

Answer 21

During DNA replication, an error occurs every 100,000 bases. DNA polymerase adds a base, moves back to check it, excises it if it is wrong, then moves on.

Question 22

Replication

Answer 22

The process by which the entire sequence of DNA is duplicated before cell division. Occurs in the nucleus.

Question 23

Transcription

Answer 23

The process by which an RNA sequence is synthesised, as encoded for by a gene. Only one strand of DNA is copied. Occurs in the nucleus.

Question 24

RNA polymerase

Answer 24

• proceeds in a 5’ to 3’ direction
• requires NTPs
• must have a DNA template
• adds 50 bases per second

Question 25

Process of transcription

Answer 25

1) DNA strands separate to form a bubble
2) RNA polymerase binds to and ‘reads’ from template strand
3) RNA chain extends as the bubble moves in a 5’ to 3’ direction
4) Primary RNA transcript binds proteins involved in RNA processing (splicing–>spliceosome)

Question 26

Translation

Answer 26

The process by which a strand of RNA is translated into a polypeptide chain to form a protein. Occurs at ribosomes in the cytoplasm or on the rough ER.

Question 27

Transfer RNA

Answer 27

tRNA is an adaptor molecule with a cloverleaf structure, stabilised by hydrogen bonding between base pairs. It has distinct functional regions, including an anticodon and a region for attaching a specific amino acid.

Question 28

Polysomes

Answer 28

More than one ribosome can attach to a single mRNA strand, resulting in multiple polypeptides being synthesised at once.

Question 29

Ribosomes

Answer 29

Complexes composed of a large and small ribosomal subunit, which assemble strings of amino acids encoded by mRNA. Contain a peptidyl site, an aminoacyl site and an mRNA binding site.

Question 30

Process of translation

Answer 30

1) Large and small ribosomal subunits assemble around the strand of mRNA.
2) First tRNA binds at the P site via its complementary anticodon.
3) Second tRNA binds at the A site.
4) Amino acids carried by tRNA are joined by covalent bonding.
5) Ribosome moves along one codon in a 5’ to 3’ direction.
6) Peptide chain extends until the ribosome reaches a STOP codon.

Question 31

The genetic code

Answer 31

• universal; common between different organisms
• one codon=a sequence of 3 bases
• degenerate; more than one codon can code for the same amino acid
• read in a 5’ to 3’ direction
• 3 possible reading frames
• START codon=AUG–> methionine
• STOP codons= UAA, UGA, UAG