UNIT 2:GENETICS –NUCLEIC ACIDRNACAMPBELL & REECE,2010. CHAPTERS 17,
STRUCTURE OF ANRNA MOLECULE• RNA is made up of:• Ribose sugar (a pentose sugar with 5carbons),• Phosphate and• A nitrogenous base; e.g. Purines (Adenineand Guanine) and Pyrimidines (Cytosineand Uracil).• RNA exists largely as single nucleotide chainsin living cells.
STRUCTURE OF ANRNA MOLECULE• The RNA strand is made up of alternatingmolecules of ribose sugar and phosphate.• The nitrogen bases are attached to the sugarmolecules in the strand and ’stick out’laterally as in DNA.• A sugar, a nitrogenous base and a phosphatetogether form a ribonucleotide.• An RNA molecule is a polymer ofribonucleotides.
STRUCTURE OF ANRNA MOLECULE• Although RNA exists generally as singlepolynucleotide chain, some segments of RNAmolecules may pair temporarily in double-helical form or may fold back on themselvesto set up extensive double-helical regions.• These fold-back double helices and theirarrangement are important to RNA functions.
DIFFERENCES BETWEEN DNA & RNACharacters DNA RNA1Molecule Doublestranded, helicalSingle stranded, straight orvariously folded and twisted.2Pentose sugar Deoxyribose Ribose3PyrimidinebaseThymine Uracil4Complementary base pairingAlways presentand existsbetween A = Tand G = CNormally absent, but may bepresent in twisted segments ofa molecule.If present, pairing is between A= U and G = C5Ratio ofPurines:PyrimidinesAlways 1:1 Not necessarily 1:1
TYPES ANDLOCATION OF RNA1. 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 accordingto 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 inthe shape of a clover leaf.• It carries a specific amino acidon one end and transfers it tothe ribosomes.• Has an anticodon on the otherend; the anticodon base-pairswith a complementary codonon mRNA.• Found in the cytoplasm of thecell.
rRNA• The two ribosomal subunits (large and small)are made of proteins and ribosomal RNA(rRNA)• The single-stranded molecule of rRNA isvariously folded and twisted upon itself incertain regions forming a secondary structure.• Constitutes 50% of a ribosome.• Help to bond mRNA to protein of theribosome.• Found in the ribosomes in the cytoplasm ofthe cell.
PROTEIN SYNTHESIS• PROTEIN SYNTHESIS CONSIST OF2 DISTINCT STAGES:• TRANSCRIPTION• TRANSLATION
TRANSCRIPTION• The 3 stages of transcription:(1) Initiation(2) Elongation(3) Termination
TRANSCRIPTION: INITIATION1. RNA-polymerase attaches to thebeginning of the DNA code called thepromotor2. It unwinds the DNA molecule andbreaks the weak hydrogen bondsbetween the complementary strands –a “bubble forms”3. The one strand now acts as a templatefor the formation of the mRNA strand.
TRANSCRIPTION : ELONGATION4. Free nucleotides in the nucleus bondsto the complementary bases of theDNA template strand. (Uracil replacesThymine in mRNA)5. More free nucleotides bond to theircomplementary bases, to elongate themRNA strand, until the entire code hasbeen transcribed.The DNA parts already transcribed rewound.
TRANSCRIPTION : TERMINATION6. The mRNA will detach from the DNAtemplate.7. The RNA polymerase detaches andstarts all over again at a differentlocation where needed.8. Now the pre-mRNA strand has toundergo a modification and RNAsplicing before it can leave the nucleus.
mRNA modification and splicingEach end of a pre-mRNA molecule ismodified in a particular way: 5’ end receives a modified nucleotide5’ cap 3’ end gets a poly-A tail
mRNA modification and splicingRNA splicing removes introns (non-codingRNA) & joins exons (coding RNA), creatingan mRNA molecule with a continuouscoding sequence.RNA splicing is carried out by spliceosomes.
Why are these modificationsnecessary?–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 andtakes it to the mRNA Amino acid (connect to formprotein) Ribosome (Location for proteinsynthesis)
THE 3 STAGES OF TRANSLATIONInitiationElongationTermination
TRANSLATION: INITIATION1. Small ribosomal subunit binds with mRNA2. Small subunit moves along mRNA until itreach the start codon (AUG)3. The matching anti-codon of the tRNA(with amino acid Methionine)bonds withthe start codon.3. Add the large subunit which completes thetranslation initiation complex.
TRANSLATION: ELONGATION1. More tRNA anticodons attach to mRNAcodons.2. The amino acids attached to the tRNA’sattach to one another by means ofpeptide bonds.3. Amino acids form a long polypeptidechain.4. tRNA releases amino acid to pick upmore amino acids
TRANSLATION: TERMINATIONTermination occurs when a stop codon inthe mRNA reaches the A site of theribosomeThe A site accepts a protein called arelease factor.The release factor causes the addition of awater molecule instead of an amino acid.This reaction releases the polypeptide, &the translation assembly then comes apart
TRANSLATION: MODIFICATIONSOften translation is not sufficient to makea functional proteinPolypeptide chains are modified aftertranslationCompleted proteins are targeted tospecific sites in the cell
CRACKING THE GENETIC CODE 64 codons have been deciphered.Of the 64 triplets, 61 code for aminoacids; 3 triplets are “stop” signals to endtranslation.No codon specifies more than one aminoacidGenes can be transcribed and translatedafter being transplanted from one species toanother.