Chapter 1 - Genetics Inheritance & Genetic Control 2024/2025 Part 2 PDF

# Chapter 1: Genetic Inheritance & Genetic Control ## 2024/2025, Part 2 ## Content - Terms in Genetics - Mendelian Genetics - Monohybrid inheritance - Dihybrid inheritance - Extension of Mendelian Genetics - Human Pedigree Analysis - Population Genetics - Terms in population g...

# Chapter 1: Genetic Inheritance & Genetic Control ## 2024/2025, Part 2 ## Content - Terms in Genetics - Mendelian Genetics - Monohybrid inheritance - Dihybrid inheritance - Extension of Mendelian Genetics - Human Pedigree Analysis - Population Genetics - Terms in population genetics - Hardy-Weinberg Equation - The Hardy-Weinberg Equilibrium: Conditions - DNA Structure and Replication - DNA double helix - Models of DNA Replication - Meselson and Stahl’s experiment - DNA Replication - Protein Synthesis: Transcription and Translation - Types of RNA - Basic Principles of Transcription & Translation - Genetic Code - Transcription of RNA - Modification of pre-mRNA in Eukaryotes - Translation - Building A Polypeptide - Regulation of Gene Expression - Regulation of gene expression - Components of an operon - Lactose operon (lac operon) - Mutation - Mutation - Small-scale mutations - Chromosome mutations - Human disorder due to chromosomal alteration ## 1.4 DNA structure and replication ### 1.4.1 DNA double helix - DNA a polymer of nucleotides. - Phosphodiester linkage is the bond between adjacent nucleotides. - The sugar-phosphate backbone is the alternating phosphates & sugar molecules from which the bases projects. - The directionality of the polynucleotide strands is from 5' end (with phosphate group) to the 3' end (with the -OH group of the sugar). ### 1.4.1 DNA Double Helix: The Structure Of A DNA Strand The image shows the structure of a DNA strand with important components labeled: - Sugar-phosphate backbone - Nitrogenous bases - Phosphate - DNA nucleotide - Sugar (deoxyribose) - Nitrogenous base **Important points:** - This is a schematic representation of the DNA molecule, not a complete structural model. - The molecule is shown as a twisted ladder structure, with the sugar-phosphate backbone forming the sides of the ladder and the nitrogenous bases forming the rungs. - In the image, the nitrogenous bases are represented by their single-letter abbreviations. - The 5' end of the DNA strand is labeled with a 5' end. - The 3' end of the DNA strand is labeled with a 3' end. ### The double helix - Watson & Crick model of DNA - The double helix model of a DNA molecule was discovered by Watson & Crick in 1953. - The 2 strands of DNA form a right-handed double helix. - The 2 DNA strands are stabilized by hydrogen bonding between the base pairs. - The bases pairs is according to the Chargaff’s rule. - The 2 polynucleotides strands are antiparallel (in opposite orientation) with regard to their 5' to 3' directionality. ### Animation: DNA and RNA Structure - The image shows a schematic representation of the DNA and RNA structure. - The DNA molecule is depicted as a double helix. - The RNA molecule is depicted as a single strand. - The bases in each molecule are represented by their single-letter abbreviations. - The image is also accompanied by a QR code. ### From DNA to Chromosome - This image shows different levels of DNA packaging that occurs in a eukaryotic cell. - Starting with the DNA double helix, the DNA is packaged into nucleosomes, which are further packaged into 30-nm fibers. - These 30-nm fibers are then organized into looped domains, which are then further packaged into the chromosome. - Each level of packaging is shown in the image, with the diameter of each structure labeled in nanometers. ### The Molecular Structure of the DNA Double Helix - The Chargaff’s rule: - DNA base composition varies between species. - The percentage of A & T bases are roughly equal, as are those of G & C. bases. - This rule implies that: base sequences within 2 DNA strands are complementary to each other. - **Example:** - The sequence of 1st strand: 5’-ATGGCGGATTT-3’ - The sequence of the opposite strand: 3'-TACCGCCTAAA-5' ### 1.4.2 Models of DNA Replication - DNA replication is the process by which a double-stranded DNA molecule