Semi-Conservative DNA Replication

Semi-Conservative Replication

What is DNA?

Deoxyribo Nucleic Acid(DNA)

What is DNA?

DNA( Deoxyribo Nucleic Acid)

DNA is mainly found in the nucleus. A small amount is laso found in mitochondria and chloroplasts. It is a macromolecule formed by the end to end polymerisation of a large number of repeated units called deoxyribonucleiotides  or simply nucleotides.

Wattson and Crick established the structure of DNA moleculein 1953 on the basis of the x-ray diffraction. For this excellent discovery, Wattson and Crick was awarded with Nobel Prize in 1962. According to them, DNA is composed of two antiparallel polynucleotide chains that form a double helix around the central axis.

The strands are made up of alternate bands of deoxyribose sugar and phospate molecules. They are joined by the phospdiester linkages.

Each deoxyribo sugar in the strand has one N-base horizonatally attached to it at carbon-1. The four N-bases can occur in any possible sequence along the length of a strand. The N-base + deoxyribose sugar + phosphate together form one unit or deoxyribonucleotide in the strand. Such many deoxyribonucleotides are linked with each other in a linear fashion, therefore, the ruslting strand is described as the polynucleotide strand and DNA moelucle as polynucleotide molecule.

What are the SOURCES OF VARIATION?

Sources of variation:

Variations are more pronounced in sexually reproducing organisms. In sexual reproduction, production of offspring’s takes place by the fusion of two types of gametes. These gametes are formed by the reduction division. Thus, sexual reproduction introduces unlimited genetic variation into population. The more closely related the sexual partners are, the lesser are the variations in their offspring’s.

1)      Recombination: Recombination results in offspring that have a combination of characteristics different from that of their parents. Different types of combinations of the characters bring about the variation.

2)      Crossing Over:  At the time of gamete formation, crossing over occurs during meiosis division, which causes variations in genetic characters.

3)      Mutation: It is sudden heritable variations in genetic characters. It occurs in nature or it can be caused artificially in an organisms.

What is HEREDITY and VARIATION?

Heredity:

                The offspring’s of all the organisms (plants and animals) resemble their parents in several aspects. This is only due to the phenomenon of heredity. Hereditary literally means “tendency of like begets like” i.e. all living organisms tend to produce offspring’s like themselves.

                Hereditary may be defined as the transmission of characters from one generation to successive generations or from parents to their offspring’s. Thus, heredity is the cause of similarities between the offspring’s, so that the individuals of the same parents resemble each other in many aspects. Heredity involves the transfer of genetic characters from parents to the offspring’s via the egg and sperm. These transferable characters are called “hereditary characters”.

Variation:

SECONDARY GROWTH IN DICOT STEM

Dicot Stem: The growth in length of main axis by the activity of apical meristem is called primary growth. Increase in thickness or girth of the aixs due to the formation of secondary tissue is called secondary growth. The secondary tissue if formed by the activity of cambium and cork-cambium. The cambium forms secondary tissues in the stelar region and cork-cambium form secondary tissues into cortical regions. Secondary growht occurs only in dicot stem and root. It is usually absent in monocot root and stem.

In dicot stem, secondary growth takes place through the following steps:

PHOTOSYNTHESIS:

 Photosynthesis is a proces in which the green parts of a palnt, mostly leaves manufacture food materials from carbon dioxide and water in the presence of sunlight. Photosynthesis is the ultimate source of food that we eat and the oxygen that we breathe.

Plants absorb water from the soil by root hairs and carbon dioxide through the atmosphere by stomata. Chlorophyll absorbs light energy from sunlight and intiates the process. The sun energy activates chlorophyll and the activated chlorophyll transfers the sun energy into chemical energy in the form of ATP and NADPH2. This chemical energy provides the energy for the second step of photosynthesis. The whole process of photosynthesis envolves a number of chemical reactions each under the action os a specific enzyme, leading finally to formation of glucaose and oxygen is released as a by-product. Photosynthesis is vital to life on earth since all animals are directly or indirectly dependentant on plants for their food and survival.

Thevolume of oxygen liberated has been found to be equal to the volume of carbon dioxide absorbed. But it is proved that all the oxygen liberated in the process is released from water and not form carbon dioxide gas.