How is a DNA instruction copied?

The first part of the DNA double helix in the cell nucleus unwinds and unzips. The DNA instruction is revealed, flanked by 'start' and 'stop' codes. The cell makes a copy of the DNA in the form of an RNA molecule. The RNA copy is trimmed, and then enters the cell cytoplasm to be decoded. It is now called messenger RNA (mRNA).

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RNA encodes 'start' and 'stop' codons.

What is RNA?

RNA is similar to DNA: it consists of strands of four different chemical building blocks, or bases, attached to a chemical backbone. But unlike DNA, in which two strands coil around each other to make a double helix, RNA remains as a single strand. Like DNA, RNA has the bases A, C and G, but instead of thymine (T), it uses another base called uracil (U).

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How RNA is made inside the cell.

What is the code?

All proteins are made up of combinations of 20 different amino acids. The RNA code (like DNA) is written in just four different chemical 'letters' - bases. How can a code with four letters identify 20 different amino acids? By reading it three letters at a time - CAU, CAG, GGA, etc. This system has 64 different combinations, or codons. Most amino acids are represented by several alternative codons. Some codons are not instructions for amino acids at all, but signal the end of the gene ('stop' codons).

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The RNA code.

What happens to the RNA copy?

The RNA copy made in the cell nucleus is usually much longer than the one that is decoded in the cell cytoplasm. In 1977, scientists discovered that the genes of most living things are divided up into exons, which are part of the instruction for the protein, and introns, which are not. No-one really knows why genes are divided up in this way. After a cell makes an RNA copy of a gene, it snips out the introns and sticks the exons together to make messenger RNA (mRNA).

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Molecular model of a ribosome showing the RNA and protein components in the form of ribbon models.

How are proteins assembled?

Once in the cytoplasm, the mRNA is snatched up by tiny protein-assembly machines called ribosomes. Each ribosome works its way along the mRNA, reading the code from 'start' to 'stop', selecting the correct amino acid building blocks and ejecting a growing protein. It takes just one 50th of a second for the ribosome to select and add each building block. At this rate, a cell can assemble a small protein like insulin in just a few seconds.

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How your cells assemble proteins.

 

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