is copied to produce 2 identical DNA molecules. - There are 3 alternatives models of DNA replication: - Conservative model - Semiconservative model - Dispersive model ### 1.4.2 Models of DNA Replication - The image shows the 3 models of DNA replication as simple diagrams: - The conservative model - The semiconservative model - The dispersive model - Based on the different models, the original parental DNA strands are colored differently from the newly synthesised DNA strands. ### DNA Replication: SEMICONSERVATIVE MODEL - Watson and Crick's SEMICONSERVATIVE MODEL of replication predicts that, when a double helix replicates, each daughter molecule will have one old strand, derived from the parent molecule, and one newly made strand. - The parental molecule serves as a template for synthesis of a new complementary strand. - The image shows a schematic representation of the semiconservative model of DNA replication: - The original parent molecule - The separation of the two strands - The two daughter molecules, each consisting of one parental strand and one new strand ### 1.4.3 Meselson and Stahl’ Experiment - In the 1950s, Matthew Meselson and Franklin Stahl carried out an experiment that supported the semiconservative model of DNA replication. - **The experiment:** - Bacterial cells were grown in media containing heavy isotope of nitrogen, 15N - Bacteria incorporated 15N into DNA - Then bacteria were switched to a media containing lighter 14N - Bacteria incorporated 14N into DNA - DNA was later extracted from the bacteria at various time intervals ### Experiment by Meselson and Stahl - The image shows the steps of the Meselson and Stahl experiment: - Bacteria were grown in a medium containing heavy nitrogen (15N). - Bacteria were then transferred to a medium containing light nitrogen (14N). - DNA samples were taken after the first and second rounds of replication and tested for the presence of heavy (15N) and light (14N) nitrogen. - The density of the DNA was measured using centrifugation. - The results of the experiment supported the semiconservative model of DNA replication. ### CONCLUSION - The image shows the prediction of the 3 models of DNA replication and the results of the Meselson and Stahl's experiment. - The conservative model is shown as incorrect since the DNA strands have 15N and 14N after the first replication. - The semiconservative model is shown as correct, depicting the appearance of hybrid DNA molecules. - The dispersive model is shown as incorrect since the DNA strands are not a mixture after the first replication. ### 1.4.4 DNA Replication - **(a) Origin of replication in an E. coli cell:** - **origin of replication** - **double-stranded DNA molecule** - **two daughter DNA molecules** - **replication bubble** - **parental (template) strand** - **daughter (new) strand** - **replication fork** - **E. coli chromosome (& other bacterial chromosomes) is circular & has a single origin.** ### DNA Replication: The Process - **(b) Origins of replication in a eukaryotic cell:** - **Origin of replication** - **double-stranded DNA molecule** - **parental (template) strand** - **daughter (new) strand** - **replication fork** - **two daughter DNA molecules** - **bubble** - **Replication of DNA molecule begins at the origin of replication.** - **Enzymes recognize DNA sequence at origin of replication → Separates the 2 strands and opens up a replication ‘bubble’.** - **From the origin, replication of DNA proceeds in both directions.** - **Multiple replication bubble forms to speed up the DNA replication.** ### Animation: Origins of Replication - This image shows a schematic representation of the origin of replication on a linear DNA molecule. - The origin of replication is labeled in red. ### DNA Replication: Getting started - At each end of the replication bubble, is a replication fork, which is a Y-shaped region where the parental strands of DNA begins to unwind. - **Some of the proteins involve during the initiation of DNA replication:** - **topoisomerase** - **primase** - **helicase** - **single-strand binding proteins** - **The synthesis of new DNA strand will start from the 3' end of the RNA primer.