Nucleotides and nucleic acids

Question 1

draw the structure of a nucleotide

Answer 1

-phosphate group (circle)
-5 carbon sugar (pentagon)
-nitrogenous base (rectangle)

Question 2

name the pentose sugars in DNA and RNA

Answer 2

DNA: deoxyribose
RNA: ribose

Question 3

describe how polynucleotide strands are formed and broken down

Answer 3

-condensation reactions between nucleotides and strong phosphodiester bonds (sugar-phosphate backbone)
-hydrolysis reactions use a water molecule to break these bonds
-enzymes catalyse these reactions

Question 4

describe the structure of DNA

Answer 4

-the molecule twists to form double helix of 2 deoxyribose polynucleotide strands (so there are 2 sugar-phosphate backbones)
-h bonds form between complementary base pairs ( AT and GC) on strands that run antiparallel

Question 5

name the purine bases and describe their structure

Answer 5

-adenine C5H5N5
-guanine C5H5N5O
(2 ring molecules)

Question 6

name the pyrimidine bases and describe their structure

Answer 6

-thymine C5H6N2O2
-cytosine C4H5N3O
-uracil C4H4N2O2

Question 7

name the complementary base pairs in DNA and RNA

Answer 7

DNA: 2 h bonds between A and T
RNA: 2 h bonds between A and U
Both: 3 h bonds between G and C

Question 8

why is DNA replication described as semi-conservative?

Answer 8

-strands from original DNA molecule act as templates
-new DNA molecule contains 1 old strand and 1 new strand (specific base pairing enables genetic material to be conserved accurately)

Question 9

explain the role of DNA helicase in semi-conservative replication

Answer 9

-breaks h bonds between pairs to form 2 single strands, each of which can act as a template

Question 10

how is a new strand formed during semi-conservative replications?

Answer 10

-free nucleotides from nuclear sap attach to exposed bases by complementary base pairing
-DNA polymerase joins adjacent nucleotides on new strand in 5’->3’ direction via condensation reactions to form phosphodiester bonds
-h bonds reform

Question 11

identify features of genetic code

Answer 11

-non-overlapping: each triplet is only read once
-degenerate: more than 1 triplet codes for the same amino acid (64 possible triplet for 20 amino acids)
-universal: same bases and sequences used by all species

Question 12

how does a gene determine the sequence of amino acids in a protein?

Answer 12

-consists of base triplets that code for a specific amino acids

Question 13

describe how DNA can be purified

Answer 13

-add ethanol and a salt to (aq) solution
-nucleic acids precipitate out of solution
-centrifuge to obtain a pellet of nucleic acid
-wash pellet with ethanol and centrifuge a again

Question 14

what does transcription produce and where does it occur?

Answer 14

-produces mRNA
-occurs in the nucleus

Question 15

outline the process of transcription

Answer 15

-RNA polymerase binds to promoter region on a gene
-section of DNA uncoils into 2 strands with exposed bases - antisense strand acta as a template
-free nucleotides are attracted to their complementary bases
-RNA polymerase joins adjacent nucleotides to form phosphodiester bonds

Question 16

what happens after a strand of mRNA is transcribed?

Answer 16

-RNA polymerase detaches at terminator region
-h bonds reform and DNA rewinds
-splicing removes introns from pre-mRNA in eukaryotic cells
-mRNA moves out of the nucleus via nuclear pore and attaches to ribosome

Question 17

what does translation produce and where does it occur?

Answer 17

-produces proteins
-occurs in cytoplasm on ribosomes (which are made of protein + rRNA)

Question 18

outline the process of translation

Answer 18

-ribosomes move along mRNA until ‘start’ codon
-tRNA anticodon attaches to complementary bases on mRNA
-condensation reactions between amino acids on tRNA form peptide bonds - requires energy from ATP hydrolysis
-process continues to form polypeptide chain until ‘stop’ codon is reached

Question 19

describe the structure of ATP (adenosine triphosphate) and ADP (adenosine diphosphate)

Answer 19

-nucleotide derivative of adenine
-ATP has 3 inorganic phosphate groups
-ADP has 2 inorganic phosphate groups

Question 20

what is a mutation?

Answer 20

-an alteration to the DNA base sequence
-mutations often arise spontaneously during DNA replication