Concept of gene and Complementation test

1. CONCEPT OF GENE
Structural & genetic definitions –
complementation test
Submitted to,
Smt. I K Nishitha
Asst.Professor
Dept.of Botany
St.Teresa’s college,EKM.
Submitted by,
AncyVarghese
1 M Sc. Botany
Roll No.6
St.Teresa’s college,EKM.

2.  Our concept of the gene has undergone a remarkable evolution as biologists have
learned more about the nature of inheritance.
 genes were considered discrete factors that were retained throughout the life of an
organism and then passed on to each of its progeny.
 Over the following century, these hereditary factors were shown to reside on
chromosomes and consist of DNA.
 A genome contains all of the genes required to “build” a particular organism.
INTRODUCTION

3. MILESTONES

4. CONTRIBUTIONS OF MENDEL – CONCEPT OF GENE
 The gene was formerly called a factor.The existence of a
gene as an independentheritable factor was first identified
by Gregor Mendel who is now credited as the father of
genetics.
 Mendel formulated the laws of heredity based on his
careful breeding experiments on garden pea plants.
 An individual plant possessed two copies of a gene
 that controlled the development of each trait,one derived
from each parent.
 The two copies could be either identical to one another
or nonidentical.
 Alternate forms of a gene are called alleles.

5. WILHELM JOHANSEN
 The factor was called gene as introduced by
Wilhelm Johannsen, a Danish botanist and plant
physiologist, in 1905.
 Also coined the terms
 Genotype to refer to genetic make up
 Phenotype to refer outward appearance of an
organism

6. T H MORGAN
 Gene theory of T.H Morgan in 1911
 Chromosomes contain hereditary units & each
chromosomes carries thousands of genes.
 Genes are arranged on the chromosome in linear
order and on the special regions of locus and looks like
beads on string.
 After discovering of DNA as carriers of genetic
information Morgan’s theory was discarded.

7. CHANGING CONCEPT
 Prior to Mendel’s work, the popular belief is that male and female parents
provide fluids that blend or mix during fertilization.
 Charles Darwin referred to the particles that mix at fertilization as gemmule.
 Mendel’s work was published in 1886. However, it was in the late 1900s that it
was accepted as it was supported by the same results from the works of Hugo
deVries, Carl Correns, and Erich von Tschermak.
 Further study in the next century led to an understanding of the molecular
structure and function of the gene.
 It was found out that the gene is comprised of expressed DNA.
 The works of Rosalind Franklin and MauriceWilkins and later on of James D.
Watson and Francis Crick identified DNA structure.

8.  Genes are discrete particles inherited in Mendelian pattern.
 Responsible for expression of specific phenotypic characters.
 eg.skin colour, texture of hair…etc
 If position of gene changes then character also changes.
 May exist in alternate forms. Eg.Human blood group A,B,AB and O.
 Genes may undergo sudden changes in position and composition- mutation.
 Capable of self duplication producing their own copies.

9. G W BEADLE & E L TATUM
 Demonstrated one gene – one enzyme hypothesis in
Neurospora crassa.
 Genes express themselves through the synthesis of
enzymes.
 Genes control a biochemical reaction.
 Later one gene – one polypeptide hypothesis was put
forward.
 Stating that an enzyme contain many polypeptides.

10.  A change even in one nucleotide of the polypeptide specifying gene may mutate
and produce a varient of wild type chain.- differs in one amino acid residue.
 Therefore functional gene is not the same as mutational gene – it consists of
many mutable sites.
 There are also many sites where recombination occurs in a gene.- studies of x-
linked lozenge locus of drosophila melanogaster by c . P. Oliver in 1940.
 He demonstrated crossing over occurred between 2 mutants such as alleles lzs
& lzy of sex linked lozenge locus of drosophila melanogaster at a low frequency of
0.2%.
 First evidence for intra genic recombination.
 Till that time it was thought gene is not subdivisible and crossing over does not
occur within a gene..

11. CHROMOSOMES – PHYSICAL CARRIERS OF GENES
 Although Mendel provided convincing evidence that
inherited traits were governed by discrete factors,or
genes,
 His studies were totally unconcerned with the physical
nature of these elements or their location within the
organism.
 During the time between Mendel’s work and its
rediscovery,a number of biologists became concerned
with this other aspect of heredity—its physical basis
within the cell.
 1903,Walter Sutton,a graduate student at Columbia
University,published a paper that pointed directly to the
chromosomes as the physical carriers of Mendel’s genetic
factors (genes).

12. GENE : STRUCTURE & FUNCTION
 A gene is the fundamental, physical, and functional unit of heredity.Tranfer
information from one generation to next generation.
 In modern view, gene is a long nucleotide sequence of nucleotide pairs that is
capable of mutating and recombining at many different sites along its length.
 The genes make up the sequences of DNA (genotypes).
 The genotypes, together with the other factors (e.g. environmental factors),
determine the phenotypic trait of an organism.
 Eg.skin colour, hair colour….etc
 These changes in the genes drive evolution and natural selection.

13.  Chemically :A gene is a segment or region of
DNA containing a sequence of nucleotides that
encodes any function – a nucleotide sequence
that can make a protein.
 If it produce pigment protein for eye – it is a
gene for eye colour .
 Here, that particular gene is expressed.
 Complete DNA has millions of genes.

