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All the proteins of an organism are made under the chemical direction of nucleic acids.
There are two types, RNA (ribonucleic acid) and DNA (deoxyribonucleic acid).
DNA forms the chemical basis through which genetic information is carried.
All physical and biological characteristics are inherited through it.
DNA molecules direct their own duplications and guide the assembly of amino acid units into a unique polypeptide sequence.
Both DNA and RNA have a similar structures.
They consist of a chain of deoxyribose or ribose sugars linked by phosphate groups with side chain bases.
One structural difference between DNA and RNA is that the sugar in DNA, deoxyribose, is lacking an oxygen atom where ribose has an OH group.
phosphate group structure
Basic backbone feature:
The side chain bases are all heterocyclic amines.
Their molecular shapes give the nucleic acids their function.
They are represented by single letters--A for adenine, T for thymine, U for uracil, G for guanine, and C for cytosine.
A, T, G, and C occur in DNA.
A, U, G, and C occur in RNA.
They make up the "letters" of the genetic alphabet.
Genes, the individual units of heredity, are unique in the length of the backbone and the sequence in which the bases occur.
All of an organism's hereditary information resides on these aspects of DNA.
The DNA Double Helix
In DNA, their are two backbones (or strands) of nucleic acids and they are intertwined like a spiral staircase called the DNA double helix.
A 3D rendered computer model of the DNA double helix is shown on the right (the purple lines are to help visualize the backbone).
Hydrogen bonds (···) between the N-H groups and the O=C units in the side chain bases hold it together.
However, hydrogen bonding only occurs between certain pairs of bases.
A always pairs with T and C always pairs with G in DNA.
A will pair with U, but U occurs in RNA only, which is important in the work of RNA.
If an A is found on one strand, a T will be found on the other.
The same goes for C and G.
Because of the limitation of base pairings, replication is very accurate.
When replicating, the two strands divide apart and the complementary nucleotides (monomers of DNA, consists of phosphate, sugar, and base) will join with the two strands, creating two separate molecules.
Nucleotides are made by cells and are always present in the cellular "soup," ready to be used for duplication or other processes.
DNA replication is used in cell reproduction, where the two identical copies are carried to the two resulting cells.
| | | | | | | | | |
|-A T-| |-A T-| |-A T-| |-A T-| |-A T-| |-A T-|
| | | | | | | | | | | |
|-G C-| |-G C-| |-G C-| C-| |-G C-| |-G C-|
| | ==> | | ==> | | | ==> | | | |
|-T A-| |-T A-| |-T A-| |-T A-| |-T A-|
| | | | | | | | | |
|-C G-| |-C G-| |-C |-C G-| |-C G-| |-C G-|
| | | | | | | | | | |
one DNA strands split spare nucleotides two identical
fit in DNA molecules
DNA-Directed Synthesis of Polypeptides
Each polypeptide in a cell is made under the direction of its own gene.
To make the polypeptide, a few basic steps are followed:
DNA =============> mRNA ===========> tRNA ==> polypeptide
In transcription, a type of RNA called messenger RNA (mRNA) is used to read the code of a gene in DNA.
The gene in the DNA strand opens up, allowing RNA nucleotides to bond in a certain sequence complementary to the bases on the DNA (though U is used instead of T in RNA).
Once completed, the mRNA goes out of the cell nucleus to a ribosome (manufacturing center for polypeptides) and the DNA reforms its original shape.
At the ribosome, transfer RNA (tRNA) reads the mRNA in a process called translation.
The tRNA has an open faced part (anticodon) for reading the mRNA consisting of three letters (codon) a time.
Each tRNA is attached to a specific amino acid, and as it reads the mRNA, the amino acids are put together in a certain sequence.
For example, the anticodon CCA on tRNA matches up with the codon GGU on mRNA.
Since CCA is always with glycine, the glycine will become part of the growing chain.
If the next codon is GCU, it matches up with anticodon CGA, which is alanine.
So, the alanine will be attached to the glycine.
Through this process, long chains of polypeptides are formed.