UNIT 3 – Gene Structure and Function (Q&A) | MZO-002 MSCZOO | IGNOU
SAQ 1
i) Define Recon, Muton and Cistron.
The terms Recon, Muton and Cistron were introduced by Seymour Benzer during the 1950s to study the detailed structure and function of genes at the molecular level. He worked on the rII region of T4 bacteriophage and used bacteriophage genetics to analyze how small changes in DNA affect phenotypes. At that time, the gene was considered as a single indivisible unit. But Benzer showed that a gene has a finer internal structure and can be divided into smaller functional units. Based on this, he proposed three molecular units: Recon, Muton and Cistron, each having a specific role related to recombination, mutation and expression.
Recon
Recon is defined as the smallest unit of recombination. It refers to the smallest segment of DNA within which crossing over cannot occur, but recombination can occur between two such units. According to modern molecular understanding, recombination between two genes or within a gene occurs at the level of nucleotides. So, a recon is considered as a single nucleotide pair, because it is the smallest unit between which recombination is possible. Recombination is a physical exchange of genetic material and it cannot happen within one nucleotide base. So, the recon helps in defining how fine or small recombination events can be at the molecular level.
Muton
Muton is the smallest unit of mutation. It is defined as the smallest portion of the genetic material where a change (mutation) can take place and be detected. Mutation means any alteration in the DNA sequence that brings a visible or detectable change in phenotype or protein function. In molecular terms, a muton is also equal to a single nucleotide pair, because even a change in one base can lead to a change in amino acid or gene function. Muton helps us to understand how small a mutation can be to affect gene function.
Cistron
Cistron is the functional unit of a gene. It is defined as the smallest stretch of DNA that codes for a complete polypeptide or functional RNA. It is basically the same as a gene in modern usage. The term cistron was derived from the "cis-trans test", which is a type of complementation test done to find out whether two mutations are present in the same gene or in two different genes. If two mutations fail to complement each other when present in trans, they are in the same gene or cistron. If they do complement, they are in different cistrons. Thus, the cistron refers to that entire region of DNA which contains all the necessary information to produce a functional product like a protein or RNA.
ii) Who gave one gene and one enzyme hypothesis?
The One Gene and One Enzyme Hypothesis was proposed by George Wells Beadle and Edward Lawrie Tatum in 1941. They demonstrated this concept through their experiments on Neurospora crassa, showing that each gene controls the production of a single specific enzyme.
iii) Alkaptonuria is a genetic disorder that occurs due to disfunctioning of which protein?
Alkaptonuria is a genetic disorder caused by the dysfunction of a protein called homogentisate 1,2-dioxygenase, which normally works as an enzyme. This protein helps break down homogentisic acid, also known as alkapton. When this protein does not function properly, alkapton accumulates in the body, leading to symptoms of the disorder.
iv) What will be the frequency of AaBB genotype in a dihybrid cross?
In a dihybrid cross, both parents have the genotype AaBb. To find the frequency of the AaBB genotype, we need to calculate the probability for each gene separately and then multiply them.
For gene A (Aa × Aa):
The possible genotypes are AA, Aa and aa with probabilities 1/4, 1/2 and 1/4 respectively.
So, the probability of getting Aa is 1/2.
For gene B (Bb × Bb):
The possible genotypes are BB, Bb and bb with probabilities 1/4, 1/2 and 1/4 respectively.
So, the probability of getting BB is 1/4.
Now, multiply the two probabilities to get the frequency of AaBB:
Frequency of AaBB = (1/2) × (1/4) = 1/8 = 0.125
Therefore, the frequency of AaBB genotype among the offspring is 1/8 or 0.125.
SAQ 2
i) The number plaques formed on lambda phage is 1200 and 300 in mutant. What is the recombination frequency?
Given:
Plaques on K12 = 1200
Plaques on B = 300
The recombination frequency is calculated using the formula:
Recombination Frequency = 2 × ( 1200/300)
= 2 × 4
= 8
ii) Who gave the cis-trans complementation test?
The cis-trans complementation test was given by Sir Seymour Benzer in 1955.
iii) What is the difference between regulatory gene and structural gene?
Genes are basic units of heredity that carry instructions for the development and functioning of living organisms. Among various types of genes, regulatory genes and structural genes are essential for the proper functioning of cells, but their roles are quite different. Regulatory genes mainly control other genes, while structural genes directly code for proteins that form the body's structures or perform specific functions. To understand their differences clearly, we will compare them based on important criteria.
