nucleic acids + DNA replication

Question 1

describe the basic functions of DNA and RNA in all living cells

Answer 1

-DNA: holds genetic information which codes for polypeptides (proteins)
- RNA: transfers genetic information from DNA to ribosomes

Question 2

name the two types of molecules from which a ribosome is made from

Answer 2

• RNA
• proteins

Question 3

what are DNA composed of?

Answer 3

a double stranded helix structure, containing a sugar phosphate backbone with a nitrogen-containing organic base pairs in the centre

Question 4

what are the structures of a nucleotide?

Answer 4

• a strand of DNA or RNA is a polynucleotide made up from joined mononucleotide
• individual nucleotides are made from 3 components
  1. phosphate group
  2. a pentose sugar (deoxyribose/ribose)
  3.an organic base
• the phosphate group and the pentose sugar are the back bones
• organic base contains nitrogen

Question 5

DNA is made from which monomer?

Answer 5

mononucleotides

Question 6

Identify the molecule.

Answer 6

ribose

Question 7

diagram of a DNA nucleotide

Answer 7

Question 8

diagram of a RNA nucleotide

Answer 8

Question 9

describe the differences between a DNA and a RNA nucleotide

Answer 9

• in DNA the pentose sugar is deoxyribose whereas its ribose in RNA
• in DNA the base contains thymine whereas in RNA its uracil instead

Question 9

describe how nucleotides join together to form polynucleotides

Answer 9

-condensation reaction, removing water molecules
-between phosphate group of one nucleotide and deoxyribose/ribose of another
-forming phosphodiester bonds

Question 10

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

Answer 10

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

Question 11

in DNA what is adenine complementary to?

Answer 11

Thymine

Question 12

in DNA what is cytosine complementary to?

Answer 12

guanine

Question 13

in RNA what is adenine complementary to?

Answer 13

uracil

Question 14

in RNA what is cytosine complementary to?

Answer 14

guanine

Question 15

describe the structure of DNA

Answer 15

● Polymer of nucleotides (polynucleotide)
● Each nucleotide formed from deoxyribose, a phosphate group and a
nitrogen-containing organic base
● Phosphodiester bonds join adjacent nucleotides
● 2 polynucleotide chains held together by
hydrogen bonds
● Between specific complementary base
pairs - adenine / thymine and cytosine / guanine
● Double helix

Question 16

describe the structure of (messenger) RNA

Answer 16

● Polymer of nucleotides (polynucleotide)
● Each nucleotide formed from ribose, a phosphate
group and a nitrogen-containing organic base
● Bases - uracil, adenine, cytosine, guanine
● Phosphodiester bonds join adjacent nucleotides
● Single helix

Question 17

compare and contrast the structure of DNA and RNA

Answer 17

Question 18

how is DNA a stable molecule?

Answer 18

• the phosphodiester backbone protects the more chemically reactive bases inside the double helix
  -C-G base pairs contain 3 hydrogen bonds, so the more C-G base pairs in a DNA molecule the more stable it is.
  -there are interactive forces between base pairs that holds the molecules together. This is called ‘base stacking’. This makes the molecules more stable

Question 19

Suggest how the structure of DNA relates to its functions

Answer 19

1. has a phosphodiester backbone : prevents organic bases reacting with environment
2. long molecule : store lots of genetic information (that codes for polypeptides)
   3.double helix structure (coiled) : makes DNA compact
   4.Hydrogen bonds between bases are weak → strands can be separated for replication
   5.Two strands → both can act as templates for semi-conservative replication
   6.Complementary base pairing (base sequence) → accurate replication, information can be stored and codes for amino acids/proteins
   7.Many hydrogen bonds between bases → stable / strong molecule
   8.base stacking : makes DNA stable

Question 20

How many hydrogen bonds are formed between Cytosine and Guanine in a DNA molecule?

Answer 20

3

Question 21

Name the bond that forms between the 2 nucleotides of a dinucleotide.