** ### Proteins involved in DNA Replication - The image shows a table with the proteins involved in DNA replication and their functions. - The proteins are grouped into five categories: - **helicase** - **singe-strand binding (SSB) protein** - **topoisomerase** - **DNA primase** - **DNA pol III** - **DNA pol I** - **DNA ligase** ### DNA Replication: Synthesizing a New DNA Strand - **To synthesize a new DNA strand, requires:** - **DNA polymerases:** - cannot initiate synthesis of a polynucleotide - catalyze the synthesis of new DNA, by adding nucleotides to the existing 3' end of an RNA primer or growing DNA strand (& never to the 5' end). - Thus, a new DNA strand elongates only in the 5' 3' direction. - **Nucleoside triphosphates (dNTPs):** - are the building blocks for DNA replication. - Types: dATP, dGTP, dCTP, dTTP ### DNA Replication: Antiparallel Elongation - **Synthesis of new DNA molecules during DNA replication involves:** - **The synthesis of leading strands:** Occurs from the Origin of replication towards the replication fork. - **The synthesis of lagging strands:** Occurs from the replication fork towards the Origin of replication. - The image shows a schematic representation of the two strands of DNA. - It shows the leading strand and lagging strand, along with their directions of synthesis in relation to the replication fork and the origin of replication. ### 1. Synthesis of LEADING STRAND - **Along 1 template strand, starting at the origin of replication (that has a 3' end), primase synthesizes a short RNA primer.** - **This enables DNA pol III to synthesize a complementary strand continuously, by elongating the new DNA strand in the 5' 3' direction towards the replication fork.** - **Only 1 primer is required for DNA pol III to synthesize the leading strand.** - **DNA pol I Then replaces the RNA primer with DNA nucleotides.** - The image shows a schematic representation of the synthesis of the leading strand of DNA. - The leading strand is synthesized continuously. - The RNA primer is shown in red and the DNA polymerase III is shown copying the DNA strand. ### 1. Synthesis of LAGGING STRAND - **Lagging strand is synthesized discontinuously as short Okazaki fragments, in the direction away from the replication fork.** - **Primase joins RNA nucleotides into a primer.** - **DNA pol III adds DNA nucleotides to the primer, forming Okazaki fragment 1, in the 5' 3' direction.** - **DNA pol III detaches once it meets RNA primer to the right.** - **DNA pol III makes Okazaki fragment 2.** - **Fragment 2 is primed.** - **DNA pol I degrades RNA primer & replaces the RNA with DNA.** - **DNA ligase joins the Okazaki fragments to form the lagging strand into continuous DNA strand.** - The image shows a schematic representation of the synthesis of the lagging strand of DNA. - The lagging strand is synthesized discontinuously with Okazaki fragments. ### Animation: Lagging Strand - This image shows a schematic representation. - It depicts the lagging strand being synthesized discontinuously, with the leading strand being synthesized continuously. ### 1.5 Protein synthesis: Transcription and Translation ### 1.5.1 Types of RNA - RNA exist as a single strand polynucleotide and important in the production of proteins. - **3 types of RNA:** - **mRNA:** A linear sequence of mRNA that copy the information contained in DNA (genes) to be translated in the translation process to make protein. mRNA travels to the ribosomes. - **rRNA:** is a component of the ribosomes. - **tRNA:** Each different tRNA carry specified amino acids to ribosome. ### 1.5.2 Basic Principles of Transcription & Translation - **TRANSCRIPTION:** - “The synthesis of RNA using a DNA template.” - The information from DNA is transcribed or COPIED into RNA molecule. - Transcription produces messenger RNA (mRNA) - Transcription leads to the translation - **TRANSLATION:** - “The synthesis of a polypeptide using information in the mRNA.” - Ribosomes are the site of translation ### Transcription and translation in prokaryotic cells - **Transcription:** - Occurs in the cytoplasm. - A gene provides the instruction for synthesizing mRNA. - **Translation:** - The information encoded in mRNA determines the sequence of amino acids to form a specific polypeptide. - Occurs