Biology Key Skills

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Biology Key Skills

Poodles make more poodles. Sheep make more sheep. Replication is a

basic fact of life. All living things make other living things that

are to one degree or another duplicates of themselves. What is the

mechanism behind all this? The answer lies in a molecule called DNA.

In 1869, Friedrich Miescher extracted a substance, which he called

nuclein from the nuclei of white blood cells. Nuclein later became

known as nucleic acid. Living cells contain two kinds of nucleic

acids-ribonucleic acid (RNA) which contains the sugar, ribose and

deoxyribonucleic (DNA) which contains the sugar, deoxyribose. Nucleic

acids are found in all living things, from the simplest protozoan to

the most complex forms of animal and plant life

.

Two young scientists-James Watson and Francis Crick-finally pieced

together the precise structure of DNA. The model proposed by Watson

and Crick for the structure of DNA is shaped like a twisted ladder.

This type of figure is known as a double helix. The sides of the

twisted ladder are made up of alternating units of deoxyribose and

phosphate. The rungs of the ladder are composed of paired nitrogen

bases. Adenine always pairs with thymine, and guanine always pairs

cytosine. The bases are held together by hydrogen bonds.

Watson and Crick's model also suggested a way in which DNA could make

copies of itself. First, the ladder untwists. Then the bases break

apart. Since and adenine nucleotide can only bond with thymine, and

guanine can only bond with cytosine, new units are assembled in

precisely the same order as old. When the splitting and pairing

processes are competed, two identical DNA molecules stand in the place

of one. The process by which DNA makes copies of itself is called

replication.

DNA replication

DNA replication begins with a partial unwinding of the double helix at

an area known as the replication fork. This unwinding is accomplished

by an enzyme known as DNA helicase. This unwound section appears under

electron microscopes as a "bubble" and is thus known as a replication

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