2.3 Nucleic Acids

Página 1

2.3.1 DNA- deoxyribonucleic acid

Key words:

Nucleotide: molecule consisting of a five-carbon sugar, a phosphate group (C5/C3) and a nitrogenous base (C1). They are linked by covalent bonds formed by condensation reactions, phosphate and sugar is the phosphodiester bond.

-They form the monomers of nucleic acids, DNA (deoxyribose) and RNA (ribose)

-Become phosphorylated nucleotides when they contain more than one phosphate group (ADP and ATP)

-Help regulate many metabolic pathways (ATP, ADP and AMP)

-May be components of many coenzymes. Adenine nucleotides are components of coenzyme NADP

Polynucleotide: large molecule containing many nucleotides.

DNA structure (a nucleic acid):

Double helix- the coiling of two polynucleotide strands into a right-handed spiral configuration.

Antiparallel sugar-phosphate backbones- the opposite directions of the two strands refers to the direction of the third and fifth carbon molecules on the pentose sugar. 5'-3'prime.

Four nitrogenous bases- Purine two rings (Adenine and Guanine) and Pyrimidine one rind (Thymine and Cytosine), they link giving equal sized rungs.

Hydrogen bonds- form between the nitrogenous bases, giving stability. A and T (2), G and C (3).

2.3.2 How DNA replicates

Key words:

Semi-conservative replication: how DNA replicates, resulting in two new molecules, each of which contains one old strand and one new strand.

-Evidence for the replication: (dispersive/conservative x) Meselson and Stahl grew E-coli in N15 for 14 generations, then transferred some into N14 to undergo one replication. Showed two bands, one hybrid and one light showing semi-conservative replication.

Semi-conservative replication:

DNA's double his unwinds catalysed by a gyrate enzyme

It then unzips, hydrogen bonds between the nucleotide bases are broken, catalysed by DNA helicase

Free phosphorylated nucleotides present in the nucleoplasm are bonded to the exposed bases

The enzyme DNA polymerase catalyses the addition of the new nucleotide bases in 5' to 3' direction.

Threading strand is synthesised continuously, whereas the lagging strand is in fragments that is later joined, catalysed by ligase enzymes.

Hydrolysis of the activated nucleotides release the extra phosphate groups and supplies the energy to make phosphodiester bonds.

2.2.3 How DNA codes for polypeptides

Key words:

Gene: a length of DNA that codes for a polypeptide or for a length of RNA that is involved in regulating gene expression.

Polypeptide: a polymer made of many amino acids units joined by a peptide bond.

Protein: a large polypeptide of 100 or more amino acids.

RNA:

The sugar molecule is ribose, the nitrogenous base is uracil, the polynucleotide chain is single-stranded, the polynucleotide chain is shorter and there are three forms: mRNA, tRNA, rRNA.

Genes and the genetic code:

-The genetic code is universal, in almost all living organisms have the triplet of DNA bases codes for the same amino acid.

-The genetic code is degenerate, for all amino acids except two there is more than one base triplet.

-The genetic code is non-overlapping, read from a fixed point in a group of three bases.

Transcription:

The DNA is too large to fit through the nuclear pores so a copy must remade. A section of the DNA unwinds, leaving the template and coding strand. The enzyme helicase breaks the hydrogen bonds for the DNA to unzip. The RNA nucleotides form next to the complementary bases on the template strand. Bonds (ester/ covalent) form between the RNA nucleotides by RNA polymerase. This forms an mRNA strand, which leaves through the nuclear pore.

Translation:

The mRNA enters the groove between the two subunits of a ribosome. The ribosome is large enough for two mRNA codons to be side by side. A tRNA molecule with an anticodon complementary to the mRNA codon lines up with the mRNA strand. Hydrogen bonds form between the mRNA strand and the anticodon of the tRNA. A second tRNA bonds to the next codon on the mRNA, then a peptide bond forms between neighbouring amino acids. IT continues till a newly synthesised polypeptide forms.