Answer 21

phosphodiester bond

Question 22

How is DNA adapted to carry out its function as a means of passing genetic information from generation to generation?

Answer 22

-Very stable - rarely mutates
-Hydrogen bonds between strands easy to separate for replication and protein synthesis.
-Very long - carries a lot of genetic information.
-Base pairs protected from chemical/physical forces by phosphodiester backbone.

Question 23

Name the 3 Scientists who discovered the structure of DNA

Answer 23

-James Watson
-Francis Crick
-Rosalind Franklin

Question 24

How many hydrogen bonds are formed between Adenine and Thymine in a DNA molecule?

Answer 24

2

Question 25

How are the 2 strands of DNA that make up the double helix held together?

Answer 25

Hydrogen bonds between complementary organic bases

Question 26

Identify the molecule

Answer 26

phosphoric acid (phosphate)

Question 27

The phosphate group attaches to which carbon atom of a ribose or deoxyribose molecule when forming a nucleotide?

Answer 27

The 5’ carbon atom (pronounced 5 prime)

Question 28

Identify the molecule

Answer 28

deoxyribose

Question 29

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

Answer 29

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

Question 30

If 19.9% of the base pairs in human DNA are guanine, calculate the percentage that is Thymine. Show your reasoning.

Answer 30

30.1% If 19.9% is guanine - 19.9% is cytosine as it is paired with it. 19.9 + 19.9 = 39.8%. The remaining DNA is made from Adenine and Thymine, which is 60.2%. Thymine = 60.2% divided by 2 = 30.1%.

Question 31

How do the organic bases help to stabilise structure of DNA?

Answer 31

-Hydrogen bonds between the base pairs holds two strands together; -Many hydrogen bonds provides strength;

Question 32

Why is semi-conservative replication important?

Answer 32

Ensures genetic continuity between generations of cells

Question 33

Describe the process of semi-conservative DNA replication

Answer 33

1. DNA helicase breaks hydrogen bonds between complementary bases, unwinding 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

Question 34

define Semi-conservative

Answer 34

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

Question 35

what 4 things are needed during DNA replication?

Answer 35

-DNA molecule to be replicated -free DNA nucleotides -DNA helicase enzymes -DNA polymerase

Question 36

suggest why DNA polymerase moves in opposite directions along DNA strands

Answer 36

● DNA has antiparallel strands ● So shapes / arrangements of nucleotides on two ends are different ● DNA polymerase is an enzyme with a specific shaped active site ● So can only bind to substrate with complementary shape (phosphate end of developing strand)

Question 37

Describe the work of Meselson and Stahl in validating the Watson-Crick model of semi-conservative DNA replication

Answer 37

1. Bacteria grown in medium containing heavy nitrogen ( 15N) so nitrogen is incorporated into DNA bases ○ DNA extracted & centrifuged → settles near bottom, as all DNA molecules contain 2 ‘heavy’ strands 2. Bacteria transferred to medium containing light nitrogen (14N) and allowed to divide once ○ DNA extracted & centrifuged → settles in middle, as all DNA molecules contain 1 original ‘heavy’ and 1 new ‘light’ strand 3. Bacteria in light nitrogen (14N) allowed to divide again ○ DNA extracted & centrifuged → half settles in middle, as contains 1 original ‘heavy’ and 1 new ‘light’ strand; half settles near top, as contains 2 ‘light’ strands

Question 38

Function of DNA helicase?

Answer 38

break H bonds between bases

Question 39

Function of DNA polymerase?

Answer 39

join adjacent nucleotides together forming phosphodiester bonds

Question 40

true or false “DNA polymerase forms phosphodiester bonds between bases.”

Answer 40

false. DNA polymerase forms phosphodiester bonds between the phosphate group of one nucleotide & the deoxyribose of another.

Question 41

true or false “Free bases attach to exposed bases.”

Answer 41

false. Free nucleotides attach to exposed nucleotide bases via complementary base pairing, but not the bases on their own.