at the ribosomes in the cytoplasm. - The image shows a schematic representation of transcription and translation in a prokaryotic cell. - Transcription takes place in the cytoplasm, and translation takes place at ribosomes. ### Transcription and Translation in Eukaryotic Cell - The inherited information flows from DNA → RNA → Protein. - **Transcription:** - Occurs in the nucleus. - A gene provides the instruction for synthesizing mRNA. - **RNA Processing:** - The original RNA transcript (primary transcript or pre-mRNA) is processed before leaving the nucleus as functional mRNA. - **Translation:** - The information encoded in mRNA determines the sequence of amino acids to form a specific polypeptide. - Occurs at ribosomes in the cytoplasm. - The image shows a schematic representation of transcription and translation in a eukaryotic cell. - Transcription takes place in the nucleus. - RNA is processed before leaving the nucleus. - Translation takes place in the cytoplasm at the ribosomes. ### 1.5.3 Genetic Code - For each gene, one DNA strand functions as a template for transcription. - During translation, the mRNA is read as a sequence of base triplets called as codons, in the 5' 3' direction. - Each codon specifies an amino acid to be added to the growing polypeptide chain. - The terms CODONS is also be use for the complementary DNA base triplet on non-template strand (known as coding strands). - **Example:** - DNA template strand : 3'- ACC- 5' - corresponding RNA codon: 5' - UGG - 3' - The image shows a schematic representation of transcription and translation. - It depicts the DNA template strand from which the mRNA is transcribed. - It shows how codons on the mRNA are translated into amino acids. ### The Codon Table for mRNA - Triplet codons are read from the 5' to 3' direction. - Triplet codons are read from the 5' to 3' direction. - Triplet codons are read from the 5' to 3' direction. - The image shows the genetic code table, which shows the amino acid that is encoded by each mRNA codon. ### Feature of the Genetic Code - One start codon (initiation codon) of mRNA: AUG (Met) - Three stop codons (termination codons) of mRNA: UAA, UAG and UGA - Linear: Codons of mRNA is read in 5' to 3' direction and in the correct reading frame (E.g.: 5' UGG UUU GGC UCA 3'). - Non-overlapping: Each base in the sequence is read only once. - Unambiguous: Each code has only one meaning. - Redundancy: Some amino acids are specified by more than one codon. - Universality: The same genetic code applies to all organisms. ### 1.5.4 Transcription of RNA - **Stages of transcription:** - **1. INITIATION:** RNA polymerase binds to a promoter, where the helix unwinds and transcription starts. - **2. ELONGATION:** RNA polymerase moves downstream, unwinding the DNA, and elongating the RNA transcript 5' 3'. DNA strands reforms a double helix. - **3. TERMINATION:** Eventually the RNA transcript is released, the RNA polymerase detaches from the DNA template. - The image shows a simplified model of transcription. - It depicts the following steps: - **Initiation:** RNA polymerase binds to a promoter. - **Elongation:** RNA polymerase transcribes the template strand of DNA into RNA. - **Termination:** RNA polymerase detaches from the DNA template and releases the completed RNA transcript. - The directions of transcription are also being shown. ### (1.) INITIATION OF TRANSCRIPTION - **The promoter of a gene:** determines where transcription start (at start point). determines which strands becomes the template strand. - **A eukaryotic promoter includes a TATA box (contains about 25 TATA nucleotides).** - **Several transcription factors binds to DNA. RNA polymerase II then binds on the promoter.** - **Transcription initiation complex forms. RNA polymerase II then unwinds the DNA double helix. RNA synthesis begins at start point on the template strand. RNA polymerase starts adding free RNA nucleotides in the 5'-3' direction.