14.  A portion of gene may contain introns and exons
as in a eukaryotic structure in a mixed way.
 Both are functional.
 Exons code for proteins.
 Introns can not make protein but it can make
other RNA- functional non coding RNA like
 tRNA, rRNA, ribozymes...etc
 eg. X – inactivation by introns in females.
 Ribozyme was discovered by Thomas Cech in
1983.
 Ribozymes include peptidyl transferase in
translation,
 Spliceosome in hnRNA processing……etc

15. MODERN CONCEPT OF GENE
 Along with introns and exons, mutable and recombinable sites, a particular gene
also contain regulatory elements in it .
 Regulatory elements like enhancers, promoters…etc,are also mixed with non-
coding sequences like introns- junk dna that is seen in between genes.
 Enhancers and promoters are not transcribed.
 So,
 Gene is a unit of heredity located on chromosomes , made up of DNA and
regulatory elements that encodes a functional protein or family of proteins or a
functional RNA molecule that influences phenotype.

16. SEYMOUR BENZER (1921-2007)
 The most extensive study on fine structure
of gene was undertaken by Seymour benzer
for a locus in T4 bacteriophage infecting e-
coli.
 This locus was known as r11 locus
 He coined the term cistron in genetics, the
basis for the genetic code (DNA) developed
 Cistron is the smallest unit of genetic
material which codes for a single
polypeptide and functions as a transmitter of
genetic information.Thus saying a gene might
consist more than one cistron.

17.  TheT4 rII System
 Seymour Benzer developed a new genetic technique involving recombination in
T4 bacteriophage rII mutants,the technique is called theT4 rII system.
 The data he collected provided the first evidence that the gene is not an
indivisible entity, as previously believed, and that genes were linear.
 He further proved that mutations were distributed in many different parts of a
single gene, and the resolving power of his system allowed him to discern
mutants that differ at the level of a single nucleotide.
 Seymour Benzer also proposed distinct classes of mutations including deletions,
point mutations,missense mutations,and nonsense mutations.
 Benzer’s work on the topic also influenced further scientist including Francis
Crick and Sidney Brenner, and Richard Feynman.

18.  Recon:
 unit of genetic recombination,the smallest section of a chromosome that is
capable of recombination.
 Consist of not more than 2 pairs of nucleotide.
 There may be more than one recon in a gene.
 Muton
 Smallest segment of genetic material that can undergo mutation.
 When changes occur to this unit phenotypic changes may occur.
 Muton can be a single nucleotide.
 Humans have approximately 20,000 protein coding genes.
 This is 1.5% of the entire genome.
 3,000,000,000 – the entire sequences of genes & based that make up our
genome.

19. PRESENT STATUS OF A GENE
 The discoveries of DNA technology, beginning in the early 1970s, have led to the second
revolution in the concept of the gene -
 in which none of the classical ( gene as the indivisible unit of genetic transmission,
genetic recombination,gene mutation and gene function- during 1910- 1930)
 or neoclassical criteria for the definition of the gene ( which prevailed from 1940 until
the 1970s. In this view the gene or cistron, as it was now called, was divided into its
constituent parts, the mutons and recons, materially identified as nucleotides)hold
strictly true -
 The discoveries concerning gene repetition and overlapping,movable genes, complex
promoters,multiple polyadenylation sites, polyprotein genes, editing of the primary
transcript,pseudogenes and gene nesting are improving the concept of gene.

20. A PEEP INTO GENE FOR EYE COLOUR
 It was thought that eye colour was a single gene trait – with the allelle for
brown eyes being dominant over blue.
 Now, scientists realize it was wrong and atleast 8 genes influencethe final eye
colour.
 The genes control the amount of melanin inside specialised cells called iris.
 The range – blue to hazel to brown depends on level of melanin pigment
stored in the melanosome packets in the melanocytes of the iris.
 OCA2 gene located on chromosome 15 produces a protein – p protein
involved in formation & processing of melanin.
 Other genes included areTYRP1,ASIALC42A5…etc.

21. COMPLEMENTATION TEST
 Complementation test determine whether different recessive mutations are in
the same gene or in different genes
 Test uses genetic complementation –for restoration of wild type phenotype by
mating 2 different mutants.
 If two recessive mutations,a and b , are in the same gene, then a diploid
organism carrying one a allelle and one b allelle will exhibit the mutant
phenotype because neither allelle provides a functional copy of the gene.
 In contrast , if mutations a and b are in separate genes, then the heterozygous
carrying a single copy of each mutant allele will not exhibit the mutant
phenotype because a wild type allelle of each gene is also present.
 In this case , the mutations are said to complement each other.

22.  If more than one gene is required for the effect of a phenotype
 Gene a + gene b =eye colour
 Occurs if there is mutation in either of these gene and if it is in homozygous recessive
condition.
 Particular phenotype will be expressed only if ,at least one copy of each gene is present;These
two genes a & b complement each other to produce a phenotype.
 During segregation , one Chromosome is from father and one is from mother.So, if both genes
are present in different chromosomes,there is high chance to get the complementation effect.
 Mutant alleles present in the same gene do not show complementation, while those located in
different genes show complementation.
 If the result is complementing with each other,then the mutations are in different genes.
 Complementation analysis can not be performed on dominant mutations because the phenotype
conferred by the mutant allelle is displayed even in the presence of a wild type allelle of the
gene.

23. TWO INDEPENDENT MUTATIONS THAT RESULT IN NOWING
DEVELOPMENT

24. REFERENCES
 Lordish,H.,Berk,A.,Zipursky,S.,Matsudaira,P., Baltimore,D.,& Darnell,J.(2018). Molecular cell biology (8th
edition).NewYork:W.H.Freeman and company.
 Pubmed.ncbi.nlm.nih.gov.- Historical development of the concept of gene.- Petter portin.2002 June
 Pubmed.ncbi.nlm.nih.gov.-The evolving defenition of the term gene- .Portin P.et al . Genetics 2017
 Medline.plus.gov.- What is a gene?