UNIT 2: GENETICS –

NUCLEIC ACIDRNA

CAMPBELL & REECE, 2010. CHAPTERS 17,

STRUCTURE OF AN RNA MOLECULE• RNA is made up of:• Ribose sugar (a pentose sugar with 5

carbons), • Phosphate and • A nitrogenous base; e.g. Purines (Adenine

and Guanine) and Pyrimidines (Cytosine and Uracil).

• RNA exists largely as single nucleotide chains in living cells.

STRUCTURE OF AN RNA MOLECULE

• The RNA strand is made up of alternating molecules of ribose sugar and phosphate. • The nitrogen bases are attached to the sugar

molecules in the strand and ’stick out’ laterally as in DNA.• A sugar, a nitrogenous base and a phosphate

together form a ribonucleotide.• An RNA molecule is a polymer of

ribonucleotides.

STRUCTURE OF AN RNA MOLECULE

• Although RNA exists generally as single polynucleotide chain, some segments of RNA molecules may pair temporarily in double-helical form or may fold back on themselves to set up extensive double-helical regions.• These fold-back double helices and their

arrangement are important to RNA functions.

BASIC STRUCTURE OF AN RNA MOLECULE

RNA STRUCTURAL ELEMENTS

DIFFERENCES BETWEEN DNA & RNA Characters DNA RNA

1 Molecule Double stranded, helical

Single stranded, straight or variously folded and twisted.

2 Pentose sugar Deoxyribose Ribose

3 Pyrimidine base

Thymine Uracil

4 Complementary base pairing

Always present and exists between A = T and G = C

Normally absent, but may be present in twisted segments of a molecule. If present, pairing is between A = U and G = C

5 Ratio of Purines: Pyrimidines

Always 1:1 Not necessarily 1:1

TYPES AND LOCATION OF RNA

1. mRNA (MESSENGER RNA) – 2. tRNA (TRANSFER RNA)3. rRNA (RIBOSOMAL RNA)4. cRNA (catalyticRNAs) 5. snRNA (Small Nuclear RNA) 6. snoRNA (Small Nucleolar RNA)

The blue RNA’s are the most important

mRNA• Single strand RNA nucleotides.• A polynucleotide strand synthesized according

to the code of the DNA. • It carries the code in base triplet (codon) form,

from the DNA to the ribosomes.• Found in the nucleus of the cell.

tRNA• A single RNA strand folded in

the shape of a clover leaf. • It carries a specific amino acid

on one end and transfers it to the ribosomes.

• Has an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA.

• Found in the cytoplasm of the cell.

tRNA

rRNA• The two ribosomal subunits (large and small)

are made of proteins and ribosomal RNA (rRNA)

• The single-stranded molecule of rRNA is variously folded and twisted upon itself in certain regions forming a secondary structure.

• Constitutes 50% of a ribosome.• Help to bond mRNA to protein of the

ribosome.• Found in the ribosomes in the cytoplasm of

the cell.

rRNA

PROTEIN SYNTHESIS

• PROTEIN SYNTHESIS CONSIST OF 2 DISTINCT STAGES:• TRANSCRIPTION• TRANSLATION

TRANSCRIPTION

• The 3 stages of transcription:(1) Initiation(2) Elongation(3) Termination

TRANSCRIPTION: INITIATION1. RNA-polymerase attaches to the

beginning of the DNA code called the promotor

2. It unwinds the DNA molecule and breaks the weak hydrogen bonds between the complementary strands – a “bubble forms”

3. The one strand now acts as a template for the formation of the mRNA strand.

TRANSCRIPTION: INITIATION

TRANSCRIPTION : ELONGATION4. Free nucleotides in the nucleus bonds to the complementary bases of the DNA template strand. (Uracil replaces Thymine in mRNA)5. More free nucleotides bond to their

complementary bases, to elongate the mRNA strand, until the entire code has

been transcribed.The DNA parts already transcribed rewound.

TRANSCRIPTION : ELONGATION

TRANSCRIPTION : TERMINATION6. The mRNA will detach from the DNA template.7. The RNA polymerase detaches and starts all over again at a different location where needed.8. Now the pre-mRNA strand has to undergo a modification and RNA splicing before it can leave the nucleus.

TRANSCRIPTION : TERMINATION

mRNA modification and splicingEach end of a pre-mRNA molecule is modified in a particular way:

5’ end receives a modified nucleotide 5’ cap

3’ end gets a poly-A tail

mRNA modification and splicingRNA splicing removes introns (non-coding RNA) & joins exons (coding RNA), creating an mRNA molecule with a continuous coding sequence.

RNA splicing is carried out by spliceosomes.

Why are these modifications necessary?

–Seems to facilitate the export of mRNA–Protect mRNA from hydrolytic enzymes–Help ribosomes attach to 5 end

TRANSLATIONoA cell translates an mRNA message into proteinoWHAT IS NEEDED TO DO THIS?

mRNA (Carries the code) tRNA (pick up amino acid and

takes it to the mRNA Amino acid (connect to form

protein) Ribosome (Location for protein

synthesis)

THE 3 STAGES OF TRANSLATION

InitiationElongationTermination

TRANSLATION: INITIATION1. Small ribosomal subunit binds with mRNA2. Small subunit moves along mRNA until it reach the start codon (AUG)3. The matching anti-codon of the tRNA (with amino acid Methionine)bonds with the start codon.3. Add the large subunit which completes the translation initiation complex.

TRANSLATION: INITIATION

TRANSLATION: ELONGATION

1. More tRNA anticodons attach to mRNA codons.

2. The amino acids attached to the tRNA’s attach to one another by means of peptide bonds.

3. Amino acids form a long polypeptide chain.

4. tRNA releases amino acid to pick up more amino acids

TRANSLATION: ELONGATION

TRANSLATION: TERMINATIONTermination occurs when a stop codon in the mRNA reaches the A site of the ribosomeThe A site accepts a protein called a release factor.The release factor causes the addition of a water molecule instead of an amino acid.This reaction releases the polypeptide, & the translation assembly then comes apart

TRANSLATION: TERMINATION

TRANSLATION: MODIFICATIONSOften translation is not sufficient to make a functional proteinPolypeptide chains are modified after translationCompleted proteins are targeted to specific sites in the cell

CRACKING THE GENETIC CODE 64 codons have been deciphered.Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation.No codon specifies more than one amino acidGenes can be transcribed and translated after being transplanted from one species to another.

CRACKING THE GENETIC CODE(codon)