Nucleic Acids

Question 1

DRAW A NUCLEOTIDE

Answer 1

DRAW IT

Question 2

DRAW A NUCLEOTIDE CHAIN

Answer 2

DRAW IT

Question 3

Describe Nucleotide

Answer 3

• Biological molecules that participate in almost all biochemical processes
• They are phosphate esters of pentose sugars, the nitrogenous base is linked to the Carbon 1 of the sugar residue, and a phosphate groups is linked to Carbon 5 or Carbon 3 of the sugar residue by covalent bonds formed by condensation reactions

Question 4

What are the uses of Nucleotides

Answer 4

• Form monomers of nucleic acids, DNA and RNA, in RNA the nucleotide pentose sugar is ribose, in DNA the nucleotide pentose sugar is deoxyribose
• Become phosphorylated nucleotides when they contain more than one phosphate group e.g. – ADP and ATP is an energy-rich end-product of most energy-releasing biochemical pathways – used to drive most energy-requiring metabolic processes in cells
• Regulates many metabolic pathways
• May be components of many coenzymes – adenine nucleotides are components of the coenzyme NADP, this is used in photosynthesis and of NAD which is a coenzyme used in respiration and of FAS and coenzyme A these are used in respiration

Question 5

where is DNA found in eukaryotic cells

Answer 5

found in the nuclei

Question 6

Where is the DNA found in prokaryotic cells and viruses

Answer 6

found in the cytoplasm

Question 7

What does DNA carry

Answer 7

• Herdiatory material and carriers coded instructions for the development and functions of all organisms

Question 8

DNA is an important …

Answer 8

Macromolecule

Question 9

Describe the structure of DNA

Answer 9

• Polymer made up of many repeating monomeric units called nucleotides,
• A molecule of DNA consist of two polynucleotide strands
• Strands run in opposite direction so as antiparallel
• Each DNA nucleotide consist of a phosphate group, a five carbon sugar called deoxyribose and one of four nitrogenous bases
• The covalent bond between the sugar residue and the phosphate group in a nucleotide is also called a phosphodiester bond – these are bonds are broken when polynucleotides break down and are formed when polynucleotides are synthesised

Question 10

what are the purine bases

Answer 10

Adenine or Guanine

Question 11

What are the pyrimidine bases

Answer 11

Thymine or Cytosine

Question 12

What is the difference between purine and pyrimidine

Answer 12

Purine has two rings whereas pyrimidine has one ring

Question 13

what are the complementary bases

Answer 13

A-T - by two hydrogen bonds

C-G - by three hydrogen bonds

Question 14

Describe the role that hydrogen bonds play in the structure of DNA

Answer 14

• Two antiparallel DNA strands are joined together by hydrogen bonds between the nitrogenous bases
• Purine always pairs with a pyrimidine this gives equal size rungs on the DNA ladder – these then twist around an imaginary axis and gives it its double helix coil giving the molecule stability
• Hydrogen bonds allow the molecule to unzip for transcription and replication

Question 15

Describe the antiparallel sugar phosphate backbones

Answer 15

• Opposite direction of the two strands refers to the direction that the third and fifth carbon molecules are facing
• The 5’ end of the molecule is where the phosphate group is attached to the fifth carbon of the deoxyribose sugar
• The 3’ end is where the phosphate group is attached to the third carbon of the deoxyribose sugar
• The rungs of the ladder consist of the complementary base pairs joined by hydrogen bonds
• Molecule is stable and the integrity of the coded information within the base sequences is protected

Question 16

How is DNA organised in an Eukaryotic cell

Answer 16

• The DNA content or the genome is in the nucleus
• Each molecule of DNA is tightly wound around histone proteins into chromosomes
• Each chromosome is therefore one molecule of DNA
• There is a loop of DNA without the histone proteins inside the mitochondria and chloroplasts

Question 17

How is DNA organised in a prokaryotic cell

Answer 17

• DNA is in a loop within the cytoplasm and is not enclosed in an nucleus
• Not wound around histone protein and is describe as naked

Question 18

How is DNA organised in a virus

Answer 18

it contains DNA in the form of a loop of naked DNA

Question 19

Describe the process of semi-conservative replication

Answer 19

1. The double helix unwinds its self but not all at once this is catalysed by a gyrase enzyme
2. The hydrogen bonds between the nucleotide bases are broken this is catalysed by DNA helicase and now there are two single strands of DNA exposed with nucleotide bases
3. Free phosphorylated nucleotides present in the nucleoplasm within the nucleus are bonded to the exposed bases following complementary base pairing rules
4. The enzyme DNA polymerase catalyses the addition of the new nucleotide bases in the 5’ to 3’ direction, to the single strands of DNA it uses each single strand of unzipped DNA as a template
5. The leading strand is continuously synthesised whereas the lagging strands which is in fragments are later joined, this is catalysed by ligase enzymes
6. Hydrolysis of the activated nucleotides to release phosphate groups sully the energy to make phosphodiester bonds between the sugar residue of one nucleotide and the phosphate group of the next nucleotide
   - The product of the replication is two DNA molecules identical to each other and the parent molecule – contains one old strand and one new strand therefore its semi-conservative
   - Loops of DNA within prokaryotes inside mitcohodria and chloroplasts also replicate semi-conservatively

