TOPIC 3.5: TRANSCRIPTION AND TRANSLATION (PROTEIN SYNTHESIS)

ASSESSMENT STATEMENTS

3.5.1 Compare the structure of RNA and DNA 3.5.2 Outline DNA Transcription in terms of

the formation of an RNA strand complementary to the DNA strand by RNA polymerase

3.5.3 Describe the genetic code in terms of codons composed of triplets of bases

3.5.4 Explain the process of translation, leading to polypeptide formation

3.5.5 Discuss the relationship between one gene and one polypeptide

PROTEIN SYNTHESIS INTRODUCTION

The control that DNA has over a cell is by a process called Protein Synthesis DNA controls the proteins produced in a cell

Some of these proteins are Enzymes The Production or Lack of Production of a

particular enzyme can have a dramatic effect on the overall biochemistry of the cell DNA indirectly controls the biochemistry of

carbohydrates, lipids, and nucleic acids by the production of enzymes

PROTEIN SYNTHESIS INTRODUCTION

Protein synthesis has two major sets of reactions Transcription (DNA to RNA) Translation (RNA to Polypeptides (protein))

Both Processes require RNA

COMPARING DNA AND RNA

DNA RNA

Contains a 5 carbon sugar Contains a 5 carbon sugar

5 carbon sugar is deoxyribose 5 carbon sugar is ribose

Each nucleotide has one of four nitrogenous bases

Each nucleotide has one of four nitrogenous bases

The nitrogenous bases are cytosine, guanine, adenine, and thymine

The nitrogenous bases are cytosine, guanine, adenine, and uracil

Double-stranded molecule Single-stranded molecule

TRANSCRIPTION PRODUCES RNA MOLECULES

Genes-are the sections of DNA that codes for polypeptides Each gene is a specific sequence of nitrogenous

bases found in a DNA molecule DNA is found inside nucleus but proteins are

synthesized outside the nucleus in the cytoplasm Messenger RNA (mRNA) is an intermediary

molecules which caries the message of DNA to the cytoplasm where he enzymes, ribosomes, and amino acids are found

In the nucleoplasm contains free RNA nucleotides along with free DNA nucleotides

TRANSCRIPTION PRODUCES RNA MOLECULES

Transcription Process: Begins when an are of DNA

of one gene becomes unzipped (similar to the unzipping of DNA replication) Only one strand of the two

DNA strands will be used as a template to create the mRNA molecule. RNA polymerase—enzyme

used as the catalyst for this process

RNA polymerase moves along the strand of DNA acting as the template, nucleotides float into place by complementary base pairing

TRANSCRIPTION PRODUCES RNA MOLECULES

The complementary base pairs are the same as in double-stranded DNA, which an exception that adenine on DNA is now paired with uracil on the newly forming mRNA.

TRANSCRIPTION PRODUCES RNA MOLECULES Facts about Transcription:

Only one of the two strands of DNA is ‘copied’, the other strand is not used

mRNA is always single-strand and shorter than the DNA that it is copied form as it is a complementary copy of only one gene

The presence of thymine in a molecule identifies it as DNA

The presence of uracil in a molecule identifies it as RNA

THE GENETIC CODE IS WRITTEN IN TRIPLETS The mRNA molecule produced by

transcription represents a complementary copy of one gene of DNA This sequence of nucleotides making up the

length of the mRNA is typically enough information to make one polypeptide

The message written into the mRNA molecule is the message that determines the order of the amino acids

THE GENETIC CODE IS WRITTEN IN TRIPLETS

The Genetic Code is written in a language of three bases Three bases is enough to code for 1 of the 20

amino acids Triplet-any set of three bases that determine the

identity of one amino acids When a triplet is found on the mRNA molecule, it

is called a codon or codon triplet

TRANSLATION RESULTS IN THE PRODUCTION OF A POLYPEPTIDE

There are three different types of RNA molecules. They are all single stranded and each is

transcribed from a gene of DNA mRNA: each mRNA is a complementary copy

of DNA gene and is enough genetic information to code for a single polypeptide

rRNA: ribosomal RNA, each ribosome is compsed of rRNA and ribosomal protein

tRNA: transfer RNA, each type of tRNA transfers 1 of the 20 amino acids to the ribosome for polypeptide formation

TYPICAL TRNA

The three bases in the middle loop are called the anticodon bases and they determine which of the 20 amino acids is attached to the tRNA

TRANSLATION RESULTS IN THE PRODUCTION OF A POLYPEPTIDE

Once an mRNA molecule has been transcribed, the mRNA detaches from the single-strand DNA template and floats free in the nucleoplasm. At some point, the mRNA will float through one

of the many holes in the nuclear membrane (nuclear pores) and will then be in the cytoplasm

TRANSLATION PROCESS

The mRNA will locate a ribosome and align with it so that the first two codon triplets are within the boundaries of the ribosome

The next step is the introduction of the tRNA. The tRNA must be complementary to the first codon triplet of the mRNA molecule Than, the first amino acid is brought into the

translation process While the first tRNA ‘sits’ in the ribosome holding

the first amino acid The second tRNA floats in and brings a second

(specific) amino acid The second tRNA matches its three anticodon bases

with the second codon triplet of the mRNA

TRANSLATION PROCESS

Two specific amino acids are now being held side by side An enzyme now

catalyses a condensation reaction between the two amino acids and the resulting covalent bond between them is called a peptide bond

TRANSLATION PROCESS

The next step in the translation process involves the breaking of the bond between the first tRNA molecule and that amino acid that it transferred in The first tRNA

floats away into the cytoplasm and invariably reloads with another amino acid of the same type

TRANSLATION PROCESS The ribosome that has

only one tRNA in it now moves one codon triplet down the mRNA molecule This creates room for a

third tRNA to float in bringing with it a third specific amino acid

This process continues until the ribosome gets to the last codon triplet—the final codon triplet will not code for an amino acid, but signals a stop codon

TRANSLATION PROCESS

The entire polypeptide breaks away from the final tRNA molecule, and becomes a free floating polypeptide in the cytoplasm of the cell

THE ONE GENE/ONE POLYPEPTIDE HYPOTHESIS

Beadle and Tatum set out to provide experimental proof of the connection between genes and enzymes.

They hypothesized that if there really was a one-to-one relationship between genes and specific enzymes, it should be possible to create genetic mutants that are unable to carry out specific enzymatic reactions.

THE ONE GENE/ONE POLYPEPTIDE HYPOTHESIS

In the last few years, researchers have discovered that at least some genes are not quite that straight forward. For example, one gene may lead to a single

mRNA molecule, but the mRNA molecule may be modified in many different ways Each modification may result in the production of a

different polypeptide during the translation portion of protein synthesis