** - The image shows a schematic representation of the initiation stage of transcription. - The image depicts the binding of transcription factors and RNA polymerase II to the promoter. - The unwinding of the DNA double helix and the beginning of RNA synthesis process are also depicted. ### (2.) ELONGATION OF RNA STRAND - **RNA Polymerase** continues to separates the DNA strand apart. add RNA nucleotides to the 3' end of the growing RNA polymer, according to the base- pairing rules along the DNA template. RNA molecule elongates in 5' → 3' direction. - **Then the DNA double helix reforms & the new RNA molecule peels away from its DNA template.** - The image shows a schematic representation of the elongation stage of transcription. - RNA polymerase transcribes the template strand of DNA into RNA. - The direction of transcription is also being shown. ### (3.) TERMINATION OF TRANSCRIPTION The mechanisms of termination are different in bacteria and eukaryotes. - **In bacteria, ** - RNA polymerase transcribes a mRNA termination sequence in the DNA. - The transcribed termination sequence (on the mRNA) signals the end of the transcription. - The RNA polymerase detaches from the DNA. - The released mRNA can be translated without further modification. - **In eukaryotes,** - RNA polymerase II transcribes the polyadenylation signal sequence (mRNA termination sequence) on the DNA. - This produces a polyadenylation signal (AAUAAA) in the pre-mRNA. - 10 – 35 nucleotides downstream from this polyadenylation sequence (AAUAAA), the pre-mRNA transcript is released. ### 1.5.5 Modification of pre-mRNA in Eukaryotes - **RNA processing (mRNA modification) involves:** - **1. ALTERATION OF MRNA ENDS:** - Each end of a pre-mRNA molecule is modified. - Addition of a 5' cap: The 5' end receives a 5' cap, a modified form of a guanine (G) nucleotide. - Addition of a 3' poly-A tail: At the 3' end, an enzyme makes a poly-A tail, consists of 50 - 250 adenine nucleotide. - **2. RNA SPLICING:** Removal of non-coding sequences (introns) / joining together of coding sequence (exons). - The addition of 5’ cap and poly-A tail has several functions: - facilitate the export of mRNA to the cytoplasm - protect mRNA from hydrolytic enzymes - help ribosomes attach to the 5' end of mRNA. ### RNA PROCESSING: - **1. Addition of the 5' cap & 3' Poly (A) tail** - **A modified guanine nucleotide added to the 5' end** - **50-250 adenine nucleotides added to the 3' end** - **polyadenylation signal** - **The image shows a schematic representation of a pre-mRNA molecule. - It shows the different regions of the pre-mRNA molecule, including: - 5’ cap - 5’ UTR - Start codon - Stop codon - 3’ UTR - Poly-A tail ### RNA PROCESSING: - **2. Split Genes & RNA SPLICING** - **Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions** - **These noncoding regions are called intervening sequences, or introns.** - **The other regions are called exons (coding region) because they are eventually expressed, usually translated into amino acid sequences** - **RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence.** - The image shows a schematic representation of RNA splicing. - It shows a pre-mRNA molecule with introns and exons. - It also shows the process of RNA splicing. ### 1.5.6 Translation - Building A Polypeptide - The image shows a schematic representation of a tRNA molecule. - It illustrates the two-dimensional structure of the tRNA molecule and highlights the different components of the tRNA, including: - Amino acid attachment site - anticodon - hydrogen bonds - The image also shows the three-dimensional structure of the tRNA molecule. ### Structure and Function of tRNA - tRNA: - are transcribed from DNA templates in the nucleus. - Travels to cytoplasm → where translation occurs. - Consists of a single RNA strand (~80 nucleotides long). - Function → to transfer amino acid to a ribosome. - Has 2 ends: - An amino acid attachment site. - An anticodon. - The anticodon triplet is unique to each tRNA type. - Each tRNA molecule can be used repeatedly. ### Structure and Function of tRNA - An amino acid attachment site allows each tRNA to carry a specific amino acid to ribosome. - Anticodons are written in 3' 5', Base pair with complementary mRNA's codon (5' 3' direction). - **Example:** - Anticodon: 3'-AAG-5' - mRNA codon: 5'-UUC-3 ### Aminoacyl-tRNA synthetases catalyzes the attachment of amino acid to their tRNA. - The image shows a simplified model of the process of aminoacyl-tRNA synthetase attaching