RNA and Protein Synthesis12-3

Protein Synthesis

• Ribonucleic acid, RNA, plays a role in protein synthesis.

• Central ConceptDNA RNA protein

A. RNA Structure & Function

• Differences in RNA than in DNA– Sugar ribose instead of deoxyribose– Nitrogenous base uracil instead thymine– Single stranded instead of double stranded– Shorter than DNA

Types of RNA

• Messenger RNA (mRNA) – carries instructions from a gene to make a protein

• Ribosomal RNA (rRNA) – part of the structure of ribosomes

• Transfer RNA (tRNA) – transfer amino acids to the ribosomes to make a protein

B. Transcription

• The process by which the genetic instructions in a specific gene are transcribed into an RNA molecule.

• Takes place in the nucleus of eukaryotic cells and in the DNA-containing region in the cytoplasm of prokaryotic cells.

• Figure 12-14 of Prentice Hall Biology

RNA Editing

• After the RNA is produced, it must be edited before it can be used.

• Introns are removed and exons are spliced together before the RNA leaves the nucleus.

C. The Genetic Code

• The term for the rules that relate how a sequence of nitrogenous bases in nucleotides corresponds to a particular amino acid.

• Three adjacent nucleotides (“letters”) in mRNA specify an amino acid (“word”)

• Three adjacent nucleotides are called a codon and encodes for an amino acid or signifies a start or stop signal.

• No codon encodes more than one amino acid. There are 64 possible codons and amino acids are can be specified by more than one codon.

• A start codon is a specific sequence of nucleotides in mRNA that indicates where translation should begin. AUG.

• Three stop codons are specific sequences of nucleotides in mRNA that indicates where translation should end.

• Figure 12-17

D. Translation

• Decoding of the genetic instructions to form a polypeptide

• Takes place on the surface of the ribosome

• Protein structure– Made of one or more polypeptides– Polypeptides are chains of amino acids linked by peptide bonds.– Only 20 different amino acids– The amino acids sequence determines how the polypeptides will twist

and fold into the protein. The shape of the protein is critical to its function.

Translation

• Figure 12-18• Beginning at the start codon, tRNA carrying an

amino acid pairs its anticodons pair up with the complementary codon on the mRNA.

• This continues as an assembly line linking the amino acids and breaking bonds between the tRNA and the amino acids.

• The linked amino acids form a polypeptide until a stop codon is reached.

E. The Human Genome

• A genome is the complete genetic content.

• Biologists have now decoded the order of the 3.2 billion base pairs in the 23 human chromosomes.

• Bioinformatics compares different DNA sequences to try to determine what information the DNA codon encodes.

DNA

Gene – Short Segment of double helix

Nitrogen Base – the “rung on the ladder”

Deoxyribose – “sugar side of the ladder”

Nucleotide – composed of 3 parts floating free in the cytoplasm

DNA – the double helix