Function of DNA & RNA Deoxyribonucleic ecid (DNA) holds genetic information Ribonucleic acid (RNA) transfers genetic information from DNA to ribosome

Function of DNA & RNA

• Deoxyribonucleic ecid (DNA) holds genetic information

• Ribonucleic acid (RNA) transfers genetic information from DNA to ribosomes

**Ribosomes are formed from RNA and proteins

Structure of DNA & RNA NUCLEOTIDES AND POLYMERS

• BOTH DNA and RNA are polymers of nucleotides. 

• Nucleotides are formed from a pentose sugar, a nitrogenous base and a phosphate group

• Nucleotides (monomers) make up DNA or RNA which are nucleic acids (polymers)

• DNA or RNA nucleotides are joined together by condensation reactions 

forming phosphodiester bonds

The difference between DNA and RNA nucleotides:

• DNA nucleotides have the pentose sugar deoxyribose, whereas RNA nucleotides have the pentose sugar ribose

• DNA nucleotides can have the base thymine, whereas RNA nucleotides have the uracil instead.

The differences between RNA and DNA molecules:

• DNA molecules are double stranded (double helix), whereas RNA molecules are single stranded

• DNA is longer whereas RNA is shorter

Exam tip: examiners can be picky when it comes to comparison questions. To get full marks, try and include ‘wheeras’ to ensure you have covered both sides.

Structure of DNA related to its functions:

• Double stranded → both strands can act as templates for semi-conservative replication

• Weak hydrogen bonds between bases → can be unzipped for replication

• Complementary base parking → accurate replication

• Many hydrogen bonds between bases → stable / strong molecule

• Double helix with sugar phosphate backbone → protects bases / H bonds

• Long molecule → stores a lot of (genetic) information (that codes for polypeptides)

• Double helix (coiled) → compact

Maths: use incomplete information about the frequency of bases on DNA strands to find the frequency of other bases. Key idea:

• % of adenine in strand 1 = % of thymine in strand 2 (vice versa)

• % of guanine in strand 1 = % of cytosine in strand 2 (vise versa)

Because of specific complementary base pairing between 2 strands

Q: A piece of DNA consists of 74 base pairs. The 2 strands of DNA, strands Aa and B, were analysed to find the number of bases of each type that were present. Some of the results are shown in the table. Fill in the gaps.

G in strand B = 26; 

G in strand A = 19; 

T in strand A = 9; 

A in strand A = 74 (total) - 19 - 26 - 9 = 20; 

T in strand B = 20

Nucleotides: the monomers that makeup of polynucleotides (DNA + RNA)

3.1.7.2 = DNA, GENES AND CHROMOSOMES

In eukaryotes, much of the nuclear DNA does not code for polypeptides. There are non-coding multiple

repeats of base sequences between genes. Even within a gene, only some sequences, called exons, code for amino acid sequences. Within the gene, these exons are separated by non-coding sequences called introns.

DNA is stored differently in eukaryotes vs. prokaryotes

• Eukaryotic DNA: 

  • Long, linear + double helix 

  • Found in nucleus  

  • Folded around / associated with proteins called histones

  • tightly coiled / folded into chromosomes (DNA molecule + its associated with proteins)

• Prokaryotic DNA: 

  • DNA is Shorter, circular / non-linear (loop) 

  • Not associated with proteins / histones

  • No nucleus 

  • Noo introns / no non-coding DNA (only exons)

  • Have plasmids 

Mitochondria and chloroplasts in eukaryotic cells

• Have their own DNA

• Similar to prokaryotic DNA - short, circular, not associated with proteins / histones

• No plasmids 

Genes

• Genes: A section of DNA bases that codes for:

  • The amino acid sequence of a polypeptide

  • The functional RNA e.g. ribosomal RNA and tRNAS

• Locus / loci: the fixed location of a gene on a chromosome on a particular DNA molecules

• DNA Triplet: a sequence of 3 DNA bases that code for a specific amino acid

• Genome: the complete set of genes (all) in a cell / organism, including those in mitochondria and / or chloroplasts 

• Proteome: the full range of proteins that a cell / genome is able to produce

• Alleles: different version (sequence of bases / triplets) of the same gene

• Homologous pair of chromosomes: same size chromosomes with same genes, but different alleles; the total number is the diploid number = 46

• Haploid number = 1/ 2 number = (23 in humans (these cells contain half the chromosomes))

