Nucleic Acids And Protein Synthesis; Outline Essay, Research Paper

Nucleic Acids and Protein Synthesis

7-1 DNA

Cells “know” how to how to produce ATP, how to build cilia and centrioles, how

to produce membranes and enzymes.

A program, or code, in living cells must be able to duplicate itself quickly and

accurately and must also have a means of being decoded and put into effect.

The Genetic Code

Biologists call the program of the cell the genetic code. The work genetic refers

to anything that relates to heredity. The genetic code is the way in which cells store

program that they seem to pass from one generation to another.

Transformation

When a strain of information is passed to another, it is called transformation.

The Transforming Factor

DNA is the nucleic acid that stores and transmits the genetic information from

one generation of an organism to the next. DNA carries the genetic code.

Bacteriophages

Some virusus are known as bacteriophages which mean bacteria eaters.

Bacteriophages are composed of a DNA core and a protein coat. They attach themselves

to the surface of a bacterium and then inject a material into the bacterium. Once inside

the bacterium, the injected material begins to reproduce, making may copies of the

bacteriophage. Soon the bacterium bursts, and several hundred bacteriophages are

released to infect other cells. Because the material injected into the bacterium produces

new bacteriophages, it must contain the genetic code.

The Structure of DNA

DNA is a polymer formed from units called nucleotides. Each nucleotide is a

molecule make up of three basic parts: a 5-carbon sugar called deoxyribose, a phosphate

group, and a nitrogenous, or nitrogen-containing, base.

DNA contains four nitrogenous bases. Two of the nitrogenous bases, adenine and

guanine, belonging to a group of compounds known as purines. The remaining two,

cytosine and thymine, are known as pyrimidines.

Individual nucleotides are joined together to form a long chain.

X-Ray Evidence

Fibers that make up DNA are twisted, like the strands of a rope. Large groups of

molecules in the fiber are spaced out at regular intervals along the length of the

The Replcation of DNA

Because each of the two strands of the DNA double hehx has all the information,

by the mechanism of base pairing, to reconstruct the other half, the strands are said to

be complementary. There are four nitrogenous bases in DNA. Each strand of the double

helix of DNA serves as a template, or pattern, against which a new strand is made.

Before a cell divides, it must duplicate its DNA. This ensures that each resulting cell will

have a complete set of DNA molecules. This copying process is known as replication.

DNA replication, or DNA synthesis, is carried out by a series of enzymes. These

enzymes separate, or “unzip,” the two strands of the double helix, insert the

appropriate bases, and produce covalent sugar-phosphate links to extend the growing

DNA chains. The enzymes even “proofread” the bases that have been inserted to ensure

that they are paired correctly.

The unzipping occurs when the hydrogen bonds between the base pairs are

broken and the two strands of the molecule unwind. Each of the separated strands

serves as a template for the attachment of complementary bases. Two DNA molecules

identical to each other and to the original molecule are made.

7-2 RNA

The double helix structure explains how DNA can be replicated, or copied.

However, it does not explain how information is contained in the molecule or how that

information is put to good use. DNA contains a set of instructions that are coded in the

sequence, or order, of nucleotides. The first step in decoding that message is to copy

part of the sequence into RNA (ribonucleic acid). RNA is the nucleic acid that acts

as a messenger between DNA and the ribosomes and carries out the process by

which proteins are made from amino acids.

The Structure of RNA

RNA, like DNA, consists of a long chain of macromolecules made up of

nucleotides. Each nucleotide is made up of a 5-carbon sugar, a phosphate group, and

a nitrogenous base. The alternating sugars and phosphate groups form the backbone

of the RNA chain.

There are three major differences between RNA and DNA. The sugar in RNA is

ribose, whereas the sugar in DNA is deoxyribose. Another difierence between RNA

and DNA is that RNA consists of a single strand of nucleotides, although it can form

double-stranded sections by folding back on itself in loops. DNA, as you will recall, is

double-stranded. Lastly, the nitrogenous bases found in DNA are adenine, thymine,

cytosine, and guanine. RNA also contains adenine, cytosine, and guanine, but uracil

(toom-uh-sihl) is present instead of thymine. Like DNA, RNA follows the base

pairing rules. Adenine bonds to uracil, and cytosine bonds to guanine. Although a cell

contains many different forms of RNA, there are three main types that are involved in

expressing the genetic code.

In its own way, an RNA molecule is a disposable copy of a segment of DNA.

The ability to copy a DNA base sequence into RNA makes it possible for a specific

place on the DNA molecule to produce hundreds or even thousands of RNA

moiecules with the same information as DNA.

