3.1.4 Nucleic Acids are Important Information - Carrying Molecules

Question 1

what are nucleotides composed of?

Answer 1

• phosphate
• a pentose sugar (deoxyribose - DNA), (ribose RNA)
• nitrogenous base (containing nitorgen

Question 2

what are the base pairs?

Answer 2

adenine and thymine
guanine and cytosine

Question 3

basic function of DNA

Answer 3

holds genetic information which codes for polypeptides (proteins)

Question 4

basic function of RNA

Answer 4

transfers genetic information from DNA to ribosomes

Question 5

DNA structure

Answer 5

• phosphate group
• nitrogenous base - either adenine, thymine, guanine or s=cytosine
• deoxyribose

Question 6

RNA structure

Answer 6

• phosphate group
• nitrogenous base - either adenine, uracil, guanine or cytosine

Question 7

how do nucleotides join together to form polynucleotides?

Answer 7

• condensation reactions, removing water molecules
• between phosphate group of one nucleotide and the deoxyribose/ribose of another
• forming phosphodiester bonds

Question 8

why did scientists initially doubt that DNA carried the genetic code?

Answer 8

the relative simplicity of DNA - chemically simple molecule with few components

Question 9

describe the structure of DNA

Answer 9

• polymer of nucleotides (or polynucleotide)
• each nucleotide formed from deoxyribose, a phosphate group and a nitrogenous base
• phosphodiester bonds join adjacent nucleotides
• 2 polynucleotide chains held together by hydrogen bonds
• between specific complementary base pairs - adenine/thymine and guanine/cytosine
• double helix

Question 10

describe the structure of (messenger) RNA

Answer 10

• polymer of nucleotides (polynucleotide)
• each nucleotide formed from ribose, phosphate gorup and a nitrogenous base
• bases - uracil, adenine, guanine, cytosine
• phosphodiester bonds join adjacent nucleotides
• single helix

Question 11

compare + contrast the structure of DNA and (messenger) RNA

Answer 11

• DNA has deoxyrbiose, RNA has ribose
• DNA has base thymine, RNA has base uracil
• DNA has a double helix, RNA has a single helix
• DNA is long, RNA is shorter
• DNA has hydrogen bonds/base pairing, RNA doesnt

Question 12

difference between DNA and RNA NUCLEOTIDE

Answer 12

• DNA has deoxyribose, RNA has ribose
• DNA has thymine, RNA has uracil

Question 13

how does the structure of DNA relate to its functions

Answer 13

• 2 strands - both can act as templates for semi-conservative replication
• hydrogen bonds between bases are weak - strands can be separated for replication
• complimentary base pairing - accurate replication
• many hydrogen bonds between bases - stable/strong molecule
• double helix with sugar phosphate backbone - protects bases/hydrogen bonds
• long molecule - stores lots of genetic information
• double helix (coiled) - compact

Question 14

how can you use incomplete information about the frequency of bases on DNA strands to find the frequency of other bases?

Answer 14

1. % of adenine in strand 1 = % of thymine in strand 2 (vise versa)
2. % of guanine in strand 1 = % of cytosine in strand 2 (and vise versa)
   because of specific complementary base pairing between 2 strands

Question 15

why is semi-conservative replication important?

Answer 15

ensures continuity between generations of cells

Question 16

describe the process of semi-conservative DNA replication

Answer 16

1. DNA helicase breaks hydrogen bonds between complementary base pairs, unwinding and unziping the double helix
2. both strands act as templates
3. free DNA nucleotides attracted to exposed bases and join by specific complementary base pairing
4. hydrogen bonds form between adenine-thymine and guanine-cytosine
5. DNA polymerase joins adjacent nucleotides on new strand by condensation reactions
6. forming phosphodiester bonds
7. 1 new strand, 1 old strand

Question 17

what does semi-conservative actually mean?

Answer 17

each new DNA molecule consists of one original/template strand and one new strand

Question 18

why does DNA polymerase move in opposite directions along DNA strands

Answer 18

• DNA has antiparallel strands
• so shapes/arrangements of nucleotides on two ends are different
• DNA polymerase in an enzyme with a specific shaped active site
• so can only bind to substrate with complementary shape

Question 19

name the 2 scientists who proposed models of the chemical structure of DNA and of DNA replication

Answer 19

watson and crick

Question 20

describe the work of meselson and stahl in validating the watson-crick model of semi-conservative DNA replication

Answer 20

1. bacteria grown in medium containing heavy nitrogen (N-15) so nitrogen in incorporated into DNA bases
   - DNA extracted and centrifuged - settles near bottom as all DNA contain 2 heavy strands
2. bacteria transferred to medium containing light nitrogen (N-14) and are allowed to deivide once
   - DNA extracted and centrifuged - settles in middle as all DNA molecules contain 1 original heavy and 1 new light strand
3. bacteria in light nitrogen allowed to divide again
   - DNA extracted and centrifuged - half settles in the middle as contains 1 og heavy and 1 new light strand, half settles near top as contains 2 light strands