LIFE SCIENCES FOR GRADE 12

GENETICS –NUCLEIC ACID

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

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

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.

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 CONSIST OF 2 DISTINCT STAGES:TRANSCRIPTIONTRANSLATION

TRANSCRIPTION

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

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

> 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

> 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

> A cell translates an mRNA message into proteinWHAT 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

> Initiation> Elongation> Termination

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

> 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

> 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

REFERENCES

http://www.slideshare.net/dreyngerous/rna