BIOL 205 Principles of Human Biology - DNA & Gene Expression PDF

University of Balamand Faculty of Health Sciences BIOL 205– Principles of Human Biology CHAPTER 5 – DNA & GENE EXPRESSION Dr. Espérance Debs  Outline 2  The structure of DNA  Genotype & phenotype  DNA replication  DNA Transcr...

University of Balamand Faculty of Health Sciences BIOL 205– Principles of Human Biology CHAPTER 5 – DNA & GENE EXPRESSION Dr. Espérance Debs  Outline 2  The structure of DNA  Genotype & phenotype  DNA replication  DNA Transcription  RNA Translation Transcription Translation  DNA >>>>>>mRNA>>>>>>Protein DNA & Gene Expression  The structure of DNA 3 ❖ Genetics: Study of genes ❖ Genome: holds the genetic information ❖ Genes: DNA fragments ❖ DNA: Double helix Composed of nucleotides (bases : A,T,C,G), ribose (deoxyribose) and phosphate group), linked by hydrogen bonds DNA & Gene Expression  Chromatin condensation in a Eukaryotic chromosome 4 DNA & Gene Expression  Genotype & phenotype 5 ❖ Genotype : Sum of all genes that encodes for the characteristics of an organism ❖ Phenotype : Refers to expressed properties. It is the manifestation of the genotype DNA & Gene Expression  DNA replication 6  DNA replication 7  Each daughter double helix consists of an original strand from the parent molecule and a newly synthesized complementary strand.  During DNA replication, the two strands of the double helix unwind. Each strand serves as a template for forming a new, complementary strand. DNA & Gene Expression  DNA replication 8  Helicase: Opens the double helix at replication forks by disrupting the hydrogen bonds that hold the two strands together.  Single-strand binding protein (SSB): Binds to single strands of DNA and prevents the helix from re-forming before it can be used as a template for replication.  DNA polymerase: Links nucleotide subunits to form a new DNA strand from a DNA template. DNA & Gene Expression  Nucleic acids and proteins 9 Genes → Messenger RNA (mRNA) → Proteins Genes → ribosomal RNA (rRNA) or transfer RNA (tRNA) → Proteins DNA & Gene Expression  Gene expression (protein synthesis) 10 It is the process by which DNA is expressed into proteins. This expression occurs into 2 stages: 1. Transcription 2. Translation DNA & Gene Expression  Overview 11 Transcription: For every gene, one strand only will be translated into mRNA. This strand is called DNA template strand. RNA bases are: C, G, U (replaces T in DNA) and A DNA & Gene Expression  Overview 12 Translation: The synthetized mRNA is read in a sequence of triplets. Every 3 nucleotides will form 1 amino acid that will be joined to form a protein. DNA & Gene Expression  Codons table 13 It includes 64 codons of which 61 encodes amino acids. Nucleotides are read in the following direction: 5’→ 3’ of the mRNA. AUG Codon : encodes methionine; Start codon. This is the starting signal for the translation. UAA / UAG / UGA Codons : encode STOP codons. They indicate the end of the translation process. DNA & Gene Expression  Transcription 14 Important molecular components of the transcription: A. Enzyme: RNA polymerase B. Proteins: Transcription factors C. DNA sequences: Promoter and Terminater Steps of transcription: 1. Initiation 2. Elongation 3. Termination DNA & Gene Expression  Translation 15 Important molecular components of the translation: A. Messenger RNA B. Transfer RNA C. Ribosomes D. Amino acids DNA & Gene Expression  Transfer RNA 16 Transfer RNA has the function to pair complementary anticodon with codon and to link it to a correspondant amino acid. The anticodon is read in the 3’→ 5’ direction. DNA & Gene Expression  Ribosomes 17 Ribosomes are composed of: 1. 3 binding sites with transfer RNA (big sub-unit) 2. 1 binding site with messenger RNA (small sub-unit) DNA & Gene Expression  Completion of a functional protein 18 Proteins may be translated by many ribosomes simultaneously forming a polyribosome. Microscopic observation of a polyribosome. DNA & Gene Expression  Summary of the transcription and translation in a eukaryotic cell 19 DNA & Gene Expression