STRUCTURE OF DNA
What is structure of DNA? The structure of DNA was first explained in 1953 by Watson and Crick. DNA was shown to be a double helix that coils around itself. The two strands are parallel and the distance between the two is constant.
Components of DNA
What are the components of DNA? DNA is made up of repeating units called nucleotides.
Each nucleotide is composed of:
- A five-carbon sugar (deoxyribose).
- Phosphate molecule.
- Nitrogenous base, four types are available i.e.
- Adenine – (A)
- Guanine – (G)
- Cytosine – (C)
- Thymine – (T)
The bases are represented by their initials as A, G, C and T respectively.
The sugar alternates with the phosphate, and the two form the backbone of the strands. The bases combine in a specific manner, such that Adenine pairs with Thymine and Guanine pairs with Cytosine.
The bases are held together by hydrogen bonds. A gene is the basic unit of inheritance consisting of a number of bases in linear sequence on the DNA.
Genes exert their effect through protein synthesis.
The sequences of bases that make up a gene determine the arrangement of amino acids to make a particular protein.
The proteins manufactured are used to make cellular structures as well as hormones and enzymes.
The type of proteins an organism manufactures determines its characteristics.
For example, albinism is due to failure of the cells of an organism to synthesise the enzyme tyrosine required for the formation of the pigment melanin.
First Law of Heredity
- It is also known as Law of Segregation (Mendel’s First Law).
- The characters of an organism are controlled by genes occurring in pairs known as Alleles.
- By definition, an allele is an alternative form of a gene controlling a particular characteristic.
- Of a pair of such alleles, only one is carried in each gamete.
- This is explained by first meiotic anaphase stage, when the homologous chromosomes are separated so that each carries one of the allelic genes.
- This is the study of the inheritance of one character trait that is represented by a pair of genes on homologous chromosomes.
- Gregor Mendel (an Austrian monk) was the first person to show the nature of inheritance.
- He did this through a series of experiments using the garden pea, Pisum sativum.
- As opposed to others before him, the success in his work lay in the fact that:
- He chose to study first a single character at a time (monohybrid inheritance).
- He then proceeded to study two characters at time (dihybrid inheritance) .
- He quantified his results by counting the number of offspring bearing each trait.
- Each character he chose was expressed in two clearly contrasting forms.
- Stem length: some plants were tall while others were short.
- Colour of unripe pods: some were green, others yellow.
- There were no intermediates.
- For each character, Mendel chose a plant that bred true.
- A true or pure breed continues to show a particular trait in all the offspring in several successive generations of self-fertilization.
- He made one plant to act as the female by removing the stamens before the ovary was mature and protecting (e.g. by wrapping with paper).
- The female plant from contact with any stray pollen.
- When the ovary was mature, he carefully dusted pollen from the anthers of the selected male plant and transferred it to the stigma of the female plant.
- Observations were then made on the resulting seeds or on the plants obtained when those seeds were planted.
- For each pair of contrasting characters he studied, Mendel obtained the same results.
- For example, when he crossed pure breeding tall plants with pure breeding short plants, the first offspring, known as the first filial generation (FI) were all tall.
- When these were selfed i.e. self-fertilization allowed to take place, the second generation offspring also known as the second filial generation or F2 occurred in the ratio of 3 tall: 1 short.
- The same ratio was obtained for each of the other characters studied.
- From this it is clear that one character i.e. tall is dominant over the short character.
- A dominant character is that which is expressed alone in the offspring even when the opposite character is represented in the genotype.
- The unexpressed character is said to be recessive.
- From these results and others obtained when he studied two characters at the same time, Mendel concluded that gametes carry factors that are expressed in the offspring.
- These factors are what we know today as genes.
- Mendel put forward the following laws of inheritance:
- Of a pair of contrasting characters, only one can be represented in a gamete.
- For two or more pairs of such contrasting characters, each factor (gene) in the gamete acts independently of the others and may combine randomly with either of the factors of another pair during fertilization.
- Genetic experiments carried out to date confirm Mendel’s Laws of inheritance e.g. T.H. Morgan’s work on inheritance in the fruit fly Drosophila melanogaster.
Terms used in Genetics Genotype:
- The genes present in an individual. The genetic constitution of an individual. It is expressed in alphabetical notation .e.g. TT, Tt.
- The observed character or appearance i.e. the expression of the genes in the structure and physiology of the organism.
- In some cases the phenotype is the product of the genotype and the environment. Phenotype is expressed in words.eg TALL, SHORT, RED WHITE .etc.
- These are alternative forms of the same gene that control a pair of contrasting characters e.g. tall and short.
- They are found at the same position or gene-locus on each chromosome in a homologous pair.
- This is a state where the alleles in an individual are similar e.g. TT (for tall)
- This is a state where the alleles are dissimilar i.e. each of the two genes responsible for a pair of contrasting characters are present
- g. Tt. (T for tall; t for short)
- This is the offspring resulting from crossing of two individuals with contrasting characters.
Hybrid vigour or Heterosis:
The hybrid develops the best characteristics from both parents i.e. it is stronger or healthier, or yields more than either parent.
Use of Symbols
- To represent genes in the chromosomes, letters are used.
- It is customary to use a capital letter for the dominant characteristic and small letter for the recessive one.
- The gametes are encircled.
- For example, a cross between a tall and a short pea plant is illustrated as follows;
- Let –T– represent gene for tallness.
- Let –t– represent gene for shortness.
Fertilization-using checker board or Punnet square
F1 genotype Tt
F1 Phenotypic ratio = All tall.
F2 Genotype TT, 2Tt, tt
F2 Phenotypic ratio: 3 Tall; 1 short
Test Cross or Back Cross
This is made between the F 1 bearing the dominant trait with the homozygous recessive parent. It is called a back cross because of using the first parent. It is also a test cross because it tests the genotype of the individual.