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Genetic Structure - Page 6

Gene Expression

Gene Expression is the process by which a gene gets turned on in a cell to make RNA and proteins. Most genes contain the information needed to make functional molecules called proteins. The journey from gene to protein is complex and tightly controlled within each cell.

Gene expression consists of two major steps:

Transcription first, then Translation

Together, transcription and translation are known as gene expression. This drawing provides a graphic overview of the many steps involved in transcription and translation:


(a) Within the nucleus of the cell (light blue), genes (DNA, dark blue) are transcribed into RNA.

(b) This RNA molecule is then subject to posttranscriptional modification and control, resulting in a mature mRNA molecule (red)

(c) It is then transported out of the nucleus and into the cytoplasm (peach), where it undergoes translation into a protein. mRNA molecules are translated by ribosomes (purple) that match the three-base codons of the mRNA molecule to the three-base anti-codons of the appropriate tRNA molecules.

(d) These newly synthesized proteins (black) are often further modified, such as by binding to an effector molecule (orange), to become fully active.

Image Courtesy of National Center for Biotechnology Information

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Lets go through this one more time in a little more detail:

Transcription: During the process of transcription, the information stored in a gene's DNA is transferred to a similar molecule called RNA (ribonucleic acid) in the cell nucleus.

Both RNA and DNA are made up of a chain of nucleotide bases, but they have slightly different chemical properties. The type of RNA that contains the information for making a protein is called messenger RNA (mRNA) because it carries the information, or message, from the DNA out of the nucleus into the cytoplasm.

Translation: the second step in getting from a gene to a protein, takes place in the cytoplasm. The mRNA interacts with a specialized complex called a ribosome, which "reads" the sequence of mRNA bases.

Each sequence of three bases, called a codon, usually codes for one particular amino acid. (Amino acids are the building blocks of proteins.)

Forming a Protein: A type of RNA called transfer RNA (tRNA) assembles the protein, one amino acid at a time. Protein assembly continues until the ribosome encounters a "stop" codon (a sequence of three bases that does not code for an amino acid).

The flow of information from DNA to RNA to proteins is one of the fundamental principles of molecular biology. It is so important that it is sometimes called the "central dogma."

This is very technical information, the important point to remember is that "gene expression" means a gene is "turned on" and can form a protein.

So although lung cells and kidney cells have the same genes, they have different functions because different genes are turned on in the lung vs. the kidney.

Note: to further complicate this, many genes come in a number of variant forms, known
as alleles. An allele is one member of a pair or series of different forms of a gene. A
dominant allele prevails over a normal allele. A recessive allele prevails if its counterpart
allele on the other chromosome becomes inactivated or lost.

For example, you might have an allele for blood type O from mom and one for blood type
B from dad. Since type B is dominant over type O, you will have blood type B. Most traits
are carried on more than one set of genes, however, and their inheritance pattern is more
complex than are blood types.



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