Not all DNA codes for proteins / polypeptides / functional RNA - In eukaryotes, much of the DNA does not code for polypeptides:

• Between genes…

  • Non-coding multiple repeats (or Variable Number Tandem Repeats (VNTRs) in second year)

• With genes

  • Only exons code for amino acid sequences, which are separated by one or more non-coding sequences, called introns

  • Introns: regions within a gene that don't code for polypeptides

EXAM TIP: INtrons INterrupt the exons, which code for protein. EXons are EXpressed when coding for proteins

3.1.7.3 = DNA REPLICATION

Cell division occurs in two main stages:

• Nuclear division is the process by which the nucleus divides. There are two types of nuclear division, mitosis and meiosis

• Cell division follows nuclear division and is the process by which the whole cell divides

Process of Semi-conservative DNA replication:

1. DNA Helicase (enzyme) unwinds / unzips DNA double helix by breaking hydrogen bonds between the complementary base pairs → strands separate

2. = both two strands act as templates

3. Free floating DNA nucleotides attracted to exposed bases and attach via specific complementary base pairing, (adenine-t; guanine-cytosine)

4. DNA polymerase joins adjacent nucleotides on new strand by condensation, forming phosphodiester bonds = sugar phosphate backbone

5. Hydrogen bonds reform

6. DNA Replication is semi-conservative; each new DNA strand formed contains one original / template strand and one new strand

7. Ensures genetic continuity between generations of cells.

DNA polymerase moves in opposite directions along the DNA strands

• DNA has antiparallel strands

• The nucleotides on each strand are arranged differently 

• DNA polymerase is an enzyme with a specific shaped active site with a specific tertiary structure which can only bind to substrate with a complementary shape and orientation and form an enzyme-substrate complex

• This means DNA replication can be continuous on one strand and on the other side DNA replication is discontinuous

• DNA polymerase in lagging strand keeps going back up because the DNA helicase is still unwinding the DNA

• This is why it only does fragments which are joined together by DNA ligase 

• DNA polymerase can only bind to and add nucleotides to the phosphate (3’) end of the developing strand (so works in a 5’ to 3’ direction for new strand)

• Note - 5’ (“5 prime”) and 3’(“3 prime”) indicate the carbon numbers in DNA’s sugar backbone; 5’ carbon has a phosphate group attached and a 3’ carbon has a hydroxyl (-OH) group

the polymerase can continuously join nucleotides together in the leading strand because it's moving in the same direction as the helicase is unravelling the strands of DNA and in the other lagging strand it has to join them in fragments as the polymerase still needs to be inline with orientation of the nucleotide but because it's the anti-parallel strand DNA is being unravelled in the opposite direction to the one that the nucleotides are being joined meaning it has to go back and do it again?. This also explains why it only moves in a 5' to 3' direction?. 

Evidence for semi-conservative replication (Meselson and Stahl):

DNA replication is semi-conservative; new DNA is made of one original strand and one new stand.

Nitrogen is used because it is found in the nitrogenous bases (A, C, T, G).

• Experiment done by meselson + stahl

• Replication cultured in growth medium containing only heavy nitrogen  (¹⁵N) only for several generations

• Nitrogen incorporated into bacterial DNA bases

• When centrifuged only one heavy band is observed

• Bacteria / Cells transferred to a medium / nutrient solution with only light nitrogen (¹⁴N) and allowed to grow and divide twice

• During this process, DNA from different samples of bacteria was extracted, suspended in a solution in separate tubes and spun in a centrifuge

• After one replication the DNA band was intermediate → twice the thickness

• After two replication in light nitrogen → intermediate and light bands were observed

• Proving DNA replication is semi-conservative

Test tube observations:

Sample 1: DNA from bacteria grown for several generations in a nutrient solution containing ¹⁵N

→ DNA molecules contain 2 ‘heavy’ strands

Sample 2: DNA from bacteria grown orginiall in a nutrient solution containing ¹⁵N, then transferred for one division to a solution containing ¹⁴N

→ DNA molecules contain 1 original ‘heavy’ and 1 new ‘light strand’

Sample 3: DNA from bacteria grown originally in a nutrient solution ¹⁵N, then transferred for two divisions to a solution containing ¹⁴N.

→ 50% DNa molecules contain 1 original ‘heavy’ and 1 new ‘light’ strand, 50% contain both ‘light’ strands