Lecture-3
DNA Structure:
(Deoxyribonucleic acid)
DNA is a long linear polymer found in the nucleus of a cell and formed from nucleotides
and shaped like a double helix; associated with the transmission of genetic information;
"DNA is the king of molecules"
Watson and Crick (1953) proposed the model of DNA structure.
Note:
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DNA is a polymer.
Monomer unit of DNA is nucleotides.
Nucleotide = Sugar + Nitrogen bases + Phosphate
DNA is a right handed double helix.
About 10 nucleotide pairs per helix.
Bases are bonded with hydrogen bonds.
Adenine always binds with Thymine and Cytosine always binds with Guanine.
Polymer:
(Macromolecule comprises repeating structural units connected with
covalent bond).
Back bone:
(Repeated pattern of a sugar and phosphate group)
Chemical structures of different units of DNA structure:
Phospho sugar
Phospho sugar
DNA types:
Division on the basis of position
Chromosomal DNA:
This type of DNA is present on chromosomes in the nucleus and is always a part
of chromosomal inheritance.
A chromosome is an organized structure of DNA and protein that is found in cells. A
chromosome is a single piece of coiled DNA containing many genes, regulatory elements
and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which
serve to package the DNA and control its functions.
Chromosomal DNA has two nuclear DNA copies. It is inherited both by male and female
parent and change by 50% each generation. In human almost 3 billion nucleotides are
present.
Plasmid DNA:
A plasmid is an extra-chromosomal DNA molecule separate from the
chromosomal DNA which is capable of replicating independently of the chromosomal
DNA. In many cases, it is circular and double-stranded. Plasmids usually occur naturally
in bacteria, but are sometimes found in eukaryotic organisms. Plasmids are considered
transferable genetic elements, or "replicons", capable of autonomous replication within a
suitable host.
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Plasmids have much less base pairs than chromosomes
Plasmids are rarely organized by chaperone proteins
Plasmids are easily transferred
Plasmids usually contain non-essential genes
Plasmids function can be lost or gained without harming the organism
Plasmids are usually found in "lower" organisms.
Mitochondrial DNA:
Mitochondria are structures within cells that convert the energy from food into a
form that cells can use. Although most DNA is packaged in chromosomes within the
nucleus, mitochondria also have a small amount of their own DNA. This genetic material
is known as mitochondrial DNA or mtDNA. In humans, mitochondrial DNA spans about
16,500 DNA building blocks (base pairs), representing a small fraction of the total DNA
in cells. Mitochondrial DNA contains 37 genes, all of which are essential for normal
mitochondrial function. Thirteen of these genes provide instructions for making enzymes
involved in oxidative phosphorylation. Oxidative phosphorylation is a process that uses
oxygen and simple sugars to create adenosine triphosphate (ATP), the cell's main energy
source. The remaining genes provide instructions for making molecules called transfer
RNA (tRNA) and ribosomal RNA (rRNA), which are chemical cousins of DNA. These
types of RNA help assemble protein building blocks (amino acids) into functioning
proteins.
Mitochondrial DNA possesses 100s to 1000s of copies of DNA and pass only from
mother parent during sexual reproduction. Very slowly change generation because there
is no involvement of recombination. In case of human almost 16500 nucleotides are
present in mtDNA.
mtDNA is useful for forensic cases when nuclear DMA is insufficient and is not
recoverable especially from fossils. Each mitochondria contain 1 to 10 mtDNA and
mitochondria can occupy up to 25% of cytoplasm.
mtDNA is a type of DNA carried by both males and females, but is only inherited from
the mother. This makes mtDNA useful for tracing one's direct maternal line (mother,
maternal grandmother, maternal great-grandmother, etc).
Autosomal DNA:
Autosomal DNA is the type of DNA responsible for most physical characteristics,
such as height, eye color, etc. Autosomal DNA is inherited by sons and daughters from
both parents (and from all four grandparents, etc).
Autosomal Chromosomes are found in the nucleus of most of the cells in our body. Most
of the cells in our body (somatic cells) contain 23 pairs of chromosomes consisting of 22
pairs of Autosomal chromosomes and 1 pair of sex chromosomes (the X and Y
chromosomes). One copy is of each pair inherited from our biological mother, and the
other copy is from our biological father.
Y-DNA:
Y-DNA is a type of DNA that is only carried by men, who inherit it from their
fathers. This means that males with a common paternal ancestor have similar Y-DNA.
Y-DNA is particularly useful for tracing one's direct paternal line (father, paternal
grandfather, etc.) because it changes slowly from generation to generation, and in most
societies, the surname of the father is also inherited by his sons.
Division on the basis of structure and funtion
A-DNA:
A-form of DNA is right-handed, but every 2.3 nm makes a turn and there are 11
base pairs per turn
B-DNA:
A long, thin form of deoxyribonucleic acid.The most
common form of DNA in living
organisms, in which the double helix twists in a right-hand direction. B-DNA posses
almost 10 bp per turn. In this, the helix makes a turn every 3.4 nm, and the distance
between two neighboring base pairs is 0.34 nm. In a solution with higher salt
concentrations or with alcohol added, the DNA structure may change to an A form,.
Z-DNA:
Another DNA structure is called the Z form, because its bases seem to zigzag.
Z DNA is left-handed. One turn spans 4.6 nm, comprising 12 base pairs. The DNA
molecule with alternating G-C sequences in alcohol or high salt solution tends to have
such structure.
cDNA: (Clonal or complementary DNA)
In genetics, complementary DNA (cDNA) is DNA synthesized from a mature
mRNA template in a reaction catalyzed by the enzyme reverse transcriptase and the
enzyme DNA polymerase. cDNA is often used to clone eukaryotic genes in prokaryotes.
Though there are several methods for doing so, cDNA is most often synthesized from
mature (fully spliced) mRNA using the enzyme reverse transcriptase. This enzyme
operates on a single strand of mRNA, generating its complementary DNA based on the
pairing of RNA base pairs (A, U, G and C) to their DNA complements (T, A, C and G
respectively). cDNA is produced from fully transcribed mRNA found in the nucleus and
therefore contains only the expressed genes of an organism.
cDNA is used to describe libraries of genetic information which is then used for testing
pharmcuticals.
Sources:
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http://wiki.answers.com/Q/How_is_plasmid_DNA_different_from_chromosomal_DNA
http://ghr.nlm.nih.gov/chromosome/MT
http://www.smgf.org/pages/how_it_works.jspx
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http://www.web-books.com/MoBio/Free/Ch3B3.htm
Autosomal DNA and its Applications: Part 1, Autosomal DNA 101. Genebase Tutorials. Retrieved
October 17, 2010, from http://www.genebase.com/tutorial/item.php?tuId=30