How does dna make proteins

Translation, the second step in getting from a gene to a protein, takes place in the cytoplasm. Each sequence of three nucleotides, called a codon, usually codes for one particular amino acid. Amino acids are the building blocks of proteins. Through the processes of transcription and translation, information from genes is used to make proteins.

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An organism's complete set of nuclear DNA is called its genome. Besides the DNA located in the nucleus, humans and other complex organisms also have a small amount of DNA in cell structures known as mitochondria. Mitochondria generate the energy the cell needs to function properly. In sexual reproduction, organisms inherit half of their nuclear DNA from the male parent and half from the female parent.

However, organisms inherit all of their mitochondrial DNA from the female parent. This occurs because only egg cells, and not sperm cells, keep their mitochondria during fertilization.

DNA is made of chemical building blocks called nucleotides. These building blocks are made of three parts: a phosphate group, a sugar group and one of four types of nitrogen bases.

To form a strand of DNA, nucleotides are linked into chains, with the phosphate and sugar groups alternating. The four types of nitrogen bases found in nucleotides are: adenine A , thymine T , guanine G and cytosine C.

The order, or sequence, of these bases determines what biological instructions are contained in a strand of DNA. The complete DNA instruction book, or genome, for a human contains about 3 billion bases and about 20, genes on 23 pairs of chromosomes. DNA contains the instructions needed for an organism to develop, survive and reproduce. To carry out these functions, DNA sequences must be converted into messages that can be used to produce proteins, which are the complex molecules that do most of the work in our bodies.

Each DNA sequence that contains instructions to make a protein is known as a gene. The size of a gene may vary greatly, ranging from about 1, bases to 1 million bases in humans. Genes only make up about 1 percent of the DNA sequence. DNA sequences outside this 1 percent are involved in regulating when, how and how much of a protein is made.

Structural genes contain sequences of DNA determining the sequences of amino acid in proteins. The genetic information stored in the nucleus of a cell needs to be delivered to the ribosomes in the cytoplasm. Inside the nucleus of cells, DNA exists in the form of huge double helices. DNA molecules are too large to be passed on directly to the ribosomes so the genetic information stored in DNA needs to be copied onto a smaller, more mobile medium — that process is called transcription.

Single-stranded RNA molecules are much smaller than DNA molecules and can therefore travel through the tiny pores of the nuclear membrane. The role of RNA molecule will be to deliver protein-building instructions to a ribosome. In RNA, thymine is replaced by another nucleotide base called uracil. Base pairing rules in transcription differ slightly, therefore, from base pairing rules in DNA replication Box 2.

The process of DNA transcription is very similar to that of DNA replication see part 2 and involves the following steps:. Not all information encoded in mRNA strands is useful for constructing a protein. A newly transcribed RNA strand consists of two elements:. The process is shown in Fig 2. The genetic code is a triplet of three nitrogenous bases coding for one amino acid.

As there are 20 naturally occurring amino acids, three bases allow for each amino acid to be represented by one triplet code and some are represented more than once. Each run of three bases triplet code on an mRNA strand is called a codon.

Since AUG also codes for the amino acid methionine, methionine is the first amino acid incorporated into a protein — if it is not actually needed, it will be removed later Xiao et al, After transcription and post-transcriptional modification, a mature, uninterrupted sequence of mRNA is generated.

On entering the cytoplasm, this sequence attaches to a ribosome and can then be used for protein synthesis in a process called translation. The crude protein usually needs to be modified before it can adopt its final 3D configuration and start performing its function in the body.

Exported proteins can either be used in a tissue locally or transported to distant regions of the body by the blood. For example, the hormone insulin, synthesised in pancreatic beta cells, is released directly into the circulation when blood-glucose levels increase. It then functions as a chemical messenger binding to receptors which are themselves proteins on many human cells, instructing them to take up glucose, thereby normalising glucose concentration in the blood.

To function correctly, proteins must have the correct sequences of amino acids, which ultimately relies on the genetic code remaining constant. However, there are so many nucleotide bases in the human genome approximately three billion base pairs that errors invariably occur. Such errors are referred to as mutations and can lead to the production of proteins that may not function correctly.

Abnormal proteins are associated with a variety of diseases, including some forms of autoimmune disease and malignancy. Genetic mutations may occur randomly following errors in DNA replication described in part 1 of this series , particularly as the body ages; alternatively, they may be caused by environmental factors that directly damage DNA molecules.

Many genetic mutations occur in sections of DNA that do not code for proteins for example, in the non-coding introns , so they usually have little impact on physiological function. If mutation occurs in the control genes that regulate cell division or in the genes that code for DNA repair enzymes, the result can be uncontrolled cell division and skin cancer Hopkins, Although human skin exposed to sunlight produces its own natural UV protection in the form of melanin the dark pigment that tans the skin , depletion of the ozone layer and excessive time in the sun can lead to damaging doses of UV radiation that increase the risk of skin cancer.

Sunscreens offer better UV protection and have been shown to significantly reduce UV-induced skin damage and skin cancers Green and Williams, Mutations such as those caused by UV radiation to DNA in the skin are not generally passed down through generations.

However, when mutations affect the germinal cells of the testes and ovaries, they can be inherited by offspring.