Nucleotides and nucleic acids

Question 1

what are the three forms of RNA

Answer 1

rRNA
mRNA
tRNA

Question 2

what does a nucleotide consist of?

Answer 2

a nitrogenous base
pentose sugar
phosphate group

Question 3

what bond holds together nucleotides

Answer 3

phosphodiester bond

Question 4

compare the structure of DNA and RNA

Answer 4

DNA
- has deoxyribose
- bases are adenine, cytosine, guamine and thymine
- dna is double stranded
RNA
- has ribose
-bases are adenine, cytosine, guamine and uracil
- rna is single stranded

Question 5

how a chromosomes formed from dna

Answer 5

DNA helix are wound around histone proteins

Question 6

define how dna is antiparallel

Answer 6

one strand runs 5’ to 3’ and the other strand runs 3’ to 5’

Question 7

describe base pairing in dna

Answer 7

A purine (two carbon rings) always pairs with a pyrimidine (one carbon ring)
Cytosine-Guanine
Adenine-Thymine/Uracil

Question 8

what is mRNA

Answer 8

it is a copy of one gene from DNA, created in the nucleus and leaves via the nuclear pore to carry the copy to a ribosome so a polypeptide chain can be made
- much shorter than DNA
- short lived
- single strand, every three bases (codon) code for a specific amino acid

Question 9

what is tRNA

Answer 9

found in the cytoplasm,
- it is single stranded but folded to create a cloverleaf shape, held in place by hydrogen bonds
- it brings a specific amino acid to the ribosome
- this is determined by the anticodon found on tRNA which is complementary to a codon on mRNA

Question 10

describe ATP structure

Answer 10

• contains three phosphate groups, adenine base and ribose sugar

Question 11

how does ATP release energy

Answer 11

ATP + H20 -> ADP + Pi + energy

ATP is hydrolysed into adenosine diphosphate and a phosphate ion, releasing energy, using the enzyme ATP hydrolase

Question 12

how is ATP made

Answer 12

made during respiration via a condensation reaction and using the enzyme ATP synthase this is called phosphorylation

ADP + Pi -> ATP + H20

Question 13

why is ATP a good immediate energy store

Answer 13

due to the instability of ATP, cells do not store large amounts of it. however ATP is rapidly reformed by the phosphorylation of ADP, this interconversion of ATP and ADP is happening constantly in all living cells, meaning cells do not need a large store of ATP, thereofre is a good immediate energy store

Question 14

what are the properties of ATP

Answer 14

• small - moves easily into, out of and within cells
• water soluble - energy requiring processes happen in aqueous environments
• contains bonds between phosphates with intermediate energy: large enough to be useful for cellular reactions but not so large that energy is wasted as heat
• easily regenerated - can be recharged with energy

Question 15

explain the method for extracting DNA

Answer 15

1- grind sample in a pestle and mortar to break down the cell walls
2- mix sample with detergent to break cell membrane
3- add salt, breaks hydrogen bonds between DNA and water molecules
4-add protease enzyme to break down any histone proteins associated with the DNA
5- add ice cold ethanol to sample, this causes the DNA to precipitate out of solution
6- the DNA can be picked up by spooling onto a glass rod, should see white strands

Question 16

How is DNA’s structure related to its functions

Answer 16

• stable structure due to sugar phosphate backbone and double helix
• double stranded so replication can occur using both strands as templates
• weak hydrogen bonds between the bases for easy separation of the two strands during replication
• large molecule so a lot of information is carried
• complementary base pairing allows identical copies to be made

Question 17

explain the term semi-conservative replication

Answer 17

during replication two new molecules of DNA are produced, each one consists of one old strand of DNA and one new strand.

Question 18

describe the difference between continuous and discontinuous replication

Answer 18

DNA polymerase can only move from 3’ to 5’, so the enzyme has to replicate each of the template strands in opposite directions.
The strand that is unzipped from the 3’ end can be continuously replicated as the strand unzip, this is called the leading strand and undergoes continuous replication
The other strand is unzipped from the 5’ end, so DNA polymerase has to wait until a section of the strand is unzipped and then work back along the strand. this results DNA produced in sections, called Okazaki fragments, which then have to be joined up. this strand is called the lagging strand and undergoes discontinuous replication

Question 19

describe the process of replication

Answer 19

1- Helicase enzyme breaks the hydrogen bonds between the complementary bases holding the double helix together, causing the strands to unwind and the two strands to separate
2- Free floating DNA nucleotides hydrogen bond to their complementary base on the exposed original DNA strand
3- DNA polymerase catalyses the condensation reaction with forms phosphodiester bonds between adjacent nucleotides to forms a new polymer chain of DNA

Question 20

describe the process of transcription

Answer 20

A copy of the gene on DNA is transcribed onto mRNA to be taken to the ribosome to be translated into a protein
1- Helicase unwinds the DNA in the region of the gene that is to be expressed
2- Free floating RNA nucleotides line up against the bases of the antisense strand and form hydrogen bonds between the base pairs (A-U & C-G)
3- RNA polymerase catalyses the formation of phosphodiester bonds between RNA nucleotides
4- mRNA detaches and leaves the nucleus via the nuclear pore and the DNA zips up again.

Question 21

what are the three features of genetic code

Answer 21

degenerate, universal and non-overlapping

Question 22

define degenerate

Answer 22

more than one codon (three bases) can code for the same amino acid

Question 23

define universal

Answer 23

the same triplet of bases codes for the same amino acid in all organisms

Question 24

define non-overlapping

Answer 24

each base in a gene is only part of one triplet of bases that codes for one amino acid. each codon is read as a discrete unit

Question 25

what are mutations

Answer 25

occur when sequences of bases are not always matched exactly, and an incorrect sequence may occur in newly copied strand. these errors are random and spontaneously and lead to a change in the sequence of bases. this can be insertion, deletion of substitution. all can impact the polypeptide and how the protein folds and carries out its function.

Question 26

describe the process of translation

Answer 26

the code on the mRNA is translated into a sequence of amino acids by the ribosome - forming a polypeptide
1- mRNA arrives from nuclear pore and attaches to the ribosome at the start codon
2- tRNA molecule with the complementary anticodon to the start codon aligns opposite the mRNA and hydrogen bonds form between codons
3- the next tRNA aligns on the next codon, carrying the correct amino acid for that codon
4- the ribosome cataylses the formation of a peptide bond between the two amino acids.
5- the ribosome will move along the mRNA molecule to the next codon and another complementary tRNA will attach, this continues until the ribosome reaches the stop codon where it detaches.
6- the polypeptide chain can now fold into a protein

Question 27

what are introns

Answer 27

introns are sequences of bases in a gene that do not code for amino acids and therefore polypeptide chains, these get removed, spliced out of mRNA molecules after transcription

Question 28

what are exons

Answer 28

are sequences of bases in a gene that code for sequence of amino acids, and so polypeptide chains

Question 29

what are start and stop codons

Answer 29

at the start of every gene, there is a start codon, which enable the ribosome to attach

at the end of every gene there are three bases that do not code for an amino acid and is a stop codon, this causes the ribosome to detach and end translation.