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DNA STRUCTURE - PowerPoint PPT Presentation


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DNA STRUCTURE. N UCLEIC ACIDS Include DNA: Deoxyribonucleic acid RNA: Ribonucleic acid. DNA ( Deoxyribonucleic acid): Site : Mostly found in the nucleus of cell and small amount is found in the mitochondria. DNA carries all the genetic informations of the individual

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slide2

NUCLEIC ACIDS

Include

DNA: Deoxyribonucleic acid

RNA: Ribonucleic acid

slide3

DNA (Deoxyribonucleic acid):

  • Site : Mostly found in the nucleus of cell and small amount is found in the mitochondria.
  • DNA carries all the genetic informations of the individual

- Nuclear DNA:carries the genetic informations in the chromosomes that encode functional proteins or functional RNA.

-

slide4

Mitochondrial DNA:

  • contains:
  • 1-genes encode proteins of the electron transport chain.
  • 2-genes encoding transfer RNA (tRNA) and the small and large subunits of ribosomal RNA (rRNA).
slide5

DNA must be to replicated precisely each time the cell divides, in such a way that each daughter cell acquires the same amount of genetic material.

slide6

Structure:

  • DNA is formed of two chains (strands).
  • The 2 strands wind around each other forming a double helix (Watson and crick model).
slide7

Each strand (chain ) of DNA is formed of nucleotides

  • Each nucleotide of DNA is formed of :
  • Sugar (deoxy ribose).

2. Phosphate group.

3. Nitrogen containing bases:

  • Purine bases (bases containing 2 rings (Adenine [A], Guanine [G])
  • Pyrimidine bases (bases containing single ring (Thymine [T], Cytosine)
slide8

Linking of the base to the sugar (at the 1’ carbon of the sugar) is called nucleoside

Figure: β-Glycosidic linkage in a deoxyribonucleoside.

slide9

Linking of nucleoside to phosphate group ( at the 5’ carbon of the sugar) is called nucleotide

slide11

Nucleotides are linked in each DNA strand through Phosphodiester bond between the sugars and phosphates

slide12

Figure: Formation of a phosphodiester bond between 2 successive nucleotidesEster bonds are strong covalent bonds formed by the reaction of an acid and alcohol(- OH).Phosphodiester bond is a group of strong covalent bonds between a phosphate group and two 5-carbon ring carbohydrates (pentoses) over two ester bonds.

slide13

Fig.: A polynucleotide chain of DNA

  • Each DNA strand (chain) has 2 ends:
  • The 5’ end has a free phosphate group.
  • The 3’ end has a free hydroxyl group.
  • Note the direction of DNA replication is in a 5’ -3’ direction.
slide14

The hydrophilic (polar) deoxyribose phosphate backbone of each chain is on the outside of the DNA molecule, whereas the hydrophobic (nonpolar bases) are stacked inside perpendicular to the axis of the helix.

  • The 2 DNA strands wind around each other in antiparallel manner; that is From any fixed position in the helix, one strand is oriented in the 5′ 3′ direction and the other in the 3′ 5′ direction.
  • The double helix made by winding of the 2 DNA strands around each other, is stabilized by hydrogen bonding between the bases of the 2 strands and by the hydrophobic interactions between the stacked bases.
slide15

Base pairing rule: Thymine always pair with adenine and cytosine always pair with guanine. So one polynucleotide chain is of the double helix is always complementary to the other.

  • Thymine and adenine bases are connected by 2 hydrogen bonds , while cytosine and guanine are connected by 3 hydrogen bonds.This makes G-C base pairs more stable than A-T base pairs
slide17

On its exterior surface, the double helix of DNA contains two deep grooves between the ribose-phosphate chains. These two grooves are of unequal size and termed the major and minor grooves. The difference in their size is due to the asymmetry of the deoxyribose rings and the structurally distinct nature of the upper surface of a base-pair relative to the bottom surface.

slide22

The chromatin in eukaryotic cellsconsists of:

Nuclear DNA

bound to

Histones proteins

&

smaller amounts of non histone proteins (Enzymes involved in DNA replication and transcription)

&

Small amount of RNA (snRNA)

slide23

The chromatin in eukaryotic cellsconsists of:

Nuclear DNA

bound to

Histones proteins

&

smaller amounts of non histone proteins (Enzymes involved in DNA replication and transcription)

&

Small amount of RNA (snRNA)

slide24

HISTONES

-Are basic proteins (having positive charges), so they form ionic bonds with the negatively charged DNA so their binding is facilitated.

  • - 5 types: H1, H2A, H2B, H3, H4.
  • - Functions:
  • They help condensation of the DNA into more compact chromosomes.
  • Protect the DNA from digestion by exonucleases.
  • Modified histones: Has an important role in changing the structure of chromatin and chromatin function e.g. acetylation of H3 and H4 is associated with activation or inactivation of gene expression.
slide26

Nucleosome consists of

1- DNA core:

Formed of supercoiled DNA surrounding histone octamer(2 molecules of each of H2A, H2B, H3, H4).

2- Linking region:

It joins one nucleosome core to the next

Formed of 60 bp of DNA and 1 molecule of histone (H1) which protects the linker DNA from digestion by exonucleases

slide29

Chromatinis built from repeating nucleosomes units

If the chromatin is placed in low salt buffer and viewed with E/M (i.e If the chromatin is pulled into a linear strucrure)→→→→ It resembles a string of beads( with the beads representing nucleosome cores and the string representing the DNA linker)

slide30

Compaction of chromatin

  • Condensation of the nucleosomes with histone H1 in the centreform the 10 nm fibril which represents 7 fold shortening of the linear ß- form DNA.
  • Supercoiling of the 10 nm fibril with 6-7 nucleosomes per turnform the 30 nm fiber which represents 50 fold shortening of the length of DNA.
  • Supercoiling of the 30 nm fiber intosuperloops(700 nm in diameter).
  • Each 6 superloops attached to a protein scaffold form a rosette.
  • Each 30 rosette form one coil.
  • Each 10 coils form one chromatid.
slide31

Chromosomes

2 identical sister chromatids attached at the centromere→→one chromosome (in the metaphase)

The centromere

- is rich in A=T and is about 130 bp long.

- is connected to specific proteins to form a complex known as kinetochore which is connected to mitotic spindle. This complex is essential structure for chromosomal segregation during mitosis.