DNA

Question 1

What are nucleic acids, what are two different types, and what do they do?

Answer 1

Nucleic acids are polymers made out of nucleotides

DNA = holds genetic information

RNA = transfers genetic information from DNA to the ribosomes

Question 2

What is the structure of DNA?

Answer 2

• a double helix with two polynucleotide chains held together by hydrogen bonds between complementary base pairs
• pentose sugar (because it has 5 carbons ) =deoxyribose
• phosphate group
• a nitrogen-containing organic base : cytosine, thymine, adenine, guanine

Question 3

What chemical reaction forms a nucleotide and what bond is created when two nucleotides undergo this chemical reaction?

Answer 3

condensation reaction

A condensation reaction between two nucleotides forms a phosphodiester bond

Question 4

What is the difference between DNA and proteins?

Answer 4

DNA is a sequence of bases but proteins are a sequence of amino acids

Question 5

What is the structure of RNA?

Answer 5

• relatively short polynucleotide chain
• pentose sugar = ribose
• phosphate group
• a nitrogen-containing organic base: cytosine, uracil, adenine, guanine

Question 6

What is the function of RNA?

Answer 6

transfers genetic information from DNA to ribosomes rather than holds information (like DNA)

Question 7

Draw the structure of a phosphodiester bond

Answer 7

Question 8

What is the base paring for DNA?

Answer 8

adenine = thymine

guanine = cytosine

Question 9

Why is DNA a stable molecule?

Answer 9

• the phosphodiester backbone protects the more chemically reactive organic bases inside the double helix
• hydrogen bonds link the organic base pairs forming bridges between the phosphodiester uprights
• because there are three hydrogen bonds between cysteine and guanine, the higher proportion of C—G pairings, the more stable the DNA molecule

Question 10

How is DNA adapted to carry out its function?

Answer 10

• very stable structure. Rarely mutates
• two separate strands are joined only with hydrogen bonds, which allows them to separate during DNA replication and protein synthesis
• extremely large molecule and therefore carries an immense amount of genetic information
• by having base pairs within the helical cylinder of the deoxyribose-phosphate backbone, the genetic information is to some extent protected from being corrupted by outside chemical and physical forces
• Base pairing leads of DNA being able to replicate and to transfer information and mRNA

Question 11

Name the two purine bases. How are they different from the pyrimidine bases? (name all the pyrimidine bases)

Answer 11

• The bases adenine and guanine are purines
  – they have a double ring structure
• The bases cytosine, thymine and uracil are pyrimidines
  – they have a single ring structure

Question 12

Which bases form two hydrogen bond?

Answer 12

adenine and thymine

Question 13

What are the two requirements for semi-conservative replication?

Answer 13

• the four types of nucleotides (each with a different base) must be present
• a source of chemical energy to drive the process

Question 14

What does semi-conservative replication of DNA ensure?

Answer 14

genetic continuity between generations of cells

Question 15

Describe the anti-parallel nature of DNA

Answer 15

As the strands run in opposite directions (they are antiparallel), one is known as the 5’ to 3’ strand and the other is known as the 3’ to 5’ strand

Question 16

Draw a dna polynucleotide

Answer 16

Question 17

How many bases make up an amino acid?

Answer 17

3

Question 18

Give two features of DNA and explain how each one is important in the semi-conservative replication of DNA? (2 marks, 3 possible answers)

Answer 18

1- Weak hydrogen bonds between bases allow two strands to unzip

2- 2 strands exists, both of them can act as template for replication

3- Complementary base pairing, this allows accurate replication

Question 19

why may a gene contain more than the necessary amount of bases it needs

Answer 19

because there are regions of the gene that are non-coding

Question 20

Describe the process of translation?

Answer 20

A molecule of mRNA leaves the nucleus via a nuclear pore and binds to a ribosome.

6 bases are attached to the small subunit of the ribosome these are read as two codons.

tRNA with complementary anticodons, form weak hydrogen bonds with the complementary codons, bringing with it a specific amino acid.

A second tRNA molecule bearing a different amino acid binds to the second codon with its complementary anticodon.

Using an enzyme to catalyst the reaction and ATP for energy, peptidebonds form between the two adjacent amino acids.

The ribosomes now moves along the mRNA, reading the next codon. A third RNA molecule brings another amino acid and a peptide bond forms.

The first tRNA leave and is able to collect another one of its amino acids. The polypeptide chain grows until a stop codon is reached

Question 21

Describe the process of transcription? (exam-based)

Answer 21

occurs in the nucleolus of the nucleus

DNA Helicase breaks the hydrogen bonds between complementary bases in the gene

the double strand of the gene unwinds and the nucleotide bases are exposed

One of the separate DNA strands acts as a template.

Free RNA nucleotides align to exposed bases on the template strand by complementary base pairing, with Uracil pairing with adenine and guanine pairing with cytosine.

RNA Polymerase moves along the strands, joining the nucleotides together with phosphodiester bonds to form the sugar-phosphate backbone of the RNA strand.

RNA polymerase reaches a stop codon and then detaches, resulting in the formation of pre-MRNA

pre- MRNA is then spliced, to remove introns and leaves only exons to produce mRNA

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