2.5: Nucleic Acids

Define nucleotides

monomers that are linked by condensation reactions to form nuclei acids

Two main types of nucleic acids

DNA - deoxyribonucleic acid

RNA - ribonucleic acid

Basic structure of a nucleotide

• pentose sugar (sugar with 5 carbon atoms)

• nitrogenous base (attached to 1’ carbon)

• phosphate group (attached to 5’ carbon)

DNA deoxyribose pentose sugar vs RNA ribose pentose sugar

Ribose sugar:

• hydroxyl (OH) group at the 2’ carbon position

deoxyribose sugar:

• hydrogen (H) at the 2’ carbon position

Nitrogenous bases of DNA

adenine (A)

thymine (T)

cystosine (C)

guanine (G)

A is complementary to T

C is complementary to G

Nitrogenous bases of RNA

• Uracil (U) replaces thymine (T)

adenine (A)

uracil (U)

cytosine (C)

guanine (G)

Type of molecule: DNA vs RNA & why?

The presence of the 2' hydroxyl group makes RNA more susceptible to hydrolysis

As a result:

• DNA is the storage molecule

• RNA is the transport molecule with a shorter molecular lifespan

The nitrogenous base molecules found in the nucleotides of DNA (A, T, C, G) and RNA (A, U, C, G) occur in two structural forms:

purines

pyrimidines

Difference between purines and pyrimidines

purines have a double ring structure

pyrimidines have a single ring structure

purines will always bond/ be complementary to a pyrimidine

Which bases are purines?

adenine

guanine

Which bases are pyrimidines?

cytosine

thymine

uracil

Separate nucleotides are joined via ____ reactions to form polymers/ polynucleotides - DNA or RNA

condensation
(releasing a water molecule)

The condensation reaction occur between..

the phosphate group of one nucleotide

and the pentose sugar of the next nucleotide

A condensation reaction between two nucleotides forms a ___ bond

phosphodiester bond

(a type of covalent bond)

The chain of alternating phosphate groups and pentose sugars produced as a result of many phosphodiester bonds is known as the…

sugar-phosphate backbone

Strands of a DNA molecule

DNA molecules are made up of two polynucleotide strands (double helix) lying side by side that are antiparallel (running in opposite directions)

- each DNA polynucleotide strand is said to have a 3’ end and a 5’ end

- as the strands are antiparallel, one strand is known as the 5’ to 3’ strand and the other is known as the 3’ to 5’ strand

Each individual polynucleotide strand is made up of…

alternating deoxyribose sugars and phosphate groups bonded together by phosphodiester bonds to form the sugar-phosphate backbone

the nitrogenous bases of each nucleotide project out from the backbone towards the interior of the double-stranded DNA molecule

The two antiparallel DNA polynucleotide strands are held together by…

hydrogen bonds between the nitrogenous bases

Hydrogen bonds always occur between the same pairs of bases. In DNA, what are these base pairs, and how many hydrogen bonds are formed between these bases?

• the purine adenine (A) always pairs with the pyrimidine thymine (T)
  → two hydrogen bonds are formed between these bases

• the purine guanine (G) always pairs with the pyrimidine cytosine (C)
  → three hydrogen bonds are formed between these bases

this is known as complementary base pairing

What structure is DNA described as

double helix

Diagram showing a DNA molecule

Summary of bonding: phosphodiester bonds vs hydrogen bonds

phosphodiester bonds: join the nucleotides in the sugar phosphate backbone

hydrogen bonds: join the bases of the two complementary strands together

Is RNA double stranded?

no, it is single stranded

Nitrogenous bases in RNA

A, G, C, U

RNA molecule diagram

RNA vs DNA: Similarities

• both are polynucleotides

• both are nucleic acids

• both contain the nitrogenous bases adenine (A), guanine (G) and cytosine

• both contain a phosphate group

• both contain a pentose sugar

• both have a sugar-phosphate backbone held together by phosphodiester bonds

DNA vs RNA: Differences

• DNA is double stranded, RNA is single stranded

• RNA polynucleotide chains are shorter than DNA chains

• DNA has a deoxyribose pentose sugar whereas RNA has a ribose pentose sugar

• DNA has the nitrogenous bases A, G, C, T whereas RNA has the nitrogenous bases A, G, C, U

• DNA is found inside the nucleus whereas RNA is found both inside and outside of the nucleus

• DNA is a storage molecule whilst RNA is a transport molecule

Examples of RNA

• messenger RNA (mRNA), which is the transcript copy of a gene that encodes for a specific polypeptide

• transfer RNA (tRNA)

• ribosomal RNA (rRNA)

Messenger RNA (mRNA)

• mRNA is a single strand

• mRNA is made in the nucleus in a process called transcription

• It then leaves the nucleus and goes out into the cytoplasm and to the ribosome where it is used to make a protein during translation

tRNA

• single nucleotide chain folded into a clover shaped molecule

• it is held in shape by hydrogen bonds between base pairs (A-U or C-G)

• at one end of the tRNA molecule there is a specific of 3 bases called an anticodon

• at the other end is an amino acid binding site

• tRNA carries amino acids to the ribosome where they are used to make proteins

Main role of DNA

stores genetic information

Main role of RNA

RNA has several roles including:

• mRNA: transferring genetic information from DNA to ribosomes

• rRNA: combining with proteins to form ribosomes

• tRNA: being involved in protein synthesis