Genes, Proteins and Chromosomes

• Genes contain instructions for making proteins
• Proteins act alone or in complexes to perform many cellular functions
• The flow of information is gene to protein

Learning Outcomes

• Information flows from gene to protein
• Gene expression processes differ between prokaryotes and eukaryotes

Chromosomes

• Chromosomes are strands of DNA containing all the genes that an organism needs to survive and reproduce
• Genes are segments of DNA that specify how to build a protein
• Genes may specify more than one protein in eukaryotes
• Chromosome maps are used to show the locus (location) of genes on a chromosome

Genetic Variation

• Phenotypic variation among organisms occurs due to genotypic variation
• Genotypic variation is due to differences in the sequence of DNA bases
• Differences exist between species and within a species
• Different genes (genomes) lead to different proteins (proteomes)
• Different versions of the same gene are called alleles
• Epigenetics are the differences in gene expression

Flow of Genetic Information

• DNA's information content exists as nucleotide sequences along DNA strands
• Inherited DNA leads to traits by dictating protein synthesis
• Gene expression is the process where DNA directs protein synthesis, including transcription and translation

Transcription and Processing

• Proteins are the links between genotype and phenotype
• Genes dictate phenotypes through enzymes
• Enzymes catalyze specific chemical reactions in the cell
• One gene-one enzyme hypothesis refined the knowledge of metabolic functions
• Not all proteins are enzymes
• Keratin is the structural protein of hair
• Insulin is a hormone
• Keratin and insulin are both proteins and gene products
• Many proteins are composed of several polypeptides, each of which has its own gene
• The idea has been restated as the one gene-one polypeptide hypothesis
• Some genes code for RNA molecules that play roles in cells but are never translated into protein
• Genes code for polypeptide chains or for RNA molecules
• Protein-coding genes exist
• RNA-only gene exists

Linking Genes to Proteins

• Transcription and translation are the two main processes linking gene to protein
• Genes provides instructions for making specific proteins
• The nucleic acid RNA bridges DNA and protein synthesis
• RNA is chemically similar to DNA, except it contains ribose as its sugar
• RNA also substitutes the nitrogenous base uracil for thymine
• RNA molecules consists of a single strand
• Nucleotide monomers in DNA or RNA act like letters of the alphabet
• Sequences of nucleotides in each gene carries the information for the primary structure of proteins
• 20 possible amino acids can exist
• Two major transitions are needed to get from DNA to protein: transcription and translation
• DNA leads to RNA which leads to protein

Transcription and Translation

• Gene expression is how DNA directs synthesis
• Transcription synthesizes RNA under the direction of DNA
• Transcription produces messenger RNA (mRNA)
• Translation is the synthesis of a polypeptide, occurring under the direction of mRNA