Transcription: RNA Synthesis

As you will recall, DNA replication is also known as DNA synthesis because the

molecule being synthesized turns out to be the same as the molecule being copied. In

RNA synthesis, the molecule being copied is just one of the two strands of a DNA

molecule. Thus the molecule being synthesized is different from the molecule being

copied. The term transcription is used to describe this process. Transcription is the

process by which a molecule of DNA is copied into a complementary strand of

RNA.

DNA is found in the nucleus and ribosomes are located in the cytoplasm.

Because DNA does not leave the nucleus, a messenger, or carrier, must bring the

genetic information from the DNA in the nucleus out to the ribosomes in the

cytoplasm. The molecule that performs this function is messenger RNA (mRNA), one

of the three main types of RNA.

During transcription, RNA polymerase attaches to special places on the DNA

molecule, separates the two strands of the double helix, and synthesizes a messenger

RNA strand. The messenger RNA strand is complementary to one of the DNA

strands. The base-pairing: mechanism ensures that the messenger RNA will be a

complementary copy of the DNA strand that serves as its template.

7-3 Protein Synthesis

The information that DNA transfers to messenger RNA is in the

form of a code. This code is determined by the way in which the four nitrogenous

bases are arranged in DNA.

The nitrogenous bases in DNA contain information that directs protein synthesis.

Because most enzymes are proteins, proteins control biochemical pathways within the

cell. Not only do proteins direct the synthesis of lipids, carbohydrates, and nucleotides,

but they are also responsible for cell structure and cell movement. Together, DNA

and its assistant, RNA, are directly responsible for making proteins.

The Nature of the Genetic Code

DNA and RNA each contain different nitrogenous bases (DNA contains A, T,

C, G; RNA contains A, U, C, G); hence, diffexent nueleotides. In order to code for

the 20 different amino acids, more than one nucleotide must make up the code

word for each amino acid. The code word of the DNA nucleotides are copied onto a

strand of messenger RNA. Each combination of three nucieo- tides on the

messenger RNA is called a codon, or three- letter code word. Each codon specifies a

particular amino acid that is to be placed in the polypeptide chain. There is more

than onecodon for each amino acid.

Translation

The decoding of a messenger RNA message into a polypeptide chain (protein)

is known as translation. There is an elaborate mechanism that involves the two other

main types of RNA — transfer RNA (tRNA) and ribosomal RNA (rRNA) — and the

cytoplasmic organelle known as the ribosome.

Transfer RNA carries amino acids to the ribosomes, where the amino acids

are joined together to form polypeptides. Transfer RNA is a single strand of RNA that

loops back on it self. Ribosomal RNA makes up the major part of the ribosomes.

THE ROLE OF TRANSFER RNA

You will notice that there are three exposed bases on each transfer RNA

molecule. These nucleotides will base pair with a codon on messenger RNA. Because

the three nucleotides on transfer RNA are complementary to the three nucleotides

on messenger RNA, the three transfer RNA nucleotides are called the anticodon.

Attached to each transfer RNA molecule is the amino acid specihed by the

codon to which it base pairs. By matching the transfer RNA anticodon to the

messenger RNA codon, the correct amino acid is put into place. Each transfer RNA acts

like a tiny beacon for its specific amino acid.

THE ROLE OF THE RIBOSOME

Messenger RNA molecules do not automatically line up transfer RNA molecules

and link their amino acids together. Instead, this process of protein synthesis takes

place in organelles known as ribosomes. Ribosomes are made up of two subunits, a

large one and a smaller one. Each subunit consists of ribosomal RNA and proteins

(about 70 different types).

The first part of protein synthesis occurs when the two subunits of the ribosome

bind to a molecule of messenger RNA. Then the initiator codon: AUG binds to the first

anticodon of transfer RNA, signaiing the beginning of a polypeptide chain.

Soon the anticodon of another transfer RNA binds to the next messenger RNA

codon. T:his second transfer RNA carries the second amino acid that will be placed

into the chain of the polypeptide.

The polypeptide chain continues to grow until the ribosome reaches a stop

codon on the messenger RNA. A stop codon is a codon for which no transfer RNA

molecules exist. When the stop codon reaches the ribosome, the ribosome releases

the newly formed polypeptide and messenger RNA, completing the process of

translation.

As you can now see, the ribosome, in its own way, is at the center of the whole

business of making the genetic code work. In the nucleus, DNA directs the formation

of three different kinds of RNA: transfer RNA, ribosomal RNA, and messenger

RNA. They all leave the nucleus and then seem to go their separate ways.