BIOL 217 Topic 11 Fall 2024 PDF

BIOL 217 Topic 11 Fall 2024 Learning Objectives – Topic 11 Gene Expression – Chapter 17 Understand the central dogma, the genetic code, and how information flows from gene to protein Compare and contrast transcription and translation Be able to explain the steps in transcription, includi...

BIOL 217 Topic 11 Fall 2024 Learning Objectives – Topic 11 Gene Expression – Chapter 17 Understand the central dogma, the genetic code, and how information flows from gene to protein Compare and contrast transcription and translation Be able to explain the steps in transcription, including the important molecules involved and the three steps (initiation, elongation, termination) Understand the different types of mutations and how they can modify the translated protein 2  The Flow of Genetic Material DNA is inherited by an organism and due to the synthesis of proteins, this leads to expression of specific traits The information content of genes is in the specific sequences of nucleotides Proteins are the links between genotype and phenotype Gene expression: the process by which DNA directs protein synthesis Includes two stages: transcription and translation 3 Figure Page 355  Principles of Gene Expression RNA is the bridge between genes and the proteins for which they code Transcription: synthesis of RNA using information in DNA Transcription produces messenger RNA (mRNA) Translation: synthesis of a polypeptide, using information in the mRNA Ribosomes are the sites of translation 4 Figure 17.4  Principles of Gene Expression In prokaryotes: Translation of mRNA can begin before transcription has finished In eukaryotes: Nuclear envelope separates transcription from translation Eukaryotic RNA transcripts are modified through RNA processing to yield the finished mRNA A primary transcript is the initial RNA transcript from any gene prior to processing 5 Figure 17.4  Central Dogma The central dogma is the concept that cells are governed by a cellular chain of command: 6 Figure from Page 359  The Genetic Code Flow of information from gene to protein: triplet code Triplet code: non overlapping, three nucleotide words Words of a gene: Transcribed into complementary non overlapping, three nucleotide words of mRNA Words in mRNA are translated into amino acids, forming a polypeptide 7 Figure 17.5  The Flow of Information Template strand: Provides a template for the sequence of complementary nucleotides in an mRNA transcript Non template strand: Nucleotides of this strand are identical to the codons of mRNA, except U in RNA replaces T in DNA 8 Figure 17.5  The Flow of Information During translation, the mRNA triplets, called codons, are read in the 5′ → 3′ direction Each codon specifies the amino acid to be added in a growing polypeptide From gene (DNA sequence) to protein (polypeptide sequence) 9 Figure 17.5  Universal Genetic Code The genetic code is nearly universal: Shared by the simplest bacteria and the most complex animals Genes can be transcribed and translated after being introduced to a new species 10 Figure 17.7 Transcription 11  Transcription Transcription is the first stage of gene expression DNA directed RNA synthesis The three stages of transcription: Initiation Elongation Termination RNA synthesis is catalyzed by RNA polymerase 12 Modified Figure 17.4  Transcription and RNA Polymerase RNA polymerase: Catalyzes RNA synthesis Opens DNA strands and joins RNA nucleotides together in 5’ – 3’ direction Does not need any primer Follows same base-pairing rules as DNA, except that uracil substitutes for thymine Produces RNA that is complementary to the DNA template strand 13 Figure 17.8  Transcription and RNA Polymerase Promoter: DNA sequence where RNA polymerase attaches In bacteria, the sequence signaling the end of transcription is called the terminator The stretch of DNA that is transcribed is called a transcription unit 14 Figure 17.8 Transcription 15  Synthesizing an RNA transcript - Initiation Promoters: Transcriptional start point Usually extend several dozen nucleotides upstream of the start point The TATA box is crucial in forming the initiation complex in eukaryotes 16 Figure 17.9  Synthesizing an RNA transcript - Initiation Transcription factors: Mediate the binding of RNA polymerase