Nature, Structure and Organisation of the Genetic Material

Chapter 10Discovery of genes, alleles and Deoxyribonucleic Acid

There have been many scientists and biologists who have contributed to our understanding of genetic material.

We will take a look at some of the important individuals and how their contributions have helped our understanding of genetics.

Gregor MendelModern genetics had

its beginnings in a monastery garden, where an Austrian monk named Gregor Mendel documented a particular mechanism of inheritance.

He discovered the basic principles of heredity by breeding garden peas in carefully planned experiments.

Mendel’s workIn order to study inheritance, Mendel chose to

use peas, probably as they are available in many varieties.

The use of plants also allowed strict control over the breeding.

Mendel’s workMendel’s work supported the “Particulate

Inheritance” model as opposed to the “Blending Inheritance” model.

Mendel’s work was ignored until the 1900s and was rediscovered by three biologists (Bateson, Punnett and Sutton).

Mendelian GeneticsMendel studied a number of characteristics in pea plants including, but not limited to:

Height - short or TALL

Seed color - green or YELLOW

Seed shape - wrinkled or ROUND

Genetic crossesTo breed two varieties

of pea plants, Mendel used a paint brush.

He transferred pollen from a true breeding (homozygous) white flower to the carpel of a true breeding (homozygous) purple flower.

Tracking heritable characteristicsMendel tracked heritable characters for 3

generations.When F1 hybrids were allowed to self-pollinate

a 3:1 ratio of the 2 varieties occurred in the F2 generation.

First filial generation

(F1)

This shows a heterozygote with one gene, two alleles: one allele for purple flowers (P) and white flowers (p).

Mendel’s Law of SegregationBy carrying out these

monohybrid crosses, Mendel determined that the 2 alleles for each character segregate during gamete production.i.e. each allele is inherited independently of the other alleles. This is called Independent Assortment of Alleles.

Mendel discovered this in 1860.

Mendel’s terminologyTrue breeding: When the plants self-pollinate, all their offspring are of the same variety.Hybridisation: Mating, or crossing of two varieties.Monohybrid cross: A cross that tracks the inheritance of a single character.P generation: True breeding parents.F1 generation: (first filial) Hybrid offspring of the P generation.F2 generation: (second filial) Offspring from the self-fertilisation of the F1 hybrids.Factors: what we now call alleles

Walter SuttonIn 1902, Walter

Sutton recognised that there were thread-like structures in cells which he called chromosomes.

He then came to the realisation that Mendel’s factors were located on these chromosomes.

William Bateson and Reginald Crundall Punnett

Also in the early 1900s W. Bateson and R.C. Punnett found that certain characteristics in peas were inherited together.

It was later found that these characteristics must be carried on the same chromosome in order to be inherited together.

Thomas Hunt MorganIn 1910 T.H Morgan confirmed the

results of Sutton, Bateson and Punnett.

Morgan’s experiments with Drosophila showed that genes were located on chromosomes.

Morgan also confirmed that when genes were located close together on homologous chromosomes, their alleles tended to be inherited together, i.e. linked.

Frederick GriffithFrederick Griffith in 1928 discovered a substance that could change a harmless organism into a disease causing killer organism.Bacteria (like pneumococci) also showed variation.The harmless avirulent form (rough) has been ‘transformed’ by something from the virulent form (smooth).

Oswald AveryOswald Avery discovered that the chemical compound containing genes was deoxyribonucleic acid (DNA).Treatment Result

protein destroying enzymes

ability to transform rough to smooth

lipid destroying enzymes ability to transform rough to smooth

carbohydrate destroying enzymes

ability to transform rough to smooth

DNA destroying enzymes

transformation ability destroyed

Discovering DNAAfter it was found that genetic information is

inherited through DNA, the race was on to find the structure of DNA.

During the 1940s and 1950s a number of different scientists studied DNA to find the proportions of the 4 different nucleotides (adenine, guanine, thymine and cytosine).

It was found that Adenine and Thymine were found in equal proportions and Guanine and Cytosine occurred in equal proportions. (Chargaff’s Rules)

James Watson and Francis CrickIn 1953 James Watson and

Francis Crick, discovered that the structure of DNA was as two nucleotide chains arranged to form a double helix.

Watson and Crick used results obtained by Rosalind Franklin.

Rosalind Franklin used X-ray crystallography to obtain the image on the right.

Deoxyribonucleic Acid

Watson and Crick’s 1954 model of DNA