LIFE SCIENCES FOR GRADE 12 GENETICS – NUCLEIC ACID RNA

WHAT IS RNA > WHAT IS RNA > 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.

> > > > 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.

TYPES AND LOCATION OF RNA > > > > > > TYPES AND LOCATION OF RNA > mRNA (MESSENGER RNA) – > tRNA (TRANSFER RNA) > rRNA (RIBOSOMAL RNA) > cRNA (catalyticRNAs) > snRNA (Small Nuclear RNA) > snoRNA (Small Nucleolar RNA)

mRNA > > > > 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 > > > > 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.

rRNA 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.

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

TRANSCRIPTION > > > TRANSCRIPTION The 3 stages of transcription: > Initiation > Elongation > Termination

TRANSCRIPTION:INITIATION > > > TRANSCRIPTION:INITIATION > RNA-polymerase attaches to the beginning of the DNA code called the promotor. > It unwinds the DNA molecule and breaks the weak hydrogen bonds between the complementary strands – a “bubble forms” >The one strand now acts as a template for the formation of the mRNA strand.

TRANSCRIPTION:ELONGATION > > TRANSCRIPTION:ELONGATION > Free nucleotides in the nucleus bonds to the complementary bases of the DNA template strand. (Uracil replaces Thymine in mRNA) > More free nucleotides bond to their complementary bases, to elongate the mRNA strand, until the entire code has been transcribed.

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

TRANSLATION > > > > > TRANSLATION > A cell translates an mRNA message into protein WHAT 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 THREE STAGES OF TRANSLATION > > THE THREE STAGES OF TRANSLATION > Initiation > Elongation > Termination

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

TRANSLATION;ELONGATION > > > TRANSLATION;ELONGATION > More tRNA anticodons attach to mRNA codons. > The amino acids attached to the tRNA’s attach to one another by means of peptide bonds. > Amino acids form a long polypeptide chain. tRNA releases amino acid to pick up more amino acids

TRANSLATION:TERMINATION > > > > TRANSLATION:TERMINATION > Termination occurs when a stop codon in the mRNA reaches the A site of the ribosome > The 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

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