an amino acid to a tRNA molecule. - The specific amino acid, tyrosine, is attached to its specific tRNA, tyrosine-tRNA. - The amino acid is attached to the 3' end of the tRNA. ### RIBOSOMES - Ribosome has : a binding site for mRNA. 3 binding sites for tRNA: - The A site (Aminoacyl-tRNA site) - Holds the tRNA carrying amino acid to be added to the chain. - The P site (Peptidyl-tRNA site) - Holds the tRNA carrying the growing polypeptide chain. - The E site (Exit site) - Discharged tRNA leaves ribosome from the E site. - The image shows a schematic representation of a ribosome. - It identifies the A, P, and E sites. - Ribosomes are composed of two subunits, a large subunit and a small subunit, and these subunits are responsible for the translation process. ### Translation – Building a Polypeptide - Translation the synthesis of a polypeptide chain. - **3 stages of translation:** - Initiation - Elongation - Termination - The image shows a schematic representation of a ribosome with an mRNA molecule and a tRNA molecule. -The A, P, and E sites are labelled. - The growing polypeptide chain and the next amino acid to be added to the polypeptide chain are also depicted. ### 1. INITIATION - The process: - A small ribosomal subunit binds: mRNA & a special initiator tRNA. - Anticodon of initiator tRNA, UAC → base-pairs with the start codon, AUG (mRNA). - The initiator tRNA carries Methionine (Met) → attaches to the initiation codon. The image shows a schematic representation of the initiation stage of translation. - The image depicts the binding of a small ribosomal subunit, an mRNA molecule, and an initiator tRNA to the start codon. ### 1. INITIATION - **Then a large ribosomal subunit is attached to produce a translation initiation complex. Consists of: Small & large subunit mRNA Initiator tRNA.** - **The initiator tRNA sits in the P site.** - **The vacant A site is ready for the next aminoacyl-tRNA.** The image shows: - The formation of a translation initiation complex from a small ribosomal subunit, a large ribosomal subunit, an mRNA molecule, and an initiator tRNA. - The image also shows that the initiator tRNA is located in the P site. ### 2. ELONGATION - During the elongation stage, amino acids are added one by one to the preceding amino acid at the C-terminus of the growing chain. Consists of 3-steps cycle: - **1) Codon Recognition:** The mRNA codon in the A site forms H bonds with the anticodon of an incoming aminoacyl tRNA. This tRNA enters the A site. - **2) Peptide bond formation:** Peptidyl transferase catalyzes the formation of a peptide bond between amino acid (of polypeptide) at the P site to the new amino acid in the A site. The polypeptide separates from the tRNA in the P site & binds to the amino acid of the tRNA in the A site. - **3) Translocation:** The tRNA (with its attached polypeptide) in the A site, is translocated to the P site, taking the mRNA along with it. This brings the next codon to be translated into the A site. The previous tRNA that was in the P site is moved to the E site & released from the ribosome. Translation proceeds along the mRNA in a 5' to 3' direction - The image shows: - The three steps involved in the elongation stage of translation: codon recognition, peptide bond formation, and translocation. - The A, P, and E sites are labeled in the image. - The image shows: the movement of the tRNA and the polypeptide chain during the elongation process. ### 3. TERMINATION - Elongation continues until: A stop codon in the mRNA reaches the A site of ribosome. 3 Stop codon: UAA, UAG & UGA - Do not code for amino acid. - Act as signal to stop translation. - **Release factor (a protein), binds directly to the stop codon in the A site. Then it hydrolyzes the bond between the tRNA (in the P site) & the last amino acid of the completed polypeptide chain. The polypeptide is released.** - **Translation complex dissociates. The 2 ribosomal subunit & the other components of the translation assembly dissociate.