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York University
BIOL 1500

Biology Chapter 5 Notes: DNA, Gene Expression and Biotechnology • Watson and Crick figured out the exact structure of the double helix deoxyribonucleic acid (DNA) • DNA is a nucleic acid: consists of individual units called nucleotides; molecule of sugar + phosphate group (4 oxygen atoms + phosphorus atom) + nitrogen-containing molecule called a base (adenine, thymine, guanine, cytosine); 1 DNA molecule can have as many 200m base pairs • backbones of DNA: sugar + phosphate sequence • Base pairs: Adenine + Thymine / Cytosine + Guanine • Genes: sections of DNA that contain instructions for making proteins; sequence of bases in a DNA molecule that carries the information necessary for producing a functional product (usually a protein or RNA molecules) full set of DNA present is called a genome; DNA exists in chromosomes; 23 unique pieces of DNA; two copies of each piece for a total of 46 • Alleles: alternative versions of a gene that codes for a feature; any feature of that organism is a trait • Not all DNA contains instructions for making proteins: in humans, genes make up less than 5% of DNA; non-coding DNA may be referred as "junk DNA" • 25% of non-coding DNA are within genes, 75% are between genes; these regions are called introns • Genotype: carrier for a particular trait; Phenotype: the appearance of the dominating trait • From Gene to Protein: Transcription & Translation Transcription: a copy of a gene's base sequence is made 1. Recognize and bind: the enzyme RNA polymerase recognizes a promoter site (a part of the DNA molecule that indicates the start of a gene); RNA polymerase binds to the DNA molecule at the promoter site and unwinds it just a bit so only one strand of DNA can be read 2. Transcribe: as the DNA strand is being processed by the RNA polymerase, the RNA polymerase builds a copy (transcript); this copy is the messenger RNA (mRNA); it can move elsewhere in the cell and its message can be translated into proteins; as the DNA is being replicated, it is being rewound; mRNA transcript is constructed from 4 different molecules called ribonucleotides; RNA polymerase transcribes a specific sequence of DNA; mRNA moves throughout the cell to where then info is needed 3. Terminate: when RNA polymerase meets a sequence of bases on the DNA at the end of the gene (termination sequence) it stops creating the transcript and detaches from the DNA molecule; mRNA molecule is released as free-floating, single-strand copy of gene after termination 4. Capping and editing: prokaryotic cells are ready to make protein after mRNA transcript separates from DNA; eukaryotes need to edit the transcript in several ways; a cap and tail may be added to protect the mRNA from damage and help the protein-making machinery recognize the mRNA; introns are snipped out if it is present; after editing, it leaves the nucleus for the cytoplasm Translation: the copy of the transcript is used for protein production • begins when mRNA molecule moves out of the cell's nucleus and into the cytoplasm • requires large numbers of free amino acids floating around; ribosomal subunits (protein production factories where amino acids are linked together in the proper order to produce the protein); molecules that can read the mRNA code and transplate it (tRNA) • Transfer RNA (tRNA): interprets the mRNA code and link specific base sequences on the mRNA with specific amino acids; a particular amino acid is attached to one side and an attachment site consisting of 3-base sequence that matches up with a 3-base sequence on the mRNA transcript; each 3-base sequence in mRNA is called a codon; 1. Recognize and initiate protein building: begins in the cell's cytoplasm when the subunits of a ribosome essentially a two-peice protein-building factory recognizes and assembles around a start sequence on the mRNA transcript; start sequence is always AUG; subunits assemble themselves into a ribosome, one side of a tRNA molecule also recognizes the start sequence on the mRNA and binds to it; the initiator tRNA has the amino acid methionine; methionine is the first amino acid in the protein produced (sometimes edited out) 2. Elongate: next codon on the mRNA specify which amino acid carrying tRNA molecule should bind to the mRNA; ribosomes then facilitate the connection of the second amino acid to the first; after this, the tRNA molecule floats away and a new tRNA attaches; this process is called protein synthesis 3. Terminate: eventually the ribosome arrives at the 3-base sequence on the mRNA that signals the end of translation; the assembly is complete once this happens; the amino acid sequence folds and bends as it is produced based on the chemical features in the side chain; the original mRNA strand might be translated a few times or a few hundred times; eventually the mRNA strand is broken down by enzymes in the cytoplasm Causes of Mutation • Mutation: a sequence of bases in an organism's DNA is altered; can lead to changes in the structure and function of the proteins produced; wide range of effects • Point Mutation: where one nucleotide base pair in the DNA is replaced with another, inserted, or deleted; insertions and deletions can be more harmful than substitutions because the amino acid sequence of a protein is determined by reading the codon on an mRNA molecule (if a single base is added or removed, the 3-base groupings get thrown off and the sequence of amino acids is stipulated "downstream" from that point will be all wrong) • Chromosomal aberrations: changes to overall organization of the genes on a chromosome; manipulation of large chunks of data; can involve complete deletion of entire section of DNA, moving of a gene from one part of a chromosome to elsewhere or duplication of a gene with the new copy inserted elsewhere on the chromosome or on a different chromosome; can be altered and affect the expression of the genes around it 1. Spontaneous mutations: arise by accident as long strands of DNA are duplicating themselves at the rate of more than a thousand bases a minute in humans; most errors are repaired by DNA repaired enzymes when cells are dividing 2. Rad
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