Section Objectives Relate the concept of the gene to the sequence of nucleotides in DNA. Sequence the steps involved in protein synthesis.

Genes and Proteins The sequence of nucleotides in DNA contain information. Proteins fold into complex, three- dimensional shapes to become key cell structures and regulators of cell functions. We know proteins are polymers of amino acids - nucleotides contains information for assembling the string of amino acids that make up a single protein.

RNA RNA structure differs from DNA structure in three ways. First, RNA is single—whereas DNA is double stranded. Ribose The sugar in RNA is ribose; DNA’s sugar is deoxyribose. rather than thymine, RNA contains a similar base called uracil (U). Uracil Hydrogen bonds Adenine

RNA three types of RNA Messenger RNA (mRNA), brings instructions from DNA in the nucleus to the cytoplasm then moves to a ribosome The ribosome, made of ribosomal RNA (rRNA), binds to the mRNA and uses the instructions to assemble the amino acids in the correct order. Transfer RNA (tRNA) delivers amino acids to the ribosome to be assembled into a protein.

RNA Click image to view movie

Transcription In the nucleus, enzymes make an RNA copy of a portion of a DNA strand - transcription. Click image to view movie

Transcription A DNA strand RNA strand RNA strand C B DNA strand

RNA Processing Not all the nucleotides in the DNA of eukaryotic cells carry instructions—or code—for making proteins. introns - a long noncoding nucleotide sequences, that are scattered among the coding sequences. Exons - regions that contain coded information When mRNA is transcribed from DNA, both introns and exons are copied.

RNA Processing The introns must be removed from the mRNA before it can function to make a protein. Enzymes in the nucleus cut out the intron segments and paste the mRNA back together. The mRNA then leaves the nucleus and travels to the ribosome.

The Genetic Code The transcribed messenger RNA acts as a genetic message we could say that the language of proteins uses an alphabet of amino acids. codon - a group of three nitrogenous bases in mRNA that codes for one amino acid. Anticodon – matching tRNA that carries the amino acid

The Genetic Code Sixty-four combinations are possible when a sequence of three bases is used; thus, 64 different mRNA codons are in the genetic code. The Messenger RNA Genetic Code First Letter Third Letter Second Letter U C A G U Phenylalanine (UUU) Serine (UCU) Tyrosine (UAU) Cysteine (UGU) U Phenylalanine (UUC) Serine (UCC) Tyrosine (UAC) Cysteine (UGC) C Leucine (UUA) Serine (UCA) Stop (UAA) Stop (UGA) A Leucine (UUG) Serine (UCG) Stop (UAG) Tryptophan (UGG) G C Leucine (CUU) Proline (CCU) Histadine (CAU) Arginine (CGU) U Leucine (CUC) Proline (CCC) Histadine (CAC) Arginine (CGC) C Leucine (CUA) Proline (CCA) Glutamine (CAA) Arginine (CGA) A Leucine (CUG) Proline (CCG) Glutamine (CAG) Arginine (CGG) G A Isoleucine (AUU) Threonine (ACU) Asparagine (AAU) Serine (AGU) U Isoleucine (AUC) Threonine (ACC) Asparagine (AAC) Serine (AGC) C Isoleucine (AUA) Threonine (ACA) Lysine (AAA) Arginine (AGA) A Methionine;Start (AUG) Threonine (ACG) Lysine (AAG) Arginine (AGG) G G Valine (GUU) Alanine (GCU) Aspartate (GAU) Glycine (GGU) U Valine (GUC) Alanine (GCC) Aspartate (GAC) Glycine (GGC) Glycine (GGC) C Valine (GUA) Alanine (GCA) Glutamate (GAA) Glycine (GGA) A Valine (GUG) Alanine (GCG) Glutamate (GAG) Glycine (GGG) G

Translation: From mRNA to Protein The process of converting the information in a sequence of nitrogenous bases in mRNA into a sequence of amino acids in protein is known as translation. Translation takes place at the ribosomes in the cytoplasm.

Chain of RNA nucleotides The role of transfer RNA Amino acid Each tRNA molecule attaches to only one type of amino acid. Chain of RNA nucleotides Transfer RNA molecule Anticondon

The role of transfer RNA Ribosome mRNA codon

The role of transfer RNA Usually, the first codon on mRNA is AUG, which codes for the amino acid methionine. AUG signals the start of protein synthesis.

The role of transfer RNA Methionine tRNA anticodon

The role of transfer RNA A new tRNA molecule carrying an amino acid pairs with the second mRNA codon. Alanine

The role of transfer RNA The amino acids are joined when a peptide bond is formed between them. Methionine Alanine Peptide bond

The role of transfer RNA A chain of amino acids is formed until the stop codon is reached on the mRNA strand. Stop codon

Question 1 What are the three chemical differences between RNA and DNA? Answer RNA consists of a single strand of nucleotides whereas DNA is a double strand. RNA contains ribose as its sugar and DNA contains deoxyribose as its sugar. Uracil in RNA replaces thymine in DNA as the nitrogenous

Question 2 What is the role of rRNA in protein synthesis? Answer Ribosomal RNA binds to messenger RNA and assembles the amino acids in the order needed for the protein to be synthesized.

Question 3 Which regions of the mRNA travel to the ribosome; introns, exons, or both? Answer Only exons, which contain coding information, travel to the ribosome. Introns, noncoding nucleotide sequences, do not travel to the ribosome.

Question 4 What is an anticodon, and what does it represent? Answer An anticodon is a sequence of three nucleotides on the tRNA molecule that binds to a codon of the mRNA strand.