xv. DNA as Vehicle of Heredity

delete UPSC Sample Notes [English]

xv. DNA as Vehicle of Heredity

About	Long before we understood the nature of DNA and its role as code of life, we discovered nucleus has units of heredity that are responsible for passing on traits from one generation to another. Mendel’s experiments with plants in his garden led to set of laws describing the laws of inheritance. The important result of this experiment was that traits passed on from one generation to another as discreet units. DNA as a vehicle of heredity is passed from one generation to another during gamete formation and subsequent fertilization of gametes.
Meiosis: The magic behind heredity	Meiosis in simple words is a story of how traits that make up an individual are assembled when the sex cells of his/her parent generated the sex cells/germline cells (sperm and egg). All that an individual is made of is, but an expression of the genetic matter donated by parents to the subsequent generation. When germline cells are produced in the body of parents, the 2 sets of chromosomes come together, cross-over and mix up randomly in process called meiosis. During meiosis chromosomes of the same type are lined up. When two versions of the same type chromosome line up, parts of the chromosome can be switched as shown in the figure. Imagine this cross over and recombination happening across all chromosomes. This is what gives genetic variation to sex cells (gametes) and thence to offspring. This genetic variation is what gives diversity to a species which is very important to adapt to the environment and evolution. When gametes come together to form a zygote during fertilization genetic diversity of both gametes are carried.
Mitosis and differentiation: How did you become a complex organism from a single cell?	Mitosis and differentiation put together is the story of how one becomes a complex organism all the way from being a single cell called zygote. Mitosis is the process of cell division which is what drives growth of organisms. The first step of cell division is DNA replication where a copy of DNA is made. This happens through 2-step process of unwinding and rebuilding. We have already seen the bases the make up the core of DNA pair in correspondence, meaning A always pairs with T and vice versa and G always pairs with C and vice versa. Under this process the double stranded DNA first unzips like a zip into single strands. This is done by an enzyme called helicase. Then another enzyme called DNA Polymerase synthesize each of the single strand into 2 double stranded DNA. In the process mistakes are made and thus we also have an inbuilt proof-reading mechanism. Once DNA is replicated other components of the cell get distributed forming 2 daughter cells. 2 becomes 4, 4 becomes 8, 8 becomes 16 and the chain reaction is set. Humans on an average have about 37 trillion cells in our body.
Differentiation	We have seen that same DNA is there in nucleus of all cells. how did we develop into heart, brain, liver, kidney, lungs, limbs etc. that came together in such sophistication to form a fully functional organism? In total there are about 220 different cell types that make up 4 types of tissues and 78 types of organs, all working in unison to make a complex organism like human possible. How? The answer lies in differentiation. From its journey from zygote to complex organism cells undergo continuous division(mitosis) and differentiation. After the zygote is formed it undergoes division through the process of mitosis. Becomes 2 cells in 30 hours after fertilization, 4 cells in 60 hours after fertilization and at the end of 6 days become blastocyst, a hollow ball of about 100 cells. Then the 1st differentiation starts. Blastocyst grow into two types of cells which make inner mass of cells and outer mass of cells. The inner mass is embryo which develops into a whole organism. The outer mass is placenta which acts as the barrier between mother and fetus. At about 3rd week different tissues start to differentiate into organs. 4th week heart, eyes begin to form, 10th week heartbeat is audible, 2nd month limbs start to develop, 3rd month liver, gallbladder, pancreas. One way of classifying cells is based on their ability to differentiate. Broadly cells are of two types, differentiated and undifferentiated. Differentiated cells are the building blocks of tissues and organs. They are specialized cells that can only divide. They cannot differentiate. They are also called somatic cells or simply adult cells. Eg: Hair cell, nail cell etc. Undifferentiated cells are cells that have the ability differentiate into many different types of cells. They are classified into two types, namely embryonic stem cells and adult stem cells.