Deoxyribonucleic Acid (DNA)
The complexity of human genetics is extensive and the sum of all the information
on this site barely begins to scratch the surface and describe all that it is.
It all begins at the cellular level with cells being the fundamental working
units of every living thing. All the instructions needed to direct their
activities are contained within a chemical known as DNA.
DNA from all living organisms is made up of the same chemical and physical
components. The DNA sequence is the particular side-by-side arrangement of bases
along the DNA strand. The order of its component chemicals spells out the exact
instructions required to create a particular organism with its own unique
traits.
DNA is the hereditary material in humans and nearly all other organisms. Most
DNA is located in the cell nucleus where it's called nuclear DNA, but a small
amount of it can also be found in the mitochondria where it is called
mitochondrial DNA or mtDNA.
The information in DNA is stored as a code and is made up of four chemical
bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). Human DNA
consists of around 3 billion bases, and more than 99% of those bases are the
same in all people. Chimpanzee and human genomes are more than 98% identical
showing that a 1% difference can be significant. The order, or sequence, that
these bases are placed into determines the information available for building
and maintaining an organism, similar to the way in which letters of the alphabet
appear in a certain order to form words and sentences.
DNA bases pair up with each other, A with T and C with G, to form units called
base pairs. Each base is also attached to a sugar molecule and a phosphate
molecule. Together, a base, sugar, and phosphate are called a nucleotide.
Nucleotides are arranged in two long strands that form a spiral called a double
helix. The structure of the double helix is somewhat like a ladder, with the
base pairs forming the ladder's rungs and the sugar and phosphate molecules
forming the vertical sidepieces of the ladder.
An important property of DNA is that it can replicate making copies of itself.
Each strand of DNA in the double helix can serve as a pattern for duplicating
the sequence of bases. Chemically, DNA consists of two long polymers called
nucleotides, with backbones made of sugars and phosphate groups joined by ester
bonds. These two strands run in opposite directions to each other and are
therefore anti-parallel.
Attached to each sugar is one of four types of molecules called bases. It's
the sequence of these four bases along the backbone that encodes information.
This information is read using the genetic code, which specifies the sequence
of the amino acids within proteins. The code is read by copying stretches of
DNA into the related nucleic acid RNA, in a process called transcription. This
is critical when cells divide because each new cell needs to have an exact copy
of the DNA present in the old cell.
Any living organism's genome is their complete set of DNA. Genomes vary widely
in size with the smallest known genome for a free-living organism (a bacterium)
contains about 600,000 DNA base pairs, while human genomes have some 3 billion.
Now except for mature red blood cells, all human cells contain a complete
genome.
Chromosomes
In the nucleus of each cell, a DNA molecule is packaged into thread-like
structures called chromosomes. Each chromosome is made up of DNA tightly coiled
many times around proteins called histones that support its structure.
DNA in each human cell is packaged into 46 chromosomes arranged into 23 pairs.
Each chromosome is a physically separate molecule of DNA that ranges in length
from about 50 million to 250 million base pairs. A few types of large
chromosomal abnormalities, including some with missing or extra copies or gross
breaks and rejoinings (translocations), can be detected by microscopic
examination. Most changes in DNA, however, are more subtle and require a closer
analysis of the DNA molecule to find perhaps single-base differences.
Now 22 of the 23 pairs of chromosomes, called autosomes, are the same in both
males and females. The 23rd pair, the sex chromosomes, differ between males and
females. Females have two copies of the X chromosome, while males have one X and
one Y chromosome.
The 22 autosomes are numbered by size. The other two chromosomes, X and Y, are
the sex chromosomes. The picture to the right of human chromosomes lined up in
pairs is called a karyotype.
Genes
Each chromosome contains many genes which are the basic physical and functional
units of heredity. Genes are specific sequences of bases that encode
instructions on how to make proteins. Genes comprise only about 2% of the human
genome with the remainder consisting of noncoding regions, whose functions may
include providing chromosomal structural integrity and regulating where, when,
and in what quantity proteins are made.
The human genome is estimated to contain around 25,000 genes. Genes vary in size
from a few hundred DNA bases to around 3 million bases.
Every person has two copies of each gene, one inherited from each parent. Most
genes are the same in all people, but less than 1% of a persons genes are
slightly different than everyone else. Alleles are forms of the same gene with
small differences in their sequence of DNA bases. As discussed earlier, these
small differences give each person their own unique physical features.
The process of inheritance of genes from each parent is very precise process.
Errors can on rare occasions happen due to outside factors such as chemicals and
viruses. A person's body usually has ways to correct these errors, but sometimes
the mistakes can result in permanent alterations or mutations of gene sequences,
leading to heritable diseases.
Some diseases, such as cystic fibrosis, are the result of the inheritance of
just a single mutated gene. With MS and autoimmune diseases, the situation is
more complex. Multiple genes acting in concert are believed to be involved,
almost certainly in a way that alters proteins in regulating immunity. Isolating
and identifying these genes is the goal of current research around the world.
Proteins
Proteins perform most life functions and even make up the majority of cellular
structures. Proteins are large, complex molecules made up of chains of small
chemical compounds called amino acids. Chemical properties that distinguish the
20 different amino acids cause the protein chains to fold up into specific
three-dimensional structures that define their particular functions in the cell.
The collection of all proteins in a cell is called its proteome. Unlike the
genome, the proteome can change from minute to minute in response to tens of
thousands of intra- and extracellular environmental signals. A protein's
chemistry and behavior are determined by the gene sequence and by the number and
identities of other proteins made in the same cell. Studies to explore protein
structure and activities, known as proteomics, look to be the focus of future
health and disease research.
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