1. Based on Function
The main function of regulatory genes is to control or regulate the expression of other genes. They produce regulatory proteins, such as repressors or activators, that influence whether structural genes are switched on or off. This control mechanism ensures that genes are expressed only when needed.
In contrast, structural genes code directly for proteins or RNA molecules that are involved in building cellular components or carrying out specific biochemical functions.
2. Based on Type of Product
Regulatory genes mainly produce regulatory proteins. These proteins bind to DNA or interact with other molecules to control gene activity.
Structural genes produce functional proteins or RNA molecules such as enzymes, structural proteins, or transport proteins, which perform various roles inside the cell.
3. Based on Role in Gene Expression
Regulatory genes play a key role in controlling gene expression by turning structural genes on or off depending on the cell's requirements and environmental conditions.
Structural genes themselves do not have this controlling ability. They are the targets of regulation and provide the final functional proteins once expressed.
4. Impact on Cellular Activities
Regulatory genes allow cells to respond flexibly to internal and external signals by controlling which genes are active at a given time. This regulation is crucial for development, metabolism and adaptation.
On the other hand, structural genes directly contribute to the cell's physical structure and metabolic processes by producing enzymes, proteins and other molecules required for survival.
TERMINAL QUESTIONS
1. Write a brief note about cis-trans complementation test.
The cis-trans complementation test is a classical genetic technique used to determine whether two mutations that cause a similar mutant phenotype are in the same gene (cistron) or in different genes. This test helps in understanding gene function and structure by showing if two mutations complement each other when combined.
The idea of this test comes from classical genetics. It was first introduced by Edward B. Lewis in the mid 1940s during his pioneering work on the genetics of the fruit fly (Drosophila melanogaster). Lewis's work laid the foundation for the concept of complementation, which is essential for grouping mutations based on their function.
Later, in the mid to late 1950s, Seymour Benzer expanded and applied this test to bacteriophage genetics, specifically studying the rII region of bacteriophage T4. Benzer used the cis-trans complementation test to analyze the fine structure of genes at the molecular level. His work helped define the cistron as the smallest functional genetic unit. This made the test a key tool in molecular genetics and gene mapping.
This test is useful only for recessive mutations and it helps scientists to group mutations into different genes or the same gene based on whether the wild-type function is restored or not.
In this test, two organisms or DNA segments having different mutations are brought together in one cell or one system. These mutations are then arranged in two ways, cis and trans, and the resulting phenotype is studied.
1. Cis Configuration
In the cis configuration, both mutations are present on the same DNA molecule and the other molecule has both wild-type alleles.
For example:
[mut1 mut2] / [wild1 wild2]
In this condition, the normal phenotype is always seen because both wild-type alleles are on one side. So, this setup does not give us information about whether the mutations are in the same gene or not. It acts as a control.
2. Trans Configuration
In the trans configuration, one mutation and one wild-type allele are present on each DNA molecule.
For example:
[mut1 wild2] / [wild1 mut2]
Now, we observe the phenotype:
- If the phenotype is normal, it means the mutations are in different genes and they complement each other.
- If the phenotype is still mutant, it means the mutations are in the same gene and they fail to complement.
This is the key part and the main result of the complementation test. The result from the trans arrangement tells us whether the mutations are in same gene or different genes.
Importance of the Cis-Trans Complementation Test
The cis-trans complementation test is useful for understanding gene function. It helps to:
- Identify if two mutations are in the same gene or different genes
- Group mutations into complementation groups
- Estimate the number of functional genes involved in a process
- Define gene function based on complementation results
- Support the one gene–one enzyme hypothesis
- Help in gene mapping and study of gene structure before DNA sequencing was available
2. Write a brief note about the gene concept given by Mendel.
The concept of gene in heredity was first introduced through the experiments of Gregor Johann Mendel, who is called the Father of Genetics. He was an Austrian monk who worked between 1856 and 1863 on garden pea plants (Pisum sativum) in the garden of a monastery in Brno (now in the Czech Republic). Mendel wanted to understand how traits like flower color and seed shape pass from one generation to the next.
In 1866, Mendel published his results in the journal of the Natural History Society of Brunn. The original title of his paper was Versuche uber Pflanzen-Hybriden (Experiments on Plant Hybridization). In this paper, he explained that traits are passed through specific units which he called "factors". These factors are now known as genes.
