Q. 24

(a) How are Mende

Answer :

(a):Mendelian inheritance: refers to the expression of monogenic traits, that is gene expression is controlled by one gene. In a pair of alleles, the expression of the recessive gene is always masked by the expression of the dominant gene.

The ability of a gene to have multiple phenotypic effects is because it influences several characters simultaneously, is called Pleiotropy.


Polygenic inheritance: is the type of inheritance controlled by one or more genes in which the dominant alleles have a cumulative effect with each dominant allele expressing a part or a unit of the trait, the full being shown only when all the dominant alleles are present.


(b): Generally, one gene is responsible for one character of an individual. In some cases there occur multiple genes to control single trait and these are said to be Polygenic genes and the trait is called as Polygenic trait.


This phenomenon shows participation of each allele and is also influenced by environment and is called quantitative inheritance as the character or phenotype can be quantified.


For example, Human skin colour which is caused by the pigment Melanin. The quantity of melanin is due to three pairs of polygenes (A, B and C).


If Black/ very dark (AABBCC) and White or very light (aabbcc) individuals marry each other, the offspring show intermediate colours also called Mulatto(AaBbCc).


OR


(a):




The whole of the DNA does not open in one stretch because of a very high energy requirement. The point of separation proceeds slowly towards both directions giving a appearance of a Y-shaped structure called Replication fork.


(b): DNA replication- is the phenomenon in which a duplicate copy of DNA is synthesised.


Below is the list of enzymes involved in the replication process:


DNA helicase


DNA polymerase


DNA clamp


Single-stranded proteins


Topoisomerase


DNA ligase


Primase


Function and property of any two enzymes as as follows:


Topoisomerase :


Also known as “DNA Gyrase”


Every cell has enzymes that increase or decrease the extent of DNA unwinding are called Topoisomerases.


Its function is it participates in unwinding of DNA helix.


There are two classes of topoisomerases:


1. Type-1 topoisomerase- this act by transiently breaking one of the two DNA strands, rotating one of the ends about the unbroken strand, and rejoining the broken ends.


2. Type-2 topoisomerase- the enzyme breaks both the DNA strands.


DNA Ligase:


is a enzyme that facilitates the joining of DNA strands together by catalysing the formation of a phosphodiester bond.


DNA ligase is used in both DNA repair and DNA replication.


The mechanism of DNA ligase is to form two covalent phosphodiester bonds between 3’ hydroxyl ends of one nucleotide (acceptor), with the 5’ phosphate end of another (donor).


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