12_18_lecture(1) (1).pptx

Chapter 18 Gene Expression: I. The Genetic Code and Transcription Lectures by Kathleen Fitzpatrick © 2016 Pearson Education, Inc. Simon Fraser University Gene Expression: I. The Genetic Code and Transcription  The coded information of DNA is used to guide production of RNA and protein molecules...

Chapter 18 Gene Expression: I. The Genetic Code and Transcription Lectures by Kathleen Fitzpatrick © 2016 Pearson Education, Inc. Simon Fraser University Gene Expression: I. The Genetic Code and Transcription  The coded information of DNA is used to guide production of RNA and protein molecules  Transcription refers to RNA synthesis using DNA as a template  Translation is the synthesis of protein using the information in the RNA © 2016 Pearson Education, Inc. Transcription and Translation Involve Many of the Same Components in Bacteria and Eukaryotes  Messenger RNA, mRNA, is RNA that is translated into protein  Ribosomal RNA, rRNA, is an integral component of the ribosome  Transfer RNA, tRNA, molecules serve as intermediaries, bringing amino acids to the ribosome  The latter two function in translation © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Where Transcription and Translation Occur Differs in Bacteria and Eukaryotes  Because bacteria do not have a nuclear envelope, translation of mRNA can begin before its transcription is completed  The compartmentalization of eukaryotic cells leads to spatial separation of transcription and translation © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. In Some Cases RNA Is Reverse Transcribed into DNA  In certain cases, RNA can serve as a template for DNA synthesis  This process of reverse transcription is catalyzed by the enzyme reverse transcriptase  Viruses that carry out reverse transcription are called retroviruses © 2016 Pearson Education, Inc. Retroviruses  Two copies of the RNA genome. Each RNA molecule has an reverse transcriptase  Once inside the cell, reverse transcriptase catalyzes synthesis of a DNA  The resulting double-stranded DNA then enters the nucleus and integrates into the genome of the host  https://www.the-scientist.com/news-opinion/ ancient-viral-dna-plays-a-role-in-humandisease-and-development-70656#:~:text=There %20are%20around%2030%20different,over %20the%20course%20of%20evolution. © 2016 Pearson Education, Inc. Retrotransposons  Reverse transcription also occurs in normal eukaryotic cells in the absence of viral infection  Much of it involves DNA elements called retrotransposons  These use reverse transcription to move from one site to another within a genome © 2016 Pearson Education, Inc. Retrotransposon Movement  Transposition begins with transcription of the retrotransposon DNA followed by translation of the resulting RNA  This produces a protein with reverse transcriptase and endonuclease activities  The retrotransposon RNA and protein then bind chromosomal DNA in a new location © 2016 Pearson Education, Inc. Retrotransposon Movement (continued)  The endonuclease cuts one of the DNA strands  The reverse transcriptase uses the retrotransposon RNA as a template to make a DNA copy that is then integrated into the target site © 2016 Pearson Education, Inc. Alu Sequences  Retrotransposons can attain high copy numbers within a genome despite transposing rarely  Alu sequences are 300 bp long and do not encode a reverse transcriptase  But using reverse transposase from elsewhere in the genome, they have increased their copy number in humans and other primates  In the human genome, about 1 million Alu sequences represent about 11% of the total DNA © 2016 Pearson Education, Inc. The Genetic Code  The relationship between the DNA base sequence and the linear order of amino acids in the protein products is based on a set of rules known as the genetic code © 2016 Pearson Education, Inc. Gene Function Is Complicated  Most eukaryotic genes contain noncoding sequences among the coding regions of the gene  Coding sequences can be read in various combinations, each coding for a unique polypeptide chain; this is called alternative splicing  Some types of genes encode functional RNAs  Genes are thus defined as functional units of DNA that encode one or more polypeptides or functional RNA © 2016 Pearson Education, Inc. The Genetic Code Is a Triplet Code  There are four DNA bases and 20 amino acids  A triplet code, in which combinations of three bases specify amino acids, would have 64 possible combinations, more than enough for all 20 amino acids © 2016 Pearson Education, Inc. Frameshift Mutations  The gene is written in a language of three-letter words  Inserting or deleting a nucleotide (indel mutations) causes the rest of the sequence to be read out of phase—this is a shift in the reading frame  Mutations that cause insertion or deletion of a nucleotide are thus called frameshift mutations © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. The Genetic Code Is Degenerate and Nonoverlapping  There are 64 combinations of nucleotide triplets and only 20 amino acids  The genetic code is a degenerate code, particular amino acid can be specified by more than one triplet  It is also nonoverlapping; the reading frame advances three nucleotides at a time © 2016 Pearson Education, Inc. Messenger RNA Guides the Synthesis of Polypeptide Chains  The genetic code refers to the order of nucleotides in the mRNA molecules that direct protein synthesis  mRNA is transcribed from DNA similarly to how DNA is replicated, but with two differences © 2016 Pearson Education, Inc. Differences Between mRNA Synthesis and DNA Replication  In mRNA synthesis, only one DNA strand is copied, called the template strand; the other strand is called the coding strand because it is similar to the mRNA sequence  In mRNA synthesis, a uracil base (U) is used instead of thymine © 2016 Pearson Education, Inc. Of the 64 Possible Codons in Messenger RNA, 61 Code for Amino Acids  All 64 codons are used in the translation of mRNA  61 of them specify a specific amino acids  One of them, AUG, plays a role as a start codon  The remaining three (UAA, UAG, UGA) are stop codons © 2016 Pearson Education, Inc.