L1: Nucleic acids

Question 1

what are nucleic acids?

Answer 1

Nucleic acids (DNA & RNA) are long chains of repeated subunits (nucleotides ).

Question 2

where are nucleotides found?

Answer 2

• present in body as poly-nucleotide (DNA,RNA ) or free nucleotides.
• nucleotide is the building block of DNA and RNA

Question 3

What are purines?

Answer 3

-Adenine
-Guanine
-Xanthine
-Hypoxanthine

Question 4

what is the structure of nucleotides?

Answer 4

1. Nitrogenous base:-
   a- purines :- ( adenine , guanine, hypoxanthine ,xanthine )
   b- pyrimidine :- (cytosine , thymine, uracil )
2. Pentose sugar :
   a- D-ribose in RNA
   b- 2-deoxy-D-ribose in DNA
3. phosphate (phosphoric acid )

N.B:- nitrogenous base + pentose sugar is called nucleoside

Question 5

-What are pyrimidines?

Answer 5

• Uracil
• Thymine
• Cytosine

Question 6

-What are the functions of nucleotides?

Answer 6

• Building block of nucleic acids.
• Source of energy as ATP and GTP.
• Mediate the action of hormones as the work as second messenger like c.AMP and c.GMP.
• Co-Enzymes help the enzyme carry hydrogen like co-enzyme A and NAD and FAD.
• Chemical group donors like PAPS which donate sulphate for sulpholipids synthesis and SAM which is a methyl donor for transmethylation reactions.
• Activate chemical compounds like UDP-glucose for synthesis of glycogen and CDP-Choline and CDP-ethanolamine for synthesis of phospholipids.
• Synthetic analogues which happens by altering the base ring or the sugar part like allopurinol and Anti-Cancer where 5-Flourouracil is used to dec the synthesis of thymine decreasing the DNA synthesis which prevents cancer cells from growth.

Question 7

How are the sugar and phosphate linked in the backbone?

Answer 7

By 3-5 phosphdiester bonds where one end is attached to the carbon atom N3 and the other end is attached to the other sugar at carbon N5

Question 8

What are the characteristics of the primary structure of DNA?

Answer 8

“ANS DP”

A: alternating sugar phosphate units makes the backbone asymmetrical.

N: Nitrogenous bases are projected to the inside at right angle.

S: Sequence of nucleotides determines the coding structure (The genetic information).

D: DNA sequence is read from the 5 end to the 3 end by the latter abb. Of the bases.

P: Each backbone has to poles one is the 3 pole which has the hydroxyl and the other is the 5 end which has the phosphate group.

Question 9

What are the characters of the secondary structure of the DNA (the physiological form)?

Answer 9

• The two helices are wounded around each other in the form of a Right-handed helix. (With diameter 2nm)
• They are antiparallel as they run in opposite direction-They are held by NBs.

Question 10

what is the definition of the melting temperature of DNA?

Answer 10

temperature at which 50 % of helical structure is lost

Question 11

What are the factors affecting the melting temperature of the DNA?

Answer 11

• types of bases : Tm decrease if DNA contains higher proportion of AT base pairs
• p.H of medium : Tm Decreases if the medium is more alkaline
• salt concentration : Tm decreases if salt concentration is decrease (salts is stabilizing agent )

Question 12

where are RNa molecules formed?

Answer 12

All RNA molecules are formed in the nucleus form DNA by RNA polymerase (transcription), then most of RNA is transported to cytoplasm.

Question 13

what are the types of RNA?

Answer 13

There are 4 main types:-
1- Messenger RNA (mRNA): 5 % of cellular RNA
2- Transfer RNA (t.RNA): 15 % of cellular RNA
3- Ribosomal RNA (r.RNA): 80 % of cellular RNA
4- Small nuclear RNA (snRNA) : 30 different species , less than 1 % of cellular RNA

• They contain 4 types of nucleotides (AMP, GMP, CMP, UMP) which are inter-connected by phosphodiester bonds.

Question 14

mRNA

Answer 14

• There are 105 different species.
• It represents 5% of the RNA in the cell.
• They are responsible for transmitting the message of the DNA to the ribosome to be translated.
• Each protein has its own mRNA.
• It is divided into three parts which are untranslated leading part which ends in a cap,Translated coding sequence which is responsible for arranging the amino acids in the polypeptide chain ,Untranslated tailing part which ends in a poly-A tail.
• It is formed by RNA polymerase II

Question 15

tRNA

Answer 15

• It is 50-60 different species.
• It represents 15% of the RNA in the cell.
• It is formed by RNA polymerase III.
• It is folded back on itself like hairpin in a clover-like appearance which is stabilized by base pairing.

Question 16

What are the arms of tRNA?

Answer 16

D-Arm : It has a stem of 3-4 bp and ends in a loop which has diflourouracil (hence its name)

TŪC arm: it has a stem of 5 bp which ends in a loop containing thymine, psuedouridine and cytosine (Hence its name)

Anti-codon arm : it has a stem of 5 bp which ends in aloop that has 7 bases where the middle three represent the anticodon. (Hence its name)

The acceptor arm : it has a stem of 7 bp and it doesn’t end in a loop but with a sequence CCA and the 3 end is where the amino acid attaches.

Question 17

What is the percentage of rRNA in the cell?

Answer 17

80%

Question 18

Why is rRNA broken into smaller pieces?

Answer 18

• In order to combine with polypeptide chains forming the ribose.

Question 19

What is the mammalian ribosome 80s composed of?

Answer 19

Consists of 2 subunits:- Large subunit 60s which consists of 50 polypeptide chain and 3 types of rRNA (5s-5.8s-28s)

-Small subunit 40s which consists of 30polypeptide chain and 18s rRNA .

Question 20

What are the polymerases that form rRNA?

Answer 20

RNA polymerase I for 5.8s,18s and 28sRNA polymerase III for 5s

Question 21

What is the stability of all types of RNA?

Answer 21

• mRNA is ranged between unstable to very stable
• tRNA is very stable
• rRNA is very stable

Question 22

what are the common features of nucleic acids?

Answer 22

Question 23

compare between DNA & RNA

Answer 23

Question 24

What are the characteristics of snRNA?

Answer 24

• 30 different species.
• Less than 1% of the cellular RNA.