and the initiation of transcription Transcription initiation complex: The completed assembly of transcription factors and RNA polymerase II bound to a promoter 17 Figure 17.9  Synthesizing an RNA transcript - Elongation As RNA polymerase moves along DNA, it untwists the double helix, 10-20 bases at a time Transcription progresses at a rate of 40 nucleotides per second in eukaryotes A gene can be transcribed simultaneously by several RNA polymerases Nucleotides are added to the 3′ end of the growing RNA molecule 18 Figure 17.10  Synthesizing an RNA transcript - Termination The mechanisms of termination are different in bacteria and eukaryotes In bacteria, the polymerase stops transcription at the end of the terminator mRNA can be translated without further modification In eukaryotes, RNA polymerase transcribes the polyadenylation (polyA) signal sequence; RNA transcript is released 10–35 nucleotides past this polyadenylation sequence 19 Figure 17.8 Translation 20  Understanding Translation Genetic information flows from mRNA to protein through the process of translation Translation creates a polypeptide from the mRNA information 21 Figure 17.15  Molecular Components involved in Translation A cell translates an mRNA message into protein with the help of transfer RNA (tRNA) tRNAs transfer amino acids to the growing polypeptide in a ribosome 22 Figure 17.15  Transfer RNA (tRNA) Each tRNA molecule enables translation of a specific mRNA codon into a certain amino acid Each carries a specific amino acid on one end Each has an anticodon on the other end Anticodon base-pairs with a complementary codon on mRNA 23 Figure 17.15  3D Structure of tRNA Single RNA strand that is ~80 nt long Flattened into one plane to reveal tRNA base pairing, looks like a cloverleaf Hydrogen bonding twists tRNA into 3D molecule tRNA is roughly L-shaped with the 5' and 3' ends both located near one end of the structure The protruding 3' end acts as an attachment site for an amino acid 24 Figure 17.16  Accurate Translation Requires Two Major Steps Correct match between a tRNA and an amino acid, catalyzed by the enzyme aminoacyl-tRNA synthetase Correct match between the tRNA anticodon and an mRNA codon 25 Figure 17.17  Ribosomes Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis The two ribosomal subunits (large and small): Made of proteins and ribosomal RNA (rRNA) 26 Figure 6.10  Ribosomal binding sites for tRNA A ribosome has three binding sites for tRNA The A site holds the tRNA that carries the next amino acid to be added to the chain The P site holds the tRNA that carries the growing polypeptide chain The E site is the exit site, where discharged tRNAs leave the ribosome 27 Figure 17.18  Building a polypeptide The three stages of translation: Initiation Elongation Termination Energy is required for some steps Factors help with each step 28 Figure 17.18 Translation 29 Translation 30  Mutations Mutations are changes in the genetic information of a cell Mutations of one or a few nucleotides can affect protein structure and function If a mutation has an adverse effect on the phenotype of the organism Referred to as a genetic disorder or hereditary disease 31 RosieLea (2015). Daisy Mutant. https://cdn.pixabay.com/photo/2015/11/13/04/33/daisy-1041464_1280.jpg New Mutations and Mutagens Spontaneous mutations can occur during errors in DNA replication, recombination, or repair Mutagens are physical or chemical agents that can cause mutations 32  DNA Mutations Point mutations: changes in just one nucleotide pair of a gene The change of a single nucleotide in a DNA template strand can lead to the production of an abnormal protein Point mutations within a gene can be divided into two general categories: Single nucleotide-pair substitutions Nucleotide-pair insertions or deletions 33 Figure 17.26  Nucleotide-pair substitution Silent mutations: No effect on the amino acid produced by a codon because of redundancy in the genetic code Due to Wobble Hypothesis 34 Figure 17.27  Nucleotide-pair substitution Missense mutations: Still code for an amino acid, but not the correct amino acid 35 Figure 17.27  Nucleotide-pair substitution Nonsense mutations: Change an amino acid codon into a stop codon Most lead to a nonfunctional protein 36 Figure 17.27