Nucleotides and nucleic acids

Question 1

What are nucleotides?

Answer 1

Building blocks of nucleic acids e.g DNA and RNA that can join together to form polynucleotides

Question 2

What are the components of a nucleotide?

Answer 2

• pentose sugar
• nitrogenous base
• phosphate group

Question 3

What is a phosphodiester bond?

Answer 3

Covalent bond between the phosphate group of one nucleotide and the sugar of another

Question 4

What is a sugar phosphate backbone?

Answer 4

Many nucleotides joined together by phosphodiester bonds to create a chain of phosphates and sugars known as the sugar-phosphate backbone

Question 5

What is DNA?

Answer 5

Deoxyribonucleic acid is a type of nucleic acid containing the instructions needed to make proteins

Question 6

What components make up DNA?

Answer 6

• deoxyribose
• A,T,G or C bases
• phosphate group

Question 7

What features allow DNA to pass genetic info from 1 generation to another?

Answer 7

Sugar-phosphate backbone = protects coding bases inside of helix
Double stranded = allows strands to act as templates in DNA replication
Large molecule = stores lots of information
Double helix = makes molecule compact
Complementary base pairing = allows accurate DNA replication
Weak H bonds = allows strands separation in DNA replication

Question 8

What are purine bases?

Answer 8

Larger bases containing 2 carbon ring structures (A and G)

Question 9

What are pyrimidine bases?

Answer 9

Smaller bases containing 1 carbon ring structure (T and C)

Question 10

What are the base pairings and how many bonds between each?

Answer 10

A with T via 2 hydrogen bonds
G with C via 3 hydrogen bonds

Question 11

What maintains the constant distance between 2 sugar-phosphate backbones?

Answer 11

Smaller pyrimidine base always binds to a larger purine base

Question 12

What is RNA?

Answer 12

Ribonucleic acid is a type of nucleic acid that uses information from DNA to synthesise proteins

Question 13

What components make up RNA?

Answer 13

• ribose
• A,U,G or C bases
• phosphate group

Question 14

How does RNA differ in structure to DNA?

Answer 14

• RNA contains ribose whereas DNA contains deoxyribose
• RNA contains base uracil inlace of thymine
• RNA is a short single stranded molecule whereas DNA is a long double stranded molecule

Question 15

How is DNA replicated and what does this produce?

Answer 15

Semi-conservatively

produces DNA molecules consisting of one original DNA strand and one newly synthesised DNA strand

Question 16

Outline the process of semi-conservative DNA replication?

Answer 16

1) DNA helicase breaks H bonds between complementary bases unwinding the double helix separating strands

2) Each strand acts as a template as free nucleotides attract to their complementary

3) enzyme DNA polymerase joins free nucleotides together via condensation reactions in the 5’ to 3’ direction

4) phosphodiester bonds form to create the sugar-phosphate backbone of the new DNA strand

5) 2 identical copies of DNA are made up of one original DNA strand and one new DNA strand

Question 17

After DNA was discovered what were the 2 theories of DNA replication proposed?

Answer 17

Conservative replication:
- original DNA molecule stays intact while completely new copy is built
- after a single replication 1 molecule has 2 original strands, and the other has 2 new strands

Semi-conservative replication:
- original DNA splits and each strand acts as a template for a new strand
- after a single replication each molecule has 1 original strand and 1 new strand

Question 18

What did Meselson and Stahl experiment?

Answer 18

Which theory of DNA replication was correct

Question 19

What principles were Meselson and Stahl experiment based off?

Answer 19

• DNA contains nitrogen
• 2 different nitrogen isotopes can be used to mark DNA strands and track them during replication (lighter and darker)
• bacteria use nitrogen from their surroundings to build new DNA molecules

Question 20

What was Meselson and Stahl experimental process?

Answer 20

1) bacteria grown in medium containing N15 so all their DNA is ‘heavy’

2) bacteria were transferred to a medium with N14 for one round of replication, (lighter nitrogen incorporated into any new DNA strands made)

3) DNA was extracted and centrifuged

4) steps 2-3 were repeated for another round of replication

5) distribution of heavy/light DNA analysed to track how DNA was replicating

Question 21

What were Meselson and Stahl’s key findings?

Answer 21

• heavier bands sink lower in test tube
• intermediate bands made of DNA (1 heavy strand + 1 light strand) are in the middle of the test tube
• lighter bands are higher up in the test tube

Question 22

What were Meselson and Stahl’s results after the first replication?

