分子生物学课件(第二章)

Cytoplasm(90%), Nucleus(10%),viruses
tRNA
rRNA
mRNA
1.1.2.1 mRNA

Life time is short

Each polypeptide has corresponding mRNA，the difference among different mRNA is quite big (in length and molecular weight)；
♬ Experimental basis
① X~ray photograph of DNA with high quality：
DNA specimens from different species have
the same results（constant width; 3.4nm）；
② Chargaff rules：the rule of the composition of
Double helix stairway in the Vatican Museum
2.2.1 key notes of DNA double helix

Two polynucleotide chains in a DNA double helix antiparallel（one strand runs in the 5 ’ → 3 ’ direction, whlie its partner runs 3’→5’）
Every Aa at least has a correspnding tRNA（tRNA、 tRNAAla）。


Different tRNA has almost the same big，73-98 Nts
1.1.2.3 rRNA

The amount is the most ， Molecular Weight is the biggest；
DNA
③ Physical chemistry studies and acid and
alkali titrate studies on DNA base ；
Chargaff rules：
Chargaff rules：
The base composition of DNA varies from one species to another, while DNA specimens isolated from different tissues of the same species have the same base composition； In nature DNA, the number of G units equals the number of C units and the number of A units equals the number of T units ( this strongly hinted towards the base pair make up of DNA )； The base composition of DNA in a given species does not change with an organism’s age, nutritional state,or changing environment。
♬DNA double-helix


A Structure for Deoxyribose Nucleic Acid was published by James D. Watson and Francis Crick in the scientific journal Nature in its 171th volume on page 737-738 (dated April 25, 1953.) ..\watson-crick.pdf It was the first publication which described the discovery of the double helix structure of DNA.

ermediaries,
carrying
genetic
information from genes to ribosomes, where the corresponding proteins can be synthesized (as
templates).
1.1.2.2 tRNA
Molecular Biology
Chapter 2
Structure and Properties of Nucleic Acid
Shihua Wang Ph.D. Prof.
1 Overview
1.1 Type and distribution of nucleic acid
1.1.1 DNA Deoxyribose Nucleic Acid

1.1.2 RNA

Ribose Ncleic Acid Function: mainly participate the expression of genetic information Type: rRNA（ribosomal RNA） mRNA（messenger RNA） tRNA（transfer RNA） Distribution:

DNA: 2’-deoxyribose + A、T、C、G

RNA: ribose + A、U、C、G

Glycosidic bond:
Linking C1’ of the ribose to the N1 of pyrimidine Linking C1’ of 2’-deoxyribose to the N9 of purine

Function: The genetic information carrier, responsible for the genetic information storage and transmission distribution： Eukaryotes: nucleus（98%）,organelle（2%） Prokaryotes: nucleoid、plasmids
phosphoric acid
nucleoside
pentose
base
nucleotide
means pentose, DNA has deoxyribose, RNA has ribose； means nitrogenous bases means phosphate
1.2.1 Pentose



backbone：phosphate + pentose；
direction： 5’ →3’ ；
Representation
A 'P 5 P
C P
T P
C
G A P P OH
3'
5'pApCpTpCpGpA -OH 3'
5'ACTCGA 3'
2.2 secondary structure of DNA -----DNA double helix


Along the same axis，two chains are wound around each other, resulting in a right-handed double helix forms a major groove and a minor groove


The bases lie on the inside，the sugarphosphate backbone is on the outside The bases are flat structure, lying in pairs perpendicular to the axis
1.2.3.2 nucleotide
base
N
O
NH
N
N
N O H
HN N H
H2O
OH O
Phosphate bond
O O P OH O
OHCH2
phosphoric acid
H2O
OH OH
glycosidic bond
pentose
Nucleoside=base+pentose Nucleotide=base+pentose+phosphate
widespread, and has a number of
specific roles.

Such as: the methylation of the N-6
position of A、the 4-amino group and
the 5-position of C
1.2.3 nucleoside and nucleotide

2 Nucleic acid structure
2.1 primary structure of nucleic acid

definition：the nucletide residue sequence of the polynucleotide chain； linkage：3’,5’-phosphodiester bond；

Functions:
structural
components
of
ribosomes, the complexes that carry out the synthesis of proteins
1.2 the components of nucleic acid
nucleic acid nucleotide


The diameter of the
double helix is 2nm

There is a complete turn every 3.4nm, with 10bp per turn.

Two chains in the double helix associate by hydrogen bonding between the bases, G with C and A with T ( base pairing , complementary). Three hydrogen bond can form specifically between G and C, but only two can form between A and T;


2.2.3 Stable factors of the double helix
In physiological conditions,the DNA double helix is very stable. stable factors： ① H bonding between complementary base pairs； ② Base-stacking interaction（hydrophobic effect, the major factor）； ③ electrovalent bond（between the negative charges carried on the phosphate groups and the positive charges carried on the proteins or metal ions）

2.2.2 Important Meanings

Explains for the first time how genetic information is encoded in DNA and passed from one generation to the next Tremendous strides in our understanding of the structure of DNA and, as a result our ability to work with and manipulate the information-rich DNA molecule Base pairing → DNA semi-conservative replication mechanism

Molecular weight is the smallest

Functions: as adapter molecules (transport amino acids), that faithfully translate the information in mRNA into a specific sequence of amino acids
ribose
2’- deoxyribose （in DNA）
（in RNA）
1.2.2 base
DNA
Pyrimidine ring
4
3N 2 N 1 6 1N 5 2 4 N 3
5 6
7 N N 8 9
RNA
purine ring
Modified bases

The chemical modification of bases is