Question 20

When does DNA self replicating happen

Answer 20

interphase - this is the moment before the cell divides, the DNA in mitochondria and chloroplasts also replicate and divide

Question 21

What is the first step of semi-conservative replication

Answer 21

firstly they are joined together at the centromere and form two sister chromatids

Question 22

Describe the Meselson and Stahl experiment - evidence for semi-conservative replication

Answer 22

• They grew bacteria E.coli for 14 generations in a medium which was has a heavy isotope of nitrogen 15N, this contains an extra neuron in every atomic nucleus – after 14 generations most of the DNA would be heavy as it would contain 15N
• They then transferred the E.coli into a medium which contained normal 14N, they left them there long enough to undergo one replicaton
• The DNA from these bacteria after one division was found to be hybrid DNA – showed that DNA does not replicate conservatively as this would have produced a heavy strand and a light strand
• Bacteria were allowed to divide once more and DNA was extracted and put in a centrifuge, this produced two bands of DNA one hybrid and one light showing that replication is semi-conservative and not dispersive

Question 23

Describe mutations

Answer 23

• During DNA replication errors may occur and wrong nucleotide may be inserted
• This could change the genetic code
• Enzyme proofread and edit out mutations this reduces the rate that mutations are produced
• Genes have changes to their nucleotide sequence – different versions of a particular gene are called alleles
• Not all mutations are harmful

Question 24

How is RNA different from DNA

Answer 24

• The sugar molecule in each nucleotide is ribose
• The nitrogenous base uracil which is pyrimidine replaces the pyrimidine base thymine
• The polynucleotide chain is usually single stranded
• The polynucleotide chain is shorter
• There are three forms of RNA – messenger RNA (mRNA) transfer RNA (tRNA) and ribosomal RNA (rRNA)

Question 25

Describe the process of transcription

Answer 25

1. A gene unwinds and unzips 2. Hydrogen bonds between complementary nucleotide bases break 3. The enzyme RNA polymerase catalyses the formation of temporary hydrogen bonds between RNA nucleotides and their complementary unpaired bases; this is called the template strand 4. A length of RNA that is complementary to the template strand of the gene is produced therefore is a copy of the other DNA strand this is called the coding strand 5. The mRNA now passes out of the nucleus through the nucleus envelope into the cytoplasm and attaches to the ribosome - Ribosomes are made in the nucleolus in two smaller subunits – they pass separately out of the nucleus through pores in the nuclear envelope and come together to form the ribosome – magnesium ions help to bind the two subunits together - made of ribosomal RNA and protein

Question 26

Describe the process of translation

Answer 26

- Transfer RNA molecules are made in the nucleolus and pass out of the nucleus into the cytoplasm – they are single stranded polynucleotides but can twist into a hairpin shape – at one end is a trio of nucleotide bases that recognises and attaches to a specific amino acid - At the loop of the hairpin there is another triplet of bases called an anticodon that are complementary to a specific codon of bases of the mRNA 1. Transfer RNA molecules bring the amino acids and find their place when the anticodon binds by temporary hydrogen bonds to the complementary codon on the mRNA molecule 2. As the ribosome moves along the length of the MRNA it reads the code, when two amino acids are adjacent peptide bonds form between them 3. Energy in the form of ATP is needed for polypeptide synthesis 4. The amino acid sequence for the polypeptide is therefore determined by the sequence of DNA triplets for nucleotide bases on the length of DNA 5. After the polypeptide is assembled the mRNA breaks down these are recycled into new lengths of MRNA with different codon sequences 6. Newly synthesised polypeptide is helped by chaperone proteins in the cell to fold correctly into its tertiary structure to carry out its function

Question 27

What does a gene contain

Answer 27

each gene contains a code that determines the sequence of amino acids in a polypeptide or protein

Question 28

What does the DNA base triplet determine

Answer 28

- Within each gene there is a sequence of DNA base triplets that determine the amino acid sequence or primary structure of a polypeptide – in the primary structure is correct then it will fold correctly and be held in its tertiary structure or shape therefore will be able to carry out its function

Question 29

How do genes pass out of the nucleus

Answer 29

- Instructions in the genes on chromosomes cannot pass out of the nucleus therefore a copy of each gene has to be transcribed into a length of mRNA, In this form the base triplets/codons can pass out of the nucleus to the ribosomes this ensures that the coded instructions are translated and the correct protein is made

Question 30

Describe the nature of the genetic code

Answer 30

- The Genetic code is near universal because in almost all living organisms the same triplet of DNA bases codes for the same amino acid - The genetic code is described as degenerate – all the amino acids except methionine and tryptophan, there is more than one base triplet – reduces the effect of point mutations as a change in one base of the triplet could produce another base triplet that still codes for the same amino acid - The genetic code is non-overlapping – it is read from a fixed point in groups of three bases, if a base is added or deleted then it causes a frame shift, as every base triplet after that and hence every amino acid coded for is changed