(a) What are general causes of congenital defects? Explain giving an example of each.

(a) The general causes of congenital defects are as follows:
(i) Variation in chromosome morphology.
(ii) Structural changes of a chromosome or chromosomal aversion a) intra chromosomal b) inter chromosomal.
(iii) Autosomal abnormalities (eg: Down syndrome).
(iv) Sex chromosomal aversion.

Example:- Down syndrome is a congenital disorder caused by having an extra 21st chromosome. People with Down's syndrome often have certain physical characteristics. Not everyone will have all of them, but they may include:-
(1) Individuals with Down syndrome have a prominent epicanthic fold in the corner of each eye.
(2) They are short with dull & happy looking, short hands with fingers showing characteristic palm & fingerprint patterns & feet with a wide gap between first & second toes.
(3) Their life expectancy is shortened.
(4) They have flat, round heads, protruding furrowed tongues which cause the mouth to remain partially open.
(5) Both physical & mental development is retarded & poor muscle tone is characteristic.
(6) No sexual maturity. The testis is small & undetected in male & in females, labia Majora tends to be large while labia minora are small or absent.

b) Discuss the various theories of cellular aging.

(b) The various theories of cellular aging
The various theories of cellular aging are given as below:

Cellular senescence:- Much of the senescence may involve the inability to respond to extrinsic growth factor. The concept of programmed death of the cell is supported by investigators. It appears that the cell synthesizes an enzyme, a nuclease that cuts the cell's DNA into fragments. Interestingly, cells that meet an untimely death such as those that are poisoned or deprived of O2, do not show precise fragmentation of their DNA. Another example of programmed cell death is the steady decline of the thymus gland with age.

DNA damage:- Cellular senescence is the somatic mutation theory which attributes cellular aging to the gradual accumulation of mutations during cell divisions. Presumably, as errors occur during cell division, cell function is adversely affected & the additive effect of these errors results in tissue aging. Certain types of errors are likely to produce a great number of subsequent errors. For example, an error in the DNA polymerase gene will make further mistakes during replication. The cause of such mutations may be free radicals or other molecules that can damage DNA.

The free radicals theory of aging matures:- Cellular damage occurs as result of highly reactive free radicals. The superoxide anion (O2), Hydrogen peroxide (H2O2) & the hydroxyl radical (OH) are all very reactive. These free radicals are generated by many normal biological reactions in the body; & are highly toxic to the cell membranes, DNA bases & proteins. Production of free radicals increases with age.

Immunological theory of aging:- Theories on cellular aging must explain the entire age related spectrum of changes. One such theory is the immunological theory of aging. There are several ways in which a less effective immune system is deleterious to the body. For instance, the ability to mount an antibody response against infections decrease. Immune systems must able to recognize & eliminate abnormal cells.