DNA structure and chromosome organization

PINSET ISLAABAD

Introduction •Chromosomes are the structures that

contain the genetic material•They are complexes of DNA and proteins•The genome comprises all the genetic

material that an organism possesses• In bacteria, it is typically a single circular

chromosome• In eukaryotes, it refers to one complete set

of nuclear chromosomes•Eukaryotes possess a mitochondrial genome•Plants also have a chloroplast genome

Introduction • The main function of the genetic material is to

store information required to produce an organism• The DNA molecule does that through its base

sequence

• DNA sequences are necessary for 1. Synthesis of RNA and cellular proteins2. Proper segregation of chromosomes3. Replication of chromosomes4. Compaction of chromosomesSo they can fit within living cells

Frederic meisher 1860

•Pus cells from bandages •Pepsin from pigs stomach •Centrifuge •Pellet results in isolation of an organelle •Nucleus

Conclusions

•DNA carries the genetic information •DNA controls the synthesis of specific

products

Life cycle of Phage Virus

Fig. 12.4, p.189

tailfiber

DNAinsideproteincoat

hollowsheath

Fig. 12.4, p.189

Hershey and chase experiment

Conclusions

•Only the phage DNA enters the bacterial cells which means that the cells DNA are responsible for the division and multiplication of the viruses

Discovery of DNA Structure

•DNA consists of two strands of nucleotides, coiled into a double helix

•Each nucleotide has ▫A five-carbon sugar (deoxyribose)▫A phosphate group▫A nitrogen-containing base (adenine,

thymine, guanine, or cytosine)

DNA Nucleotides

Base Pairing

•Bases of two DNA strands pair in only one way▫Adenine with thymine (A-T)▫Guanine with cytosine (G-C)

•The DNA sequence (order of bases) varies among species and individuals

The DNA Double Helix

2-nanometer diameter overall0.34-nanometer distancebetween each pair of bases

3.4-nanometerlength of eachfull twist of thedouble helix

In all respects shown here, theWatson–Crick model for DNAstructure is consistent with theknown biochemical and x-raydiffraction data.

The pattern of basepairing (A with T,and G with C) isconsistent with theknown compositionof DNA (A = T,and G = C).

THE DNA DOUBLE HELIX

• A DNA molecule consists of two chains of nucleotides, hydrogen-bonded together along their length and coiled into a double helix

• Four kinds of nucleotides make up the chains: adenine, thymine, guanine, and cytosine

• The order in which one kind of nucleotide base follows the next along a DNA strand encodes heritable information

• The order in some regions of DNA is unique for each species

Watson, Crick, and Franklin

•Rosalind Franklin’s research produced x-ray diffraction images of DNA▫Helped Watson and Crick

build their DNA model, for which they received the Nobel Prize

THE FRANKLIN FOOTNOTE

•Like any race, the one that led to the discovery of DNA’s structure had its winners—and its losers

Genomic Organization And Function in the Cell Nucleus

Interphase Nucleus Mitotic Chromosomes

Chromosome territories

DNA Replication and Repair

•A cell replicates its DNA before dividing▫Enzymes unwind the double helix ▫DNA polymerases assemble complementary

DNA strands on templates from free nucleotides

▫DNA ligase seals gaps in new DNA strands

•Two double-stranded DNA molecules result▫One strand of each is new

Semiconservative DNA Replication

new

newold

old

Details of DNA Replication

Any gaps between basesof the “new” DNA are joinedto form a continuous strand.The base sequence of eachhalf-old, half-new DNAmolecule is identicalto that of the parent.

Part of a parentDNA molecule, with twocomplementary strands ofbase-paired nucleotides.

Replication starts.The strands are unwoundat many sites alongthe molecule’s length.

Each of the two parentstrands guides the assemblyof new DNA strands from freenucleotides, according tobase-pairing rules.

direction ofunwinding

New DNA is assembledcontinuously on only oneof the two parent templatestrands. It is assembled onthe other parent templatestrand in short fragments.DNA ligase seals the gapsbetween the fragments.

Why discontinuous assembly? DNAsynthesis occurs only in the 5’ to 3’ direction. Free nucleotides can beadded only to the —OH group atthe 3’ end of a growing strand.

direction of

synthesis

Part of a parentDNA molecule, with twocomplementary strands ofbase-paired nucleotides.

Any gaps between basesof the “new” DNA are joinedto form a continuous strand.The base sequence of eachhalf-old, half-new DNAmolecule is identicalto that of the parent.

Each of the two parentstrands guides the assemblyof new DNA strands from freenucleotides, according tobase-pairing rules.

Replication starts.The strands are unwoundat many sites alongthe molecule’s length.

Replication Errors

•DNA repair mechanisms fix DNA damaged by chemicals or radiation

•Proofreading by DNA polymerases corrects most base-pairing errors

•Uncorrected errors are mutations

HOW CELLS DUPLICATE THEIR DNA

•Before a cell divides, enzymes and other proteins copy its DNA

•Newly forming DNA strands are monitored for errors, most of which are corrected

•Uncorrected errors are mutations

Cloning

•Clones▫Genetically identical individuals▫Produced by artificial twinning, nuclear

transfers

•To clone an adult animal▫Cell’s DNA must be reprogrammed to

function like an embryonic cell and direct development

Cloning Methods

Nuclear Transfer

Thanks