Answer 22

• original heavy DNA strands separate
• each heavy strand acts as template for a new complementary strand
• new light strands form alongside original strands
• resulting DNA molecule consists of one old heavy strand and one new light strand

Question 23

What were Meselson and Stahl’s results after the second replication

Answer 23

• both original strands and the two new strands act as templates
• new light strands form alongside all four templates
• half the resulting molecules have one original heavy strand and one new light strand
• other half are completely made of light strands

Question 24

How is DNA condensed into chromosomes?

Answer 24

DNA molecules are wound around proteins known as histones to form a DNA-histone complex

complexes coil further to form chromatin helping pack the DNA into chromosomes, each containing a single DNA molecule

Question 25

Why is DNA condensed into chromosomes?

Answer 25

Molecules are incredibly long and must be tightly packed to fit within the nucleus of a eukaryotic cell

Question 26

What is a gene?

Answer 26

Short section of DNA coding for a polypeptide (protein)

Question 27

What is a gene locus?

Answer 27

Specific position along a chromosome known as a locus where a gene is located

Question 28

What is a genome?

Answer 28

Complete set of genes

Question 29

What is a proteome?

Answer 29

Full range of proteins that a cell is capable of producing

Question 30

What is the genetic code?

Answer 30

Sequence of bases that code for amino acids

Question 31

What are the features of the genetic code?

Answer 31

Universal -= each DNA triplet codes for same amino acid in all organisms Non-overlapping = each base in DNA sequence is only read once Degenerate = most amino acids are coded for by more than one triplet

Question 32

What is mRNA?

Answer 32

Messenger RNA is a type of RNA synthesised during transcription. It carries genetic info from DNA to the ribosomes (where proteins are made)

Question 33

What are the features of mRNA?

Answer 33

- single-stranded linear molecule - contains abase sequence complementary to DNA sequence - contains codons (sets of 3 bases) coding for an amino acid - small enough to leave nucleus

Question 34

What is tRNA?

Answer 34

Transfer RNA is a type of RNA used in the process of translation It transports amino acids to ribosomes to build up a polypeptide chain

Question 35

What are the features of tRNA?

Answer 35

- single-stranded molecule folded into clover-leaf shape - uses H bonds between complementary base pairs to hold shape - contains specific sequence of 3 bases at one end (anticodon) - contains amino acid binding site at opposite end

Question 36

What is transcription and where does it take place in the cell?

Answer 36

Initial step in protein synthesis involving creating mRNA copy of a gene from the DNA template takes place within nucleus, then mRNA exits the nucleus and enters the cytoplasm where translation occurs

Question 37

What are the key events in transcription?

Answer 37

1) RNA polymerase enzyme binds to DNA 2) H bonds between DNA bases break, 2 strands of the double helix separate 3) antisense strand acts as the template for mRNA synthesis 4) free RNA nucleotides align with DNA template through complementary base pairing 5) in RNA molecule U pairs with A and C with G 6) RNA polymerase catalyses formation of phosphodiester bonds between adjacent RNA nucleotides 7) complementary mRNA strand forms carrying same base sequence as the DNA sense strand 8) process ends when RNA polymerase reaches a stop codon detaching from DNA and terminates transcription 8) RNA is released detaching from DNA, DNA rewinds into double helix structure

Question 38

What is translation and where does it take place in the cell?

Answer 38

Process of decoding information in mRNA to synthesise a polypeptide chain with the help of tRNA (chain folds into functional protein) occurs in cytoplasm specifically on the ribosome

Question 39

What are the key events in translation?

Answer 39

1) ribosome attaches to mRNA strand at a start codon 2) tRNA molecule carrying specific AA and with an anticodon complementary to the start codon binds to mRNA 3) 2nd tRNA molecule with anticodon complementary to next mRNA codon also carrying a specific AA attaches to mRNA linked together via a peptide bond using ATP 5) 1st tRNA molecule detaches from mRNA and is free to collect another AA for future use 6) ribosome moves along mRNA allowing another tRNA molecule carrying the next amino acid to bind to next codon on mRNA 7) process from step 4 to 6 is repeated elongating the polypeptide chain 8) any point during this process 2 tRNA molecules can be attached to the ribosome 9) sequence continues until the ribosome reaches a stop codon on mRNA 10) completed polypeptide chain detaches from the ribosome