Principles of Molecular Genetics PDF

Principles of molecular genetics DNA (= deoxyribonucleic acid): - the carrier of genetic information - discoverer of DNA: Miescher (1869) - the first model of DNA was built by Watson and Crick (1953) The structure of DNA: - nucleotide - the basic building block o...

Principles of molecular genetics DNA (= deoxyribonucleic acid): - the carrier of genetic information - discoverer of DNA: Miescher (1869) - the first model of DNA was built by Watson and Crick (1953) The structure of DNA: - nucleotide - the basic building block of DNA; is an organic compound - the nucleotide consists of: 1. a nucleobase (= nitrogenous base): a) DNA – adenine (A), guanine (G), cytosine (C), thymine (T) b) RNA – adenine (A), guanine (G), cytosine (C), uracil (U) 2. a sugar (pentose) a) DNA – deoxyribose b) RNA - ribose 3. a phosphate group (H3PO4) Polynucleotide chain: - a chain of many nucleotides - nitrogenous bases are held together by hydrogen bonds - the 2 strands of DNA (= polynucleotide chains) are held together by weak hydrogen bonds between the nitrogenous bases according to the principle of complementarity - complementary base pairing: (DNA – (C-G), (A-T) and vice versa, RNA – (C-G), (A-U), (T-A – only from DNA) Function of DNA: - to store and transmit genetic information Function of RNA: - to convert the information stored in DNA into proteins The transmission of genetic information: - involves: 1. replication – the process of making copies of DNA molecules in the cell nucleus 2. transcription (mRNA synthesis) - a process that involves transcribing genetic information from DNA to mRNA = messenger RNA (DNA is transcribed or re- written into RNA) 3. translation (protein biosynthesis): – happens on the ribosomes in the cytoplasm and in the endoplasmic reticulum – the synthesis of protein from RNA – the coded information from mRNA (nucleotide sequence) is translated into chains of amino acids  DNA REPLICATION (SYNTHESIS): - before a cell enters the process of mitosis, its DNA replicates itself - DNA replication occurs during the S phase of the cell cycle - equal copies of the DNA pass into the daughter cells at the end of mitosis - in human cells, this means that 46 chromosomes (or molecules of DNA) replicate to form 92 chromosomes Mechanism of DNA replication: - the double helix opens and a complementary strand of DNA is synthesized along each strand - DNA replication begins with the "unzipping" of the parent molecule - specialized enzymes unzip the DNA double helix (the hydrogen bonds between the base pairs are broken) = the two strands separate - the sequence of bases on each of the separated strands serves as a template (= a pattern) to guide the insertion of a complementary set of bases on the strand being synthesized - the enzyme DNA polymerase joins all the nucleotide components to one another, forming a long strand of nucleotides - the old strand of DNA directs the synthesis of a new strand of DNA through complementary base pairing - the nucleotides are assembled in the order of bases on the strand serving as the template - the old strand then unites with the new strand to reform a double helix The genetic code: - the sequence of nucleotides in DNA and RNA that serve as instructions for synthesizing proteins - the set of rules by which information encoded within genetic material (DNA or mRNA sequences) is translated into proteins (amino acid sequences) by living cells Properties of the genetic code: 1. the genetic code is a three-letter (triplet) code: - triplet – a series of three nucleotides (eg. CCA, TGG) = codon - codon – specifies the insertion of an amino acid in a specific structural position in a polypeptide chain during the synthesis of proteins 2. the genetic code is degenerate: - 20 main amino acids can be coded for by 64 different triplet combinations – codons - one amino acid is coded by more than one codon - 61 codons encode amino acids and 3 codons are nonsense = stop codons (do not code for any amino acids) - start and stop codons: 1. start codon – AUG (the first codon of mRNA; always codes for methionine) 2. stop codons – UAA, UAG, UGA (signal a termination of translation, or punctuation) 3. the genetic code is non-overlapping: - each nucleotide is part of only one triplet codon 4. the genetic code is almost universal: - almost all organisms in nature (from bacteria to humans) use exactly the same genetic code The function of genes at a molecular level 1. structural genes: - genes that specify the amino acid sequence of a polypeptide chain (of a specific protein) 2. genes for RNA: - genes for RNA are transcribed into the nucleotide sequence of tRNA and rRNA 3. regulator(y) genes: - genes which determine whether or not genes will be expressed - genes which regulate the activity of other genes