Molecular Basis of Inheritance 1 Flashcards
what are nucleotide
A nucleotide has three components – a nitrogenous base, a pentose sugar (ribose in case of RNA, and deoxyribose for DNA), and a phosphate group.
types of nitrogenous bases
There are two types of nitrogenous bases – Purines (Adenine and Guanine), and Pyrimidines (Cytosine, Uracil and Thymine). Cytosine is common for both DNA and RNA and Thymine is present in DNA. Uracil is present in RNA at the place of Thymine.(thymine = 5-methyl uracil)
explain the bonding of i) sugar and base
ii)sugar and phosphate
A nitrogenous base is linked to the OH of 1’ C pentose sugar through a N-glycosidic linkage to form a nucleoside, such as adenosine or deoxyadenosine, guanosine or deoxyguanosine, cytidine or deoxycytidine and uridine or deoxythymidine.
When a phosphate group is linked to OH of 5’ C of a nucleoside through phosphoester linkage, a corresponding nucleotide (or deoxynucleotide depending upon the type of sugar present) is formed.
how is a polynucleotide chain formed? what are the 2 ends of the polynucleotide chain?
Two nucleotides are linked through 3’-5’ phosphodiester linkage to form a dinucleotide. More nucleotides can be joined in such a manner to form a polynucleotide chain. A polymer thus formed has at one end a free phosphate moiety at 5’ -end of ribose sugar, which is referred to as 5’-end of polynucleotide
chain. Similarly, at the other end of the polymer the ribose has a free
3’ -OH group which is referred to as 3’-end of the polynucleotide chain.
what is the:
i)dna backbone
ii)pairs
The backbone of a polynucleotide chain is formed due to sugar and phosphates. The nitrogenous bases linked to sugar moiety project from the backbone
what is nuclein
DNA as an acidic substance present in nucleus was first identified by
Friedrich Meischer in 1869. He named it as ‘Nuclein’. However, due to
technical limitation in isolating such a long polymer intact, the elucidation
of structure of DNA remained elusive for a very long period of time.
who helped make the double helix model
It was
only in 1953 that James Watson and Francis Crick, based on the X-ray
diffraction data produced by Maurice Wilkins and Rosalind Franklin,
proposed a very simple but famous Double Helix model for the structure
of DNA. One of the hallmarks of their proposition was base pairing between
the two strands of polynucleotide chains. However, this proposition was
also based on the observation of Erwin Chargaff that for a double stranded
DNA, the ratios between Adenine and Thymine and Guanine and Cytosine
are constant and equals one.
what do you mean by complementarity of base
The base pairing confers a very unique property to the polynucleotide
chains. They are said to be complementary to each other, and therefore if
the sequence of bases in one strand is known then the sequence in other
strand can be predicted. Also, if each strand from a DNA (let us call it as a
parental DNA) acts as a template for synthesis of a new strand, the two
double stranded DNA (let us call them as daughter DNA) thus, produced
would be identical to the parental DNA molecule.
(i) It is made of two polynucleotide chains, where the backbone is
constituted by sugar-phosphate, and the bases project inside.
(ii) The two chains have anti-parallel polarity. It means, if one
chain has the polarity 5’à3’, the other has 3’à5’.
(iii) The bases in two strands are paired through hydrogen bond
(H-bonds) forming base pairs (bp). Adenine forms two hydrogen
bonds with Thymine from opposite strand and vice-versa.
Similarly, Guanine is bonded with Cytosine with three H-bonds.
As a result, always a purine comes opposite to a pyrimidine. This
generates approximately uniform distance between the two
strands of the helix (Figure 5.2).
(iv) The two chains are coiled in a right-handed fashion. The pitch
of the helix is 3.4 nm (a nanometre is one billionth of a
metre, that is 10-9 m) and there are roughly 10 bp in eachturn. Consequently, the distance
between a bp in a helix is
approximately 0.34 nm.
(v) The plane of one base pair stacks
over the other in double helix. This,
in addition to H-bonds, confers
stability of the helical structure
(Figure 5.3).
vi)Base pairing is highly specific, A pairs with T, C pairs with G. Purine-pyrimidine complementarity.
what is the central dogma of molecular biology
Francis Crick proposed the central dogma which states the flow of genetic information is unidirection from DNA to RNA and to protein
What are retroviruses
Retroviruses have RNA as their genetic material. In order to synthesize viral proteins in the host cell, it has to convert its RNA into DNA. This DNA attaches to the host DNA in the nucleus of the cell. This dna is then transcripted and sent to the ribosomes for viral protein synthesis
The process of formation of DNA from RNA is called as Reverse Transcription. The enzyme required for this process is called as Reverse Transcriptase.
length of human dna and no of bp
length of DNA double
helix in a typical mammalian cell is calculated (simply
by multiplying the total number of bp with distance
between two consecutive bp, that is, 6.6 × 109 bp ×
0.34 × 10-9m/bp), it comes out to be approximately
2.2 metres.
describe packing of DNA in prokaryote
In prokaryotes, such as, E. coli, though they do
not have a defined nucleus, the DNA is not scattered
throughout the cell. DNA (being negatively charged)
is held with some proteins (that have positive
charges) in a region termed as ‘nucleoid’. The DNA
in nucleoid is organised in large loops held by
proteins.
what are histones
There is a set of positively charged, basic
proteins called histones. A protein acquires charge
depending upon the abundance of amino acids
residues with charged side chains. Histones are rich
in the basic amino acid residues lysine and arginine.
Both the amino acid residues carry positive charges
in their side chains.
histone octamer
Histones are organised to form
a unit of eight molecules called histone octamer
