Molecular Biology: Transcription PDF

Chapter FIVE Molecular Biology Charmo University 2024-2025 Dr Rebin M. Salih The Central Dogma Translation of mRNA and Post-translational Modifications Part-I...

Chapter FIVE Molecular Biology Charmo University 2024-2025 Dr Rebin M. Salih The Central Dogma Translation of mRNA and Post-translational Modifications Part-I Learning objectives explain the rest of central dogma explain Genetic code and types of codons illustrate structure and function of tRNA understand structure and function of ribosomes The translation of the mRNA codons into amino acid sequences leads to the synthesis of proteins. A variety of cellular components play important roles in translation. These include proteins, RNAs and small molecules. Proteins are the active participants in cell structure and function. Genes that encode polypeptides are termed structural genes which are transcribed into messenger RNA (mRNA). The main function of the genetic material is to encode the production of cellular proteins in the correct cell, at the proper time, and in suitable amounts. 1 Chapter FIVE Molecular Biology Charmo University 2024-2025 Dr Rebin M. Salih An Overview of Gene Expression Archibald Garrod First to propose (at the beginning of the 20th century) a relationship between genes and protein production Beadle and Tatum’s Experiments In the early 1940s, asked this question Is it one gene–one enzyme or one gene–many enzymes? Beadle and Tatum’s conclusion: A single gene controlled the synthesis of a single enzyme o This was referred to as the one gene–one enzyme theory o ? 2 Chapter FIVE Molecular Biology Charmo University 2024-2025 Dr Rebin M. Salih The Genetic Code Translation involves an interpretation of one language into another -Each of the 20 amino acids found in proteins is uniquely specified by one or more codons. -The symbols used by the genetic code are the mRNA bases. + Function as “letters” of the genetic alphabet which has only four “letters” (U, A, C, G) + Genetic language only has 64 “words” -Codons in are all three bases (symbols) long + Function as “words” of genetic information 3 Chapter FIVE Molecular Biology Charmo University 2024-2025 Dr Rebin M. Salih Special codons: o AUG (which specifies methionine) = start codon  AUG specifies additional methionine within the coding sequence o UAA, UAG and UGA = stop codons The code is degenerate o More than one codon can specify the same amino acid -For example: GGU, GGC, GGA and GGG all code for Glycine o In most instances, the third base is the degenerate base (wobble base). The code is nearly universal o Only a few rare exceptions have been noted 4 Chapter FIVE Molecular Biology Charmo University 2024-2025 Dr Rebin M. Salih There are 20 amino acids that may be found in polypeptides. Each contains a different side chain, or R group Levels of Structures in Proteins There are four levels of structures in proteins: 1. Primary 2. Secondary 3. Tertiary 4. Quaternary A protein’s primary structure is its amino acid sequences. Within the cell, the protein will not be found in this linear state. Rather, it will adapt a compact 3-D structure. Indeed, this folding can begin during translation The amino acid sequence of the enzyme lysozyme The progression from the primary to the 3-D structure is dictated by the amino acid 129 amino acids long sequence within the polypeptide 5 Chapter FIVE Molecular Biology Charmo University 2024-2025 Dr Rebin M. Salih The primary structure of a protein folds to form regular, repeating shapes known as secondary structures There are two types of secondary structures: -a helix and -b sheet These are stabilized by the formation of hydrogen bonds. The short regions of secondary structure in a protein fold into a three- dimensional tertiary structure. This is the final confirmation of proteins that are composed of a single polypeptide. Proteins made up of two or more polypeptides have a quaternary structure. This is formed when the various polypeptides associate together to make a functional protein. A protein subunit To a great extent, the characteristics of a cell depend on the types of proteins it makes. Proteins can perform a variety of functions...? 6 Chapter FIVE Molecular Biology Charmo University 2024-2025 Dr Rebin M. Salih A key category of proteins are enzymes.  Accelerate chemical reactions within a cell  Can be divided into two main categories Anabolic enzymes  Synthesize molecules and macromolecules Catabolic enzymes  Break down large molecules into small ones Important in generating cellular energy Structure and Function of tRNA The secondary structure of tRNAs contains: Three stem-loop structures; Variable region An acceptor stem and 3’ single strand region In addition to the normal A, U, G and C nucleotides, tRNAs commonly contain modified nucleotides. Found in all tRNAs Not found in all tRNAs 7 Chapter FIVE Molecular Biology Charmo University 2024-2025 Dr Rebin M. Salih tRNA has two functions: -Recognizing a 3-base codon in mRNA -Carrying an amino acid that is specific for that codon -Recognition of mRNA During mRNA-tRNA recognition, the anticodon in tRNA binds to a complementary codon in mRNA tRNAs are named according to the amino acid they bear Proline anticodon 8 Chapter FIVE Molecular Biology Charmo University 2024-2025 Dr Rebin M. Salih -Charging of tRNAsm (Carrying an amino acid) The enzymes that attach amino acids to tRNAs are known as aminoacyl-tRNA synthetases. There are 20 types: one for each amino acid. The amino acid is attached to the 3’ end by an ester bond Aminoacyl-tRNA synthetases catalyze a two- step reaction involving three different molecules: Amino acid, tRNA and ATP tRNAs and the Wobble Rule The genetic code is degenerate. With the exception of serine, arginine and leucine, this degeneracy always occurs at the codon’s third position. Wobble position and base pairing rules 9 Chapter FIVE Molecular Biology Charmo University 2024-2025 Dr Rebin M. Salih Francis Crick proposed in 1966 the wobble hypothesis o In the codon-anticodon recognition process, the first two positions pair strictly according to the A – U /G – C rule o However, the third position can “wobble” or move a bit  Thus, tolerating certain types of mismatches Ribosome Structure and Assembly  Translation occurs on the surface of a large macromolecular complex termed the ribosome  Bacterial cells have one type of Ribosome. Found in their cytoplasm  Eukaryotic cells have two types of ribosomes. One type is found in the cytoplasm. The other is found in organelles: Mitochondria, Chloroplasts. A ribosome is composed of structures called the large and small subunits. Each subunit is formed from the assembly of Proteins and rRNA. 10 Chapter FIVE Molecular Biology Charmo University 2024-2025 Dr Rebin M. Salih Functional Sites of Ribosomes  During bacterial translation, the mRNA lies on the surface of the 30S subunit o As a polypeptide is being synthesized, it exits through a hole within the 50S subunit Ribosomes contain three discrete sites o Peptidyl site (P site) o Aminoacyl site (A site) o Exit site (E site) ……End of Chapter Five 11