The word "gene" was introduced later in 1909 by a Danish scientist Wilhelm Johannsen. The importance of Mendel's work was not understood during his lifetime. But later in 1900, three scientists Hugo de Vries, Carl Correns and Erich von Tschermak, rediscovered his experiments and confirmed his findings.
Mendel's ideas changed the way science understood heredity. He showed that inheritance follows clear rules and is not a random process. His work became the base of modern genetics, even before DNA and chromosomes were discovered.
Key Principles of Mendel's Gene Concept
Mendel's experiments showed that inheritance is not a blending of traits but is based on definite rules. His work laid the foundation for modern genetics. Later, his concepts were summarised as four main postulates, also known as laws of inheritance.
1. Principle of Paired Factors
Mendel stated that each trait is controlled by a pair of factors, one from each parent. These factors are now called genes. Each gene exists in two forms called alleles. If both alleles are same, it is homozygous. If different, it is heterozygous.
2. Principle of Dominance
When two different alleles of a trait are present, one allele masks the expression of the other. The visible allele is called dominant and the hidden one is recessive. For example, in a cross between tall (TT) and dwarf (tt) pea plants, all F1 plants are tall because tall is dominant over dwarf.
3. Law of Segregation (Mendel's First Law of Inheritance)
This law states that the two alleles of a gene pair separate or segregate during the formation of gametes. So, each gamete gets only one allele. This is why recessive traits can reappear in the next generation. This law explains the 3:1 ratio seen in monohybrid crosses.
4. Law of Independent Assortment (Mendel's Second Law of Inheritance)
This law applies when two or more traits are studied together. It states that alleles of different genes are inherited independently if they are located on different chromosomes. This results in new combinations of traits, as seen in the 9:3:3:1 ratio in dihybrid crosses.
3. What is the difference between the classical concept and the modern concept of genes?
The concept of the gene has evolved from a simple unit of heredity to a complex molecular entity. During Mendel's time, genes were understood only through the inheritance of physical traits. This formed the classical concept. With the discovery of DNA, its double helix structure and the development of molecular biology, the gene is now defined by its chemical structure and functional properties. This is known as the modern concept.
For understanding the difference between the classical and modern concept of genes, the following criteria are used:
1. Based on Definition or Concept
Classical Concept: Gene was defined as an abstract unit of heredity responsible for controlling a single trait. It was known through breeding experiments and inheritance patterns.
Modern Concept: A gene is a segment of DNA that contains the information to produce a functional product, either a protein or an RNA. It is both a structural and functional unit of heredity.
2. Based on Structure
Classical Concept: Genes were imagined as tiny particles or "beads" arranged linearly on chromosomes. No internal structure was known.
Modern Concept: Genes have a complex structure. They include coding regions (exons), non-coding regions (introns), promoter sequences, enhancers, silencers and terminators.
3. Based on Function
Classical Concept: Each gene was believed to affect only one character or trait. The relationship was simple and one-to-one.
Modern Concept: A gene may code for multiple proteins due to alternative splicing. One gene can influence several traits (pleiotropy) and multiple genes can control a single trait (polygenic inheritance).
4. Based on Mutation
Classical Concept: Mutation was described as a sudden change in a trait, without knowing what exactly changed inside the gene.
Modern Concept: Mutation is a change in the nucleotide sequence of DNA. It can be precisely studied and classified into types like point mutation, deletion, insertion, frameshift etc.
5. Based on Location and Mapping
Classical Concept: Genes were located through breeding experiments and crossing over data. Mapping was rough and symbolic.
Modern Concept: Genes are located using molecular tools. The exact position, sequence and length of a gene on the chromosome can be determined.
6. Based on Gene Expression and Regulation
Classical Concept: No clear idea about how genes were turned on or off. Regulation was not well understood.
Modern Concept: Gene expression is well studied. Transcription and translation are controlled by regulatory sequences, transcription factors, epigenetic modifications and environmental factors.
7. Based on Gene Interaction
Classical Concept: Genes were thought to act independently. Interaction was only known through phenotypic ratios.
Modern Concept: Gene networks and interactions are well studied. Concepts like epistasis, co-dominance, incomplete dominance and gene silencing are explained at molecular level.
Comments
Post a Comment