CHAPTER 5 GENETIC CONTROL Leonardus, S.Si.

Structure and replication of DNA Role of DNA in protein synthesis Content Structure and replication of DNA Role of DNA in protein synthesis

Learning Outcomes Candidates should be able to: (a) describe the structure of RNA and DNA and explain the importance of base pairing and the different hydrogen bonding between bases; (b) explain how DNA replicates semi-conservatively during interphase;

Learning Outcomes (c) state that a gene is a sequence of nucleotides as part of a DNA molecule, which codes for a polypeptide and state that a mutation is a change in the sequence that may result in an altered polypeptide; (d) describe the way in which the nucleotide sequence codes for the amino acid sequence in a polypeptide with reference to the nucleotide sequence for HbA (normal) and HbS (sickle cell) alleles of the gene for the β-haemoglobin polypeptide;

Learning Outcomes (e) describe how the information on DNA is used during transcription and translation to construct polypeptides, including the role of messenger RNA (mRNA), transfer RNA (tRNA) and the ribosomes (for genetic dictionaries see pages 37–39); (f) use the knowledge gained in this section in new situations or to solve related problems.

Summary DNA and RNA are polynucleotides, made up of long chains of nucleotides. A nucleotide contains a pentose sugar, a phosphate group and a N-containing base. A DNA molecule consists of 2 polynucleotide chains, linked by H bonds between bases. Adenine always pairs with thymine, and cytosine with guanine. RNA, which comes in several different forms (mRNA, tRNA & rRNA), has only 1 polynucleotide chain, although this may be twisted back on itself, as in tRNA. In RNA, the base thymine is replaced by uracil

DNA molecules replicate during interphase DNA molecules replicate during interphase. The H bonds between the bases break, allowing free nucleotides to fall into position opposite their complementary ones on each strand of the original DNA molecule. Adjacent nucleotides are then linked, through their phosphates and sugars, to form new strands. Two complete new molecules are thus formed from one old one, each new molecule containing one old strand and one new. The sequence of bases (or nucleotides) on a DNA molecule codes for the sequence of amino acids in proteins (or polypeptide). Each amino acid is coded for by 3 bases. A length of DNA coding for one complete protein or polypeptide is gene.

During protein synthesis, a complementary copy of the base sequence on a gene is made, by building a molecule of mRNA against one DNA strand. The mRNA then moves to a ribosome in the cytoplasm. tRNA molecules with complementary triplets of bases temporarily pair with the base triplets on mRNA, bringing appropriate amino acids. As 2 amino acids are held side by side, a peptide bond forms between them. The ribosome moves along the mRNA molecule, so that appropriate amino acids are gradually linked together, following the sequence laid down by the base sequence on the mRNA

Structure of DNA and RNA DNA : deoxy ribonucleic acid RNA : Ribonucleic acid DNA & RNA are macromolecules DNA & RNA are polymer The smaller molecules from which DNA & RNA are made are nucleotide ----- therefore DNA & RNA are polynucleotides 3 components of nucleotides are : A nitrogen containing base DNA (adenine, guanine, cytosine & thymine) RNA (adenine, guanine, cytosine & uracil)

A pentose sugar DNA (deoxy ribose) RNA (Ribose) A phosphate group

DNA components

Purine base

Pyrimidine base

Deoxyribose vs ribose

Doble helix structure of DNA

Two strands of DNA run antiparalel

DNA double stranded DNA has two strands A phosphate-deoxyribose polymer composes the backbone of the DNA adjacent sugars are connected by phosphodiester bonds nitrogenous bases are convalently bonded to the 1' carbon of the deoxyribose the two DNA strands are antiparallel the two strands are held together by hydrogen bonds between complementary bases adenine hydrogen bonds (base pairs) to thymine guanine hydrogen bonds to cytosine

RNA structure

mRNA

tRNA

rRNA

DNA vs RNA DNA RNA Double polynucleotide chain Single polynucleotide chain Bases are : adenine, guanine, cytosine & thymine Bases are : adenine, guanine, cytosine & uracil Pentose sugar is deoxyribose Pentose sugar is ribose Only one type 3 types (mRNA, tRNA & rRNA) Amount per diploid cell is constant in any one species Amount varies from cell to cell