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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 RNAContains a 5 carbon sugar Contains a 5 carbon sugar5 carbon sugar is deoxyribose 5 carbon sugar is riboseEach 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 Thus, the first amino acid is brought into the
translation process