** The image shows the steps involved in the termination stage of translation: - The release factor binds to the stop codon in the A site. - The polypeptide chain is released from the ribosome. - The ribosome dissociates into its two subunits. ### POLYRIBOSOMES - In bacteria and eukaryotes, multiple ribosomes translate an mRNA at the same time. - To produce many copies of polypeptides simultaneously. The image shows: - A schematic representation of a polyribosome, which is a complex of multiple ribosomes translating a single mRNA molecule. - The mRNA molecule is being shown with multiple ribosomes bound to it. - Each ribosome is translating a polypeptide chain and the growing polypeptides are shown as arrows. - The image also shows complete polypeptides that have been released from the polyribosome. ### 1.6 REGULATION OF GENE EXPRESSION - Regulation of gene expression - Components of an operon - Lactose operon (lac operon) ### 1.6.1 Regulation of gene expression - Prokaryotes and eukaryotes use different mechanisms to regulate the expression of their genes in response to different environmental conditions. - Regulation of gene expression is important to allow the cell to respond to different environmental conditions; e.g.: availability of nutrients - The main requirement of bacterial gene regulation is the production of enzymes and other proteins when needed. ### Regulation of gene expression in prokaryotes - In prokaryotes, the basic mechanism for the control of gene expression is the operon. - Operon = a gene complex/ a group of related genes with related functions that are clustered together on the bacterial chromosome. - Advantage: - Allows the genes to function as one transcription unit. - Can be controlled and switched on and off together = coordinately controlled. - Example of operon is lac operon ### 1.6.2 Components of an operon - Operon consists of (i) promoter (ii) operator and (iii) structural genes. - Promoter = the binding site for RNA polymerase to initiate transcription of the structural genes. - Operator = the on-off switch in a segment of DNA that controls the mRNA synthesis. - Structural genes = made up of few genes that code for different enzymes. - Outside and upstream of the operon is the location of the regulatory gene. - Regulatory gene codes for the synthesis of mRNA that is translated into repressor protein. - Repressor protein binds to the operator of a particular operon. ### 1.6.3 Lactose operon (lac operon) - In E. coli, lactose operon codes for the 3 different enzymes that function in the uptake and metabolism of lactose. - There are 3 structural genes that is transcribed into a single mRNA which is then translated into separate enzymes. - **Structural genes** **Enzyme produced** **Function** - lac Z ẞ-Galactosidase Hydrolyze lactose into glucose & galactose, which are utilized for cellular metabolism. - lac Y Permease Facilitate the uptake of lactose into the cell. - lac A Transacetylase Detoxifies other molecules entering the cell via the permease. - Lac I, the regulatory gene for lac operon codes for the synthesis of mRNA which is then translated into repressor protein (lac repressor). - Allolactose, an inducer to repressor protein of lac operon, is an isomer of lactose (formed in small amounts from lactose that enters the cell). ### Component of lac operon - The image shows a schematic representation of the lac operon. - It shows: - The regulatory gene *lacI*, which codes for the repressor protein. - The promoter, which is the binding site for RNA polymerase. - The operator, which is the binding site for the repressor protein. - The three structural genes: *lacZ*, *lacY*, and *lacA*. - The start codon and stop codon. - It also shows how RNA polymerase binds to the promoter and initiates transcription. ### Mechanisms of lac operon in the presence of lactose - Some lactose is converted into allolactose - Allolactose acts as inducer and binds to repressor protein. This causes: - the repressor protein to change shape into inactive form. - and cannot bind to the operator. - Operator is activated. - Lac operon is switched on. - RNA polymerase binds to the promoter. - Transcription of structural genes (lac Z, lac Y and lac A) produces a single, long mRNA strand. - Translation of mRNA produces 3 separate proteins for metabolism of lactose, namely ß-galactosidase, permease and transacetylase. The image shows: - The lac operon in the presence of lactose. - Allolactose binds