BIO 101 Lecture 101, Fall 2017

Topic 8 Gene Activity: How Genes Work Function of Genes A gene is a segment of DNA that specifies the sequence of amino acids in a polypeptide of a protein. From DNA to RNA to Protein Central Dogma of Molecular Biology: DNA  RNA  Protein Genes code for the sequence of nucleotides in RNA molecules. RNA brings about formation of a protein coded for by DNA of a particular gene. RNA Uracil (U) replaces thymine (T) of DNA. Types of RNA  Messenger (mRNA) - Takes message from DNA in nucleus to ribosomes in cytoplasm.  Transfer (tRNA) - Transfers amino acids to ribosomes.  Ribosomal (rRNA) - Help make up ribosomes. Gene Activity 2 Structure of RNA Steps of Gene Expression • During transcription, DNA serves as a template for RNA formation. • During translation, an mRNA transcript directs the sequence of amino acids in a polypeptide. Gene Expression Gene Activity 3 The Genetic Code There is a genetic code for each of the 20 amino acids found in proteins. Genetic code is a triplet code, with each codon consisting of three nucleotide bases.  Code Properties  Universal  Degenerate  Unambiguous  Contains start and stop signals Transcription During transcription, an mRNA molecule is formed with a sequence of bases complementary to a portion of one DNA strand. RNA polymerase joins the nucleotides together in 5’  3’ direction. Promoter defines start of a gene, the direction of transcription, and the strand to be transcribed. Elongation continues until polymerase comes to a DNA terminator sequence. Gene Activity 4 Processing Messenger RNA Newly formed mRNA molecule, primary mRNA transcript, is modified before it leaves the eukaryotic nucleus. Modifications include adding a cap on 5’ end, and poly-A tail on 3’ end. Non coding parts of this mRNA called introns are removed during RNA processing by a process called splicing, leaving only exons to produce mature mRNA. Translation Transfer RNA molecules transfer amino acids to the ribosomes. Each tRNA molecules bind with one particular amino acid, and bear an anti-codon complementary to the codon for that amino acid. A group of enzymes, tRNA synthetases attach correct amino acid to the correct tRNA molecule. Gene Activity 5 Transfer RNA Role of Ribosomal RNA Ribosomal RNA (rRNA) is produced off a DNA template in the nucleolus of a nucleus. It is packaged with proteins into ribosomal subunits, one larger than the other. Ribosomes contain binding sites to facilitate complementary base pairing between tRNA anti-codons and mRNA codons. Gene Activity 6 Ribosome Structure and Function Overview of Protein Synthesis in the Cell