Nucleic Acids: Structure + DNA Replication

Question 1

What is the function of DNA?

Answer 1

• to hold + store genetic info coding for sequences of AAs in primary structure of a protein, + so determines final 3D structure + function of protein in all living cells

Question 2

What is the function of RNA?

Answer 2

• to copy + transfer genetic code from DNA in nucleus to ribosomes in cytoplasm

Question 3

Describe the structure of a DNA monomer.

Answer 3

• DNA monomers are called nucleotides + are made up of deoxyribose (a pentose sugar), a nitrogenous base (e.g. guanine, cytosine, adenine or thymine) + 1 phosphate group
• deoxyribose sugar has a H at 2’ position

Question 4

Draw the structure of a nucleotide.

Answer 4

• pentagon [pentose sugar (deoxyribose/ribose)]
• rectangle connected to 2nd point (nitrogenous base)
• circle connected to 5th point (phosphate group

Question 5

Describe the structure of an RNA monomer.

Answer 5

• RNA monomers are also called nucleotides + are made up of ribose (a pentose sugar), a nitrogenous base (e.g. guanine, cytosine, adenine or uracil) + 1 phosphate group
• ribose sugar has a OH at 2’ position so is easier to hydrolyse

Question 6

What are the 2 structural forms of the nitrogenous bases?

Answer 6

• purines (adenine + guanine) have a double ring structure
• pyrimidines (cytosine, thymine + uracil) have a single ring structure

Question 7

Describe the formation of DNA + RNA polymers.

Answer 7

• a polynucleotide chain forms when many nucleotides are bonded together by phosphodiester (strong covalent) bonds, between the pentose sugar + phosphate groups, via condensation reactions
• sugar-phosphate backbone describes strong covalent bonds between sugar + phosphate groups

Question 8

Describe the structure of a DNA molecule.

Answer 8

• 2 polynucleotide strands held together by hydrogen bonds between specific complementary base pairs (C+G or A+T), creating a double helix structure
• each polynucleotide strand is made of many nucleotides bonded together by phosphodiester (strong covalent) bonds
• adenine + thymine form 2 H bonds, whereas cytosine + guanine can form 3 H bonds

Question 9

How does the structure of DNA relate to its function?

Answer 9

• stable bc of covalent bonds holding sugar-phosphate backbone together + many weak H bonds holding 2 strands together in a double helix
• double stranded so replication can occur using 1 strand as a template
• weak H bonds allow double helix to be easily unzipped into 2 strands during replication
• large molecule to carry lots of info
• complementary base pairing allows identical copies to be made

Question 10

Describe the structure of an RNA molecule.

Answer 10

• a relatively short, single stranded polynucleotide chain

Question 11

What are ribosomes?

Answer 11

• small organelles that are either free in cytoplasm or attached to rough endoplasmic reticulum
• they are larger in eukaryotic cells (80S) than in prokaryotic cells + mitochondria/chloroplasts of eukaryotic cells (70S)

Question 12

What is the structure of ribosomes?

Answer 12

• formed from ribosomal RNA (rRNA) + proteins, + is composed of a small + large subunit
• large subunit is the site of translation + holds tRNA molecules in place whilst condensation enzymes catalyse formation of peptide bonds between AAs
• not surrounded by membrane

Question 13

What is the function of ribosomes?

Answer 13

• site of protein synthesis

Question 14

Why did many scientists at the time doubt the newly discovered DNA molecule could carry the genetic code?

Answer 14

• bc of the relatively simple chemical composition of DNA as it was only made up of simple repeating nucleotides

Question 15

Why is semi-conservative replication important?

Answer 15

• bc it allows 2 daughter cells produced to each retain half of original DNA, which is used to create a newly synthesised strand
• this ensures there’s genetic continuity between cell generations, ensuring new cells inherit all genes from parent cells during cell division
• important for growth + replacing old + damaged cells

Question 16

Describe the process of semi-conservative replication of DNA.

Answer 16

• DNA helicase breaks H bonds linking complementary base pairs of DNA, causing double helix to unwind to form 2 strands, exposing the nucleotide bases
• both strands act as a template for free DNA nucleotides to join by complementary base pairing
• DNA polymerase joins adjacent DNA nucleotides by condensation reactions, forming complementary new strands of DNA
• original + new strand are joined by H bonding between base pairs forming 2 identical molecules of DNA (each w half original DNA + half newly synthesised DNA)

Question 17

Who discovered/confirmed the double helix structure of DNA?

Answer 17

• Watson + Crick, w the help of research on x-ray diffraction

Question 18

How did Meleson + Stahl prove Watson + Crick’s theory of semi-conservative DNA replication to be correct?

Answer 18

• by using bacteria + 2 nitrogen isotopes, 15N (heavy) + 14N (light)
• bacteria was grown in a broth w 15N, so as bacteria replicated, 15N was used to make new DNA nucleotides, so DNA was heavier
• sample of this DNA was spun in a centrifuge, showing heavy DNA settled near bottom of tube
• the next gen of bacteria was grown in a broth w 14N + spun in a centrifuge, causing DNA to settle in middle of tube
• this proved semi-conservative replication occurred as 100% of DNA was 15N14N (contained 1 stand each of heavy + light DNA)

Question 19

How did Meselson + Stahl’s experiment disprove the conservative replication theory?

Answer 19

• bc 100% of DNA settled in middle of tube after 15N bacteria was grown in broth w 14N
• if conservative replication occurred, 50% of DNA would settle at bottom of tube [bc has only heavier (15N15N) DNA] + 50% of DNA would settle at top of tube [bc has only lighter (14N14N) DNA]