The Process of Translation

Key Words

-Anticodon -Codon -Genetic Code -Translation

Objectives

-Explain how the genetic code is read -Summarize the process of translation

How many possible three-letter words can be formed from four bases?

64

What is an anticodon?

A three-bae sequence that is complementary to a codon

What is a codon?

A three-letter sequence in mRNA that specifies a particular amino acid.

The Roles of tRNA and rRNA in Translation

All three major forms of RNA--mRNA, tRNA, and rRNA--come together in the ribosome during translation. The mRNA molecule, of course, carries the coded message that directs the process. The tRNA molecules deliver exactly the right amino acid called for by each codon on the mRNA. The tRNA molecules are, in effect, adaptors that enable the ribosome to "read" the mRNA's message accurately and to get the translation just right. Ribosomes themselves are composed of roughly 80 proteins and three or four different rRNA molecules. These rRNA molecules help hold ribosomal proteins in place and help locate the beginning of the mRNA message. They may even carry out the chemical reaction that joins amino acids together.

Polypeptide Chain and Codons

Amino acids are assembled into a polypeptide chin at the ribosome. This polypeptide chain then assumes a three-dimensional structure based on its amino aicd sequence, and the structure determines tha protein's function. The order and number oa mino acids in this protein are determined by the three nucleotide "triplet" letter sequences known as codons found on the mRNA molecule.

Nucleotides in Codons

BBecause there are three nucleotides in each codon and four different nucleotides, there are 4 to the third power (4 cubed), or 64, possible triplet combinations. Since there are only 20 mino acids for which to code, many amino acids have more than one codon. In addition, there are some codons that serve to begin and end translation. tRNA molecules bring the correct amino acid to the ribosome that correspond to the triplet codon on mRNA.

Molecular Biology

Before long, with the genetic code in hand, a new scientific field called molecular biology had been established. Molecular biology seeks to explain living organisms by studying them at the molecular level, using molecules like DNA and RNA. One of the earliest findings came to be known, almost jokingly, as the field's "central dogma."

What type of chemical reaction occurs to form a peptide bond?

Dehydration synthesis

Codon

Every set of three consecutive bases is called a codon. Every codon codes for a specific amino acid to add to the polypeptide chain.

Methionine Codon

In the case of the tRNA molecule for methionine, the anticodon is UAC, which pairs with the methionine, AUG. The ribosome has a second binding site for a tRNA molecule for the next codon. If that next codon is UUC, a tRNA molecule with an AAG anticodon fits against the mRNA molecule held in the ribosome. That second tRNA molecule brings the amino acid phenylalanine into the ribosome.

Messenger RNA

Messenger RNA is transcribed in the nucleus and then enters the cytoplasm

Near-Universal Nature of the Genetic Code

One of the most interesting discoveries of molecular biology is the near-universal nature of the genetic code. Although some organisms show slight variations in the amino acids assigned to particular codons, the code is always read three bases at a time and in the same direction. Depite their enormous diversity in form and function, living organisms display remarkable unity at life' most basic level, the molecular biology of the gene.

Ribosomes

Ribosomes use the sequence of codons in mRNA to assemble amino acids into polypeptide chains.

Genetic Code

Set of instructions for synthesis of twenty amino acids.

What happens following transcription?

Specific transfer RNAs bring specific amino acids to the ribosome where the polypeptide chain is assembled.

What are terminators?

Terminators are codes for the end of a gene.

Central Dogma

The central dogma of molecular biology is that information is transferred from DNA to RNA to protein. In reality, there are many exceptions to this "dogma," including viruses that transfer information in the opposite direction, from RNA to DNA. Nonetheless, it serves as a useful generalization that helps to explain how genes work.

Genetic Code Order

The genetic code is read three "letter" at a time, so that each "word" is three bases long and corresponds to a single amino acid.

Completing the Polypeptide

The process continues until the ribosome reaches one of the three stop codons. Once the polypeptide is complete, it and the mRNA are released from the ribosome.

The Translation Process

The process that converst the sequence of bbases in an mRNA into a sequence of amino acids in a polypeptide chain called translation. mRNA and tRNA are complementary to one another. A tRNA molecule carries a set of three unpaired bases called an anticodon that pair specifically with the matching bases of the mRNA codon. The complementary pair of the codon and anticodon creates an amino acid.

The Polypeptide "Assembly Line"

The ribosome joins the two amino acids--methionine and phenylalanine--and breaks the bond between methionine and its tRNA. The tRNA floats away from the ribosome, allowing the ribosome to bind another tRNA. The ribosome moves along the mRNA, from right to left, binding new tRNA molecules and amino acids.

Which molecule bears an anticodon?

The tRNA molecule

Gene Expression

The way in which DNA, RNA, and proteins are involved in putting genetic information into action in living cells.

Why are there multiple genetic code words for each amino acid?

There are 64 possible code words and only 20 amino acids.

What is the first step?

Transcription of RNA

Transfer RNA

Translation begins at AUG, the start codon. Each transfer RNA has an anticodon whose bases are complementary to the bases of a codon on the mRNA strand. The ribosome positions the start codon to attract its anticodon, which is part of the tRNA that binds methionine. The ribosome also binds the next codon and its anticodon.

Steps in Translation

Translation begins when a ribosome attaches to an mRNA molecule in the cytoplasm. As each codon passes through the ribosome, tRNAs bring the proper amino acids into the ribosome. One at a time, the ribosome then attaches these amino acids to the growing chain.

What does a ribosome consist of?

rRNA and protein