Nucleotides and Nucleic acids

Question 1

What is a nucleotide?

Answer 1

The monomer from which nucleic acids are made

Question 2

What are the differences between RNA and DNA nucleotides?

Answer 2

RNA contains the pentose sugar ribose
DNA contains the pentose sugar deoxyribose
RNA contains uracil base whereas DNA contains thymine base

Question 3

Which bases are purines?

Answer 3

Adenine and guanine

Question 4

Which bases are pyramidines?

Answer 4

Cytosine, thymine and uracil

Question 5

How do nucleotides join together?

Answer 5

Nucleotides join together to form polynucleotides.
The phosphate group of one nucleotide and the sugar of another join via a condensation reaction to form a phosphodiester bond

Question 6

What is the chain of sugars and phosphates in a polynucleotide known as?

Answer 6

Sugar - phosphate backbone

Question 7

What reaction breaks polynucleotides back down into nucleotides?

Answer 7

Hydrolysis

Question 8

What are ADP and ATP examples of?

Answer 8

Phosphorylated nucleotides

Question 9

What is ADP made of?

Answer 9

Nitrogenous base: adenine
Sugar: ribose
2 phosphate groups

Question 10

What is ATP made of?

Answer 10

Nitrogenous base: adenine
sugar: ribose
3 phosphate groups

Question 11

Explain the structure of DNA.

Answer 11

Two polynucleotide chains. 2nd strand must be upside down in order for the base pairs to join. This means they are anti parallel. Chain is then twisted every 10 nucleotides to form the double helix structure of DNA

Question 12

How many hydrogen bonds form between the base pairs on two antiparallel DNA polynucleotides?

Answer 12

Between A and T 2 hydrogen bonds form
Between C and G 3 hydrogen bonds form

Question 13

Practical: how to purify/ extract DNA using a precipitation reaction.

Answer 13

1) Break up the cells in your sample e.g banana (you van do this using a blender
2) Make up a solution of detergent, salt and distilled water
3) Add the broken up cells to a beaker containing the detergent solution. Incubate the beaker in a water bath at 60 degrees C for 15 minutes
4) Once incubated, put your beaker in an ice bath to cool the mixture down. When its cooled, filter the mixture. Transfer a sample of your mixture to a clean boiling tube
5) Add protease enzymes to the mixture to break down some proteins in the mixture
6) Slowly dribble some cold ethanol down the side of the tube so that it forms a layer on top of the DNA - detergent mixture
7) If you leave the tube for a few minutes the DNA will form a white precipitate

Question 14

What is the purpose of the detergent, salt and temperature within the practical?

Answer 14

The detergent breaks down the cell membrane
The salt binds to the DNA and causes it to clump together
The temperature of the water bath should stop enzymes in the cells from working properly and breaking down the DNA

Question 15

Explain semi conservative replication of DNA.

Answer 15

1) Takes place in the nucleus
2) The double helix is untwisted by the enzyme gyrase
3) Hydrogen bonds are broken by the enzyme helicase
4) Each strand acts as a template
5) Free DNA nucleotides in the nucleus start to align with exposed nuceotides
6) Complimentary base pairing occurs - A-T forming 2 hydrogen bonds and C-G forming 3
7) phosphodiester bonds form between the phosphate of one nucleotide and the sugar of another using the enzyme DNA polymerase ( this is forming the sugar phosphate backbone)
8) The result: 2 identical strands each containing half the original DNA and half the new DNA.

Question 16

Where can semi conservative replication take place besides the nucleus?

Answer 16

In the nucleoid and plasmids of bacteria
In chloroplasts and mitochondria their plasmids are copied through semi conservative replication

Question 17

What is a gene?

Answer 17

A sequence of DNA which codes for a protein

Question 18

What is the meselson stahl experiment?

Answer 18

It was used to prove semi conservative replication
- Grow E.coli in nitrogen 15 ( has 7 protons 8 neutrons)
- Put through a centrifuge
- All e coli in first generation (Gen 0) moved down
- Then place the e coli in N14
- Hybrid strand is then formed in “gen 1” ( 50% n14 50% n15)
- in gen 2 50% hybrid 50% n14

Question 19

2 main stages of protein synthesis

Answer 19

1)Transcription of a gene into a strand of mRNA - this occurs in the nucleus
2) Translation of the mRNA strand into a polypeptide chain - occurs on the ribosome

Question 20

What are the 3 main types of RNA?

Answer 20

messenger RNA - mRNA
transfer RNA - tRNA
ribosomal RNA - rRNA

Question 21

Describe mRNA

Answer 21

• a copy of a template strand of DNA
• Has “codons” ( 3 adjacent nucleotides that code for an amino acid)
• Involved in transcription and translation
• Made in nucleus

Question 22

Describe tRNA

Answer 22

• Small single strand found in cytoplasm
• Has binding site for specific amino acids
• Has unpaired triplet of bases called an anticodon
• Carries an amino acid to the ribosome where they are bonded together to form a polypeptide during translation

Question 23

Describe rRNA

Answer 23

• Half the mass of ribosome
• Involved in translation
• Made in nucleolus

Question 24

Describe transcription

Answer 24

• Gene to be transcribed unwinds and helicase breaks the hydrogen bonds
• Free RNA nucleotides bind to complimentary bases on the template strand
• Only one DNA strand is used as the template
  RNA polymerase catalyses the formation of phosphodiester bonds between RNA nucleotides
• mRNA is complimentary to the template strand and a is a copy of the coding strand ( but not identical)
• mRNA released from DNA leaves nucleus through nuclear pore to ribosome

Question 25

Describe translation

Answer 25

• mRNA binds to the ribosome
• A tRNA molecule attaches to the ribosome and its anticodon binds to the first codon of the mRNA by complimentary base pairing
• The tRNA is carrying a specific amino acid
• A second tRNA molecule with its specific amino acid binds to the second exposed codon via its anticodon
• Peptide bonds form between the two amino acids
• Ribosome moves along the mRNA
  A third tRNA molecule brings another amino acid
• The polypeptide chain grows until a stop codon is reached
• Stop codons do not code for an amino acid so translation stops

Question 26

What is a codon?

Answer 26

A codon is a trinucleotide sequence of DNA or RNA which corresponds to a specific amino acid

Question 27

What are three properties of the genetic code?

Answer 27

It is universal - all living organisms use the same code
It is degenerate - for most amino acids there is more than one base triplet
It is non-overlapping - It is read starting from a fixed point in groups of three

Question 28

What is meant by the term phosphorylated nucleotide?

Answer 28

It has had a phosphate group addedW

Question 29

What is the difference between a nucleoside and a nucleotide?

Answer 29

A nucleoside is a base and a pentose sugar
A nucleotide is a base, a pentose sugar and a phosphate group

Question 30

Who determined the structure of DNA and how?

Answer 30

Rosalind Franklin, James Watson and Francis Crick. Using X-Ray crystallography

Question 31

What are the three models of DNA replication?

Answer 31

Semi conservative
Dispersive
Conservative

Question 32

What does hydrolysis of ATP do?

Answer 32

Produces ADP and an inorganic phosphate and releases energy

Question 33

What are the similarities between RNA and DNA

Answer 33

Both have sugar
Both have phosphodiester bonds