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The cell and the DNA

The DNA consists of two strands, called double helix, which wind clock-wise (to the right) around an imaginary axis.
Helix = the ladder.

The DNA is in the nucleus and is in each body cell.

The DNA carries the genetic information.

The DNA is a chain molecule consisting of many repeating units (= Desoxyribonucleotides).
Each nucleotide is a chemical conjunction consisting of a sugar, called Desoxyribose, a nucleo-base and a phosphor-acid molecule.

The 4 nucleo-bases of the DNA

The DNA is made up by only four nucleo-bases held together by a hydrogen bride:

A ... Adenine pairs only with Thymine
C ... Cytosine pairs only with Guanine
G ... Guanine
T ... Thymine

DNA and Chromosomes

The DNA is enveloped in chromosomes together with proteins.

The Chromosomes carry the Genes (= a segment of the DNA), each chromosome may carry thousands of genes.

The material, which builds up the chromosomes, is called Chromatin.
The term got its name, because the material can be colored by using special bases (Chroma = the color).

Each organism has its specific number of chromosomes:

Chromosomes of the cat:

DNA and Genes

The chromosomes carry the genes.
Genes consist of DNA-molecules.
A gene is an information unit for a specific trait. A gene consists of two alleles.

Alleles
An allele is a modification of a gene due to mutation.
The "normal gene" is called the wild type of the gene.

Locus
Locus = location
The location of a gene in the chromosome is called locus.

Finally, let us make a comparison:

The DNA consists of letters (Bases):
ATGCTCGAATAAATGTCAATTTGA

These letters join to words (= Triplets):
ATG CTC GAA TAA ATG TCA ATT TGA

These words build sentences (= Genes):
< ATG CTC GAA TAA ><ATG TCA ATT TGA>

Thus the genes contain segments of the DNA.