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Heredity Essay, Research Paper

Deoxyribonucleic acid and ribonucleic acid are two chemical substances


in transmitting genetic information from parent to offspring. It was

known early into the

20th century that chromosomes, the genetic material of cells, contained

DNA. In 1944,

Oswald T. Avery, Colin M. MacLeod, and Maclyn McCarty concluded that DNA

was the

basic genetic component of chromosomes. Later, RNA would be proven to


protein synthesis. (Miller, 139)

DNA is the genetic material found in most viruses and in all

cellular organisms.

Some viruses do not have DNA, but contain RNA instead. Depending on the


most DNA is found within a single chromosome like bacteria, or in several


like most other living things. (Heath, 110) DNA can also be found

outside of

chromosomes. It can be found in cell organelles such as plasmids in

bacteria, also in

chloroplasts in plants, and mitochondria in plants and animals.

All DNA molecules contain a set of linked units called

nucleotides. Each

nucleotide is composed of three things. The first is a sugar called

deoxyribose. Attached

to one end of the sugar is a phosphate group, and at the other is one of


nitrogenous bases. DNA contains four nitrogenous bases. The first two,

adenine and

guanine, are double-ringed purine compounds. The others, cytosine and

thymine, are

single-ringed pyrimidine compounds. (Miller, 141) Four types of DNA

nucleotides can

be formed, depending on which nitrogenous base is involved.

The phosphate group of each nucleotide bonds with a carbon from


deoxyribose. This forms what is called a polynucleotide chain. James D.

Watson and

Francis Crick proved that most DNA consists of two polynucleotide chains

that are

twisted together into a coil, forming a double helix. Watson and Crick

also discovered

that in a double helix, the pairing between bases of the two chains is

highly specific.

Adenine is always linked to thymine by two hydrogen bonds, and guanine is

always linked

to cytosine by three hydrogen bonds. This is known as base pairing.

(Miller, 143)

The DNA of an organism provides two main functions. The first

function is to

provide for protein synthesis, allowing growth and development of the

organism. The

second function is to give all of it s descendants it s own

protein-synthesizing information

by replicating itself and providing each offspring with a copy. The

information within the

bases of DNA is called the genetic code. This specifies the sequence of

amino acids in a

protein. (Grolier Encyclopedia, 1992) DNA does not act directly in the

process of

protein synthesis because it does not leave the nucleus, so a special

ribonucleic acid is used

as a messenger (mRNA). The mRNA carries the genetic information from the

DNA in the

nucleus out to the ribosomes in the cytoplasm during transcription.

(Miller, 76)

This leads to the topic of replication. When DNA replicates, the

two strands of

the double helix separate from one another. While the strands separate,

each nitrogenous

base on each strand attracts it s own complement, which as mentioned

earlier, attaches

with hydrogen bonds. As the bases are bonded an enzyme called DNA


combines the phosphate of one nucleotide to the deoxyribose of the

opposite nucleotide.

This forms a new polynucleotide chain. The new DNA strand stays attached

to the old

one through the hydrogen bonds, and together they form a new DNA double


molecule. (Heath, 119) (Miller, 144-145)

As mentioned before, DNA molecules are involved in a process

called protein

synthesis. Without RNA, this process could not be completed. RNA is the


material of some viruses. RNA molecules are like DNA. They have a long

chain of

macromolecules made up of nucleotides. Each RNA nucleotide is also made

up of three

basic parts. There is a sugar called ribose, and at one end of the sugar

is the phosphate

group, and at the other end is one of several nitrogenous bases. There

are four main

nitrogenous bases found in RNA. There are the double-ringed purine

compounds adenine

and guanine, and there is the single-ringed pyrimidine compounds of uracil

and cytosine.

(Miller, 146)

RNA replication is much like that of DNA s. In RNA synthesis, the


being copied is one of the two strands of a DNA molecule. So, the

molecule being

created is different from the molecule being copied. This is known as


Transcription can be described as a process where information is

transferred from DNA to

RNA. All of this must happen so that messenger RNA can be created, the

actual DNA

cannot leave the nucleus. (Grolier Encyclopedia, 1992)

For transcription to take place, the RNA polymerase enzyme is

needed first

separate the two strands of the double helix, and then create an mRNA

strand, the

messenger. The newly formed mRNA will be a duplicate of one of the

original two

strands. This is assured through base pairing. (Miller, 147)

When information is given from DNA to RNA, it comes coded. The

origin of the

code is directly related to the way the four nitrogenous bases are

arranged in the DNA. It

is important that DNA and RNA control protein synthesis. Proteins control

both the cell s

movement and it s structure. Proteins also direct production of lipids,

carbohydrates, and

nucleotides. DNA and RNA do not actually produce these proteins, but tell

the cell what

to make. (Heath, 111-113)

For a cell to build a protein according to the DNA s request, a

mRNA must first

reach a ribosome. After this has occurred, translation can begin to take

place. Chains of

amino acids are constructed according to the information which has been

carried by the

mRNA. The ribosomes are able to translate the mRNA s information into a


protein. (Heath, 116) This process is also dependent on another type of

RNA called

transfer RNA (tRNA). Cytoplasm contains all amino acids needed for


construction. The tRNA must bring the correct amino acids to the mRNA so

they can be

aligned in the right order by the ribosomes. (Heath, 116) For protein

synthesis to begin,

the two parts of a ribosome must secure itself to a mRNA molecule.

(Miller, 151)

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