SEVİMLİ YARATIKLAR

Legend:

Representation of the 23 paired chromosomes of the human male.

Chromosome: a very long DNA molecule and associated proteins, that carry portions of the hereditary information of an organism.

    a. Structure of a chromosome (Typical metaphase chromosome):
A chromosome is formed from a single DNA molecule that contains many genes. A chromosomal DNA molecule contains three specific nucleotide sequences which are required for replication: a DNA replication origin; a centromere to attach the DNA to the mitotic spindle.; a telomere located at each end of the linear chromosome.

The DNA molecule is highly condensed. The human DNA helix occupy too much space in the cell. Small proteins are responsible for packing the DNA into units called nucleosomes.

    b. Stained chromosomes:
Chromosomes are stained with A-T (G bands) and G-C (R bands) base pair specific dyes.
When they are stained, the mitotic chromosomes have a banded structure that unambiguously identifies each chromosome of a karyotype. Each band contains millions of DNA nucleotide pairs which do not correspond to any ********al structure.
Adapted from  K.F. Jorgenson, J.H. van de Sande, and C.C. Lin, Chromosoma 68:287-302, 1978.

    c. Karyotype of a male:
The human haploid genome contains 3,000,000,000 DNA nucleotide pairs, divided among twenty two (22) pairs of autosomes and one pair of sex chromosomes.

The major types of proteins, which must work together during the replication of DNA, are illustrated,
showing their positions.

When DNA replicates, many different proteins work together to accomplish the following steps:
 

1. The two parent strands are unwound with the help of DNA helicases.
   
2. Single stranded DNA binding proteins attach to the unwound strands, preventing them from winding back together.
   
3. The strands are held in position, binding easily to DNA polymerase, which catalyzes the elongation of the leading and lagging strands. (DNA polymerase also checks the accuracy of its own work!).
   
4. While the DNA polymerase on the leading strand can operate in a continuous fashion, RNA primer is needed repeatedly on the lagging strand to facilitate synthesis of Okazaki fragments. DNA primase, which is one of several polypeptides bound together in a group called primosomes, helps to build the primer.
   
5. Finally, each new Okazaki fragment is attached to the completed portion of the lagging strand in a reaction catalyzed by DNA ligase.

In the past, bacteria was the standard for cloning sequences of DNA, and although it is still used in research today, new and faster methods of duplicating DNA have been developed.  With the development of the polymerase chain reaction technique, usually abbreviated PCR, replication of DNA sequences was revolutionized.  Using this method a researcher develops small sequences of DNA that will allow him or her to isolate specific sequences of DNA.  These small sequences are called primers, mainly because they signal for the start of the cloning process.  One primer is designed for each strand of the DNA double helix, and they are positioned at opposite ends of the helix as well.  This positioning allows the polymerase protein (the protein responsible for replicating the complementary strand of DNA from the template strand) to replicate only within the primers, isolating that one specific sequence.  The PCR techniqueis performed through a series of cycles, depending on the quantity of DNA being replicated

The structure of deoxyribonucleic acid (DNA) was discovered by James Watson and Francis Crick in 1953.  DNA was determined to be a right handed double helix based on x-ray crystallographic data provided to Watson and Crick by Maurice Wilkins and Rosalind Franklin.  DNA is composed of repeating subunits called nucleotides.   Nucleotides are further composed of a phosphate group, a sugar, and a nitrogenous base.  Four different bases are commonly found in DNA: adenine (A), guanine (G), cytosine (C), and thymine (T).  In their common structural configurations, A and T form two hydrogen bonds while C and G form three hydrogen bonds.  Because of the specificity of base pairing, the two strands of DNA are said to be complementary.   This characteristic makes DNA unique and capable of transmitting genetic information.

Once a specific sequence of DNA has been isolated and placed into a recombinant plasmid (circular bacterial DNA with the sequence being studied inserted) the bacteria is allowed to replicate itself producing millions of clones that contain the DNA sequenced being used for the protein research.   One way to separate bacteria that have the inserted piece of DNA being studied from those who do not, is to add an antibiotic resistance into the plasmid, and as a result, the colonies that grow are the ones that contain the DNA that will be used later to translate the protein