ONE GENE – ONE POLYPEPTIDE HYPOTHESIS

It is known that genes control the synthesis of all proteins. Some proteins are enzymes, some are hormones.

As we know that, proteins are made up of polypetides- long chains of amino acids. Some proteins consist of two or more polypeptides linked and twisted around each other. Ex: hemoglobin

ONE GENE – ONE POLYPEPTIDE HYPOTHESIS

It was found that the synthesis of each polypeptide is controlled by a different gene. Because of this fact, the one gene – one enzyme hypothesis was changed to one gene – one polypeptide hypothesis.

THE DNA CODE

There are 20 amino acids in the proteins of human. Therefore, there must be at least 20 different code to specify these amino acids.

There are 4 types of nucleotides in DNA to form the genetic code;

Adenine (A)

Guanine (G)

Cytosine (C)

Thymine (T)

THE DNA CODE

using a two-letter code 42 = 16 different 2 letter code sequences can be made and this is not enough.

using a three-letter code 43 =64 different 3 – base sequences can be made – more than needed.

Each group of three bases on the mRNA that specifies an amino acid is called a codon.

TYPES OF RNA:

Messenger RNA (mRNA): It is found in cell nucleus. It is synthesized on DNA by RNA polymerase enzyme. This process is called transcription ( = the copying of a genetic message into a molecule of mRNA)

mRNA carries the genetic code (information) that is needed to sytnhesize a polypetide molecule from DNA to ribosome .

Each group of three bases on the mRNA that specifies an amino acid is called a codon.

1 codon

Codes for the addition of 1 amino acid to the polypeptide chain

AUG Beginning codone

UAA UGA UAG Stop codones

Transfer RNA (tRNA): It carries the amino acids from the cytoplasm to the ribosomes.

Each tRNA is specific for an amino acid. So at least 20 different tRNA molecules must be present in the cells.

Anticodon is complement of a mRNA codon that is, tRNA anticodon binds with the specific codon on the mRNA during protein synthesis.

The codon, which anticodon matches, is the one that specifies the amino acid that each tRNA carries.

Thus, tRNA is a device for bringing a certain amino acid to a certain place specified by mRNA.

Ribosomal RNA (rRNA): It is formed in the nucleoli of the cell.

Ribosome is where a polypeptide is assembled during protein synthesis.

A ribosome consist of protein and rRNA. Ribosomal protein is synthesized in the cytoplasm and travels to nucleus. In the nucleoli the protein and the rRNA join to form ribosomes.

60% rRNA + 40% protein

ribosome

Ribosomal RNA

PROTEIN SYNTHESIS Some of the materials that are used in protein synthesis are;

DNA

RNA

Amino acids

Ribosomes

Enzymes

ATP

Some ions

During protein synthesis, a copy of the gene on a DNA that contains the information for building a polypeptide is built into an RNA molecule, which then carries the information to the ribosomes where proteins are synthesized from the amino acid building blocks.

PROTEIN SYNTHESIS

PROTEIN SYNTHESIS

Protein synthesis has two phases:

1.Transcription

in which the information on DNA is copied into mRNA

2. Translation

in which the mRNA molecule is used to arrange amino acids within the polypeptide

Sites of Protein Synthesis

In prokaryotes transcription and translation takes place in the cytoplasm.

While transcription continues, translation begins on the ribosomes.

Sites of Protein Synthesis

In eukaryotes transcription takes place in the nucleus (also in mitochondria and chloroplasts).

mRNA then moves to the cytoplasm through the nuclear pores and translation takes place in the cytoplasm.

The first step in directing protein synthesis is to copy the DNA code for a polypeptide into a molecule of RNA. To copy the code, DNA strands separate for a short time and serve as template for RNA.

The copying of a genetic message into a mRNA molecule is called transcription.

1. Transcription

Some of the Materials Used in Transcription

Material What the material does

Template strand of DNA Provides recipe for building the mRNA

RNA nucleotides Building blocks of RNA

RNA polymerase Enzyme that promotes construction of mRNA

ATP Provides energy for forming bonds between RNA nucleotides

At the attachment site of the enzyme RNA polymerase, the DNA opens up, and as the RNA polymerase moves along the DNA molecule, the two strands of the molecule separate.

Nucleotide building blocks are assembled into RNA in 5' to 3 ' direction as the enzyme reads the template DNA strand in a 3 ' to 5‘ direction.

Note that the RNA strand is complementary –not identical- to the template strand from which it is transcribed; it is sequence is however, identical to that of the inactive (untranscribed) DNA strand, except for the replacement of thymine (T) by uracil (U).

A)Initiation

2. TransLATION

B) Elongation

C) Termination

AUG

UAA

UGA

UAG

Some of the Materials Used in Translation

Material What material does

Ribosome Holds and orients materials, promotes the formation of bonds between amino acids

Amino acids Building blocks of proteins

mRNA Provides instructions for linking a.a into protein

tRNA Brings a.a to the ribosome and lines them up according to the information on mRNA

Enzymes Bind materials to the ribosome; bind a.a to the tRNA; initiate translation

ATP Provides energy for the reactions

A)Initiation:

B) Elongation:

C) Termination:

When the ribosome reaches a termination (stop) codon the polypeptide is cleaved from the last tRNA is released from the P site. (UAA, UGA, UAG)

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Overview of transcription and translation

The information flows from DNA to RNA to protein.

Replication of the DNA occurs only once in each cell cycle, during the S phase prior to mitosis or meiosis.

Transcription and translation, however, occur repeatedly throughout the interphase portion of the cell cycle.