to the repressor protein, causing the repressor protein to change shape and become inactive. - The inactive repressor can not bind to the operator and this allows RNA polymerase to bind to the promoter and initiate transcription of the structural genes. - The enzymes produced by the lac operon are shown in the image. ### Mechanisms of lac operon in the absence of lactose - No allolactose binds to repressor protein. This causes: - the repressor protein to bind to operator, forms repressor-operator complex. - Blocks part of the promoter. - Prevents RNA polymerase binding to promoter. - Lac operon is switched off. - Structural genes is not transcribed. - No enzymes are produced. - The image shows: - The lac operon in the absence of lactose. - The repressor protein is active and binds to the operator, preventing RNA polymerase from binding to the promoter. - As a result, the structural genes of the lac operon are not transcribed, and no enzymes are produced. ### Lactose operon (lac operon) - In the context of gene regulation, lac operon is a negative control of genes because the operons are switched off by the active form of repressor protein. - Lac operon = inducible operon = an operon that is always off but can be induced to be on when a small, specific molecule interacts with different regulatory protein. - Enzymes of lactose pathway = inducible enzymes, because their synthesis is induced by a chemical signal (allolactose). - Enzyme usually function in catabolic pathways (breakdown a nutrient into simpler molecules). - Advantage: Cells avoid wasting resources by producing enzymes only when the nutrient is available. ### 1.7 MUTATION - Mutation - Small-scale mutation - Chromosome mutations - Human disorder due to chromosomal alteration ### 1.7.1 Mutation - Mutation is a change in the nucleotide sequence of an organism’s DNA. - Mutations provide the variation of alleles and are responsible for the huge diversity of genes found among organisms. - If mutation occurs in reproductive cells, it can be passed from parents to offspring and to future generations. - A change in sequences of bases in DNA leads to a change in the sequence of bases in mRNA, which leads to a change in the amino acid sequence of the polypeptide and a change in the function of the protein. ### Scales of mutation - Small-scale mutations involves one or a few nucleotide pairs, including point-mutations. - Point mutation involves changes in a single nucleotide pair of a gene. - Two categories of point mutations within a gene: - Single nucleotide-pair substitutions - Nucleotide-pair insertions or deletions. - Large-scale mutations include chromosome mutations. - Chromosome mutations involve changes in the number of chromosomes or changes to a large section of chromosome. ### 1.7.2 Small-scale mutations - **1. Nucleotide-pair substitution:** is the replacement of one nucleotide and its partner with another pair of nucleotides. This leads to three types of mutation: - Silent mutation - Missense mutation - Nonsense mutation - **2. Nucleotide-pair insertions or deletions:** are addition or losses of nucleotide pairs in a gene that may alter the reading frame of the genetic message, codon on the mRNA during translation. This leads to frameshift mutation. ### Effect of nucleotide-pair substitution - **1. SILENT MUTATION:** A change in a nucleotide pair that changes one codon into another codon that is translated into the same amino acid. Has no observable effect on the phenotype. The image shows: - A silent mutation in the nucleotide, changing a G to an A. - The mutation has no effect on the aamino acid sequence. - It is considered a silent mutation because it does not alter the amino acid sequence. ### Effect of Nucleotide-pair substitution - **2. MISSENSE MUTATION:** A nucleotide-pair substitution that results in a codon that codes for a different amino acid. The image shows: - A missense mutation, substituting C to T, causes a change in amino acid sequence from glycine (G) to serine (S). - It is known as a missense mutation because it leads to a change in the amino acid sequence. ### Effect of nucleotide-pair substitution - **3. NONSENSE MUTATIONS:** A mutation that changes an amino acid codon to a stop