Gene Expression: From Gene to Protein PDF

ROMEL C. MUTYA | Instructor Cebu Technological University – Danao Campus  A NA e m R N D o so m Ri b s i n r o te...

ROMEL C. MUTYA | Instructor Cebu Technological University – Danao Campus  A NA e m R N D o so m Ri b s i n r o te o P i n s Am i d c  OVERVIEW: CONDUCTING THE GENETIC ORCHESTRA Prokaryotes and eukaryotes alter gene expression in response to their changing environment In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell types RNA molecules play many roles Gene Expression the process by which DNA directs the synthesis of proteins (or, in some cases, just RNAs). The expression of genes that code for proteins includes two stages transcription and translation. there are three main steps: transcription, RNA processing, and  Genes specify proteins via transcription and translation Archibald Garrod (1902) first to suggest that genes dictate phenotypes through enzymes that catalyze specific chemical reactions in the cell One gene–one enzyme hypothesis (postulated that the symptoms of an inherited disease reflect a person’s inability to make a particular enzyme.)  Basic Principles of Transcription and Translation Transcription the process by which DNA makes RNA the synthesis of RNA using information in the DNA the DNA code is transcribed into a codon sequence in messenger RNA (mRNA), following the base-pairing rules: A with U and C with G there is no thymine in RNA. Uracil replaces thymine. MESSENGER-RNA (mRNA)  made from a DNA template during a process called transcription  carries the DNA code to the ribosomes where the message is translated  molecules are broken down after they carry out transcription. If the DNA sequence is: AAA TAA CCG GAC Then the complementary sequence of codons in mRNA is: UUU AUU GGC CUG Translation the synthesis of a polypeptide using the information in the mRNA. During this stage, there is a change in language: The cell must translate the nucleotide sequence of an mRNA molecule into the amino acid sequence of a polypeptide. The sites of translation are ribosomes, molecular complexes that facilitate the orderly linking of amino acids into polypeptide The Genetic Code dons: Triplets of Nucleotides  a sequence of three DNA or RNA nucleotides that corresponds with a specific amino acid or stop signal during protein synthesis  mRNA nucleotide triplets and they are customarily written in the 5’  3’ direction The codon table for mRNA. The three nucleotide bases of an mRNA codon are designated here as the first, second, and third bases, reading in the 5’  3’ direction along the mRNA. The codon AUG not only stands for the amino acid methionine (Met) but also functions as a “start” signal for ribosomes to begin translating the mRNA at that point. Three of the 64 codons function as “stop” signals, marking where CENTRAL DOGMA BY FRANCIS CRICK IN 1956Genes program protein synthesis via genetic messages in the form of messenger RNA. Put another way, cells are governed by a molecular chain of command with a directional flow of genetic information, shown here by arrows:  DNA mRNA Proteins Francis Crick You developed the “Central Dogma”  TRANSCRIPTION is the DNA-directed synthesis of RNA: A closer look Transcription begins when the enzyme RNA polymerase binds to one strand of DNA at the promoter region. Next, RNA polymerase pries apart the two DNA strands and adds nucleotides to the growing end of the new strand, a process called elongation. When the newly forming RNA strand reaches a STOP codon, it terminates elongation, and peels away from its DNA template, allowing the DNA double helix to reform. Notice that whereas replication involved both strands of DNA, transcription only  The stages of transcription: initiation, elongation, and termination. This general depiction of transcription applies to both bacteria and eukaryotes, but the details of termination differ, as described in the text. Also, in a bacterium, the RNA transcript is immediately usable as mRNA; in a eukaryote, the RNA transcript must first undergo processing. 1. RNA Polymerase Binding and Initiation of Transcription  The promoter of a gene includes within it the transcription start point (the nucleotide where RNA synthesis actually begins  RNA polymerase binds in a precise location and orientation on the promoter  in eukaryotes, a collection of proteins called transcription factors mediate the binding of RNA polymerase and the initiation of transcription (TATA box) 2. Elongation of the RNA Strand RNA polymerase moves along the DNA template strand, joining complementary RNA nucleotides to the 3’ end of the growing RNA transcript. Behind the polymerase, the new RNA peels away from the template strand, which re-forms a double helix with the nontemplate strand. 3. Termination of Transcription  In bacteria, transcription proceeds through a terminator sequence in the DNA  The transcribed terminator (an RNA sequence) functions as the termination signal, causing the polymerase to detach from the DNA and release the transcript, which requires no further modification before translation  In eukaryotes, RNA polymerase II transcribes a sequence on the DNA called, the polyadenylation signal sequence which specifies a Alteration of mRNA Ends plit Genes and RNA Splicing Introns (intervening sequences)  these noncoding regions that are removed Exons (expressed sequences or coding regions)  are pieced back together to form the final transcript. Translation is the RNA-directed synthesis of a polypeptide: A closer look  the process by which the mRNA sequence is converted into an amino acid sequence.  occurs at the ribosome.  Amino acids present in the cytoplasm are carried by tRNA molecules to the codons of the mRNA strand at the ribosome according to the base-pairing rules (A with U and C with G).  Several codons can code for the same amino acid. For example, codons UCU, UCC, UCA, and UCG all code for a single amino acid, serine. TRANSFER-RNA (tRNA)  a clover-leaf-shaped molecule that carries specific amino acid molecules to mRNA at the ribosome to help form a polypeptide during the process of translation  transfer amino acids from the cytoplasm to the ribosome An anticodon is a trinucleotide sequence complementary to that of a corresponding codon in a messenger RNA (mRNA) sequence. An anticodon is found Example:at one end of a transfer RNA Consider the (tRNA) mRNA codon molecule. 5’-GGC-3’, which is translated as the amino acid glycine. The tRNA that base-pairs with this codon by hydrogen bonding has 3’-CCG-5’ as its anticodon and carries glycine at its other end RIBOSOMAL-RNA (rRNA) a structural molecule that makes up part of a ribosome Each ribosome consists of a large and small subunit, each made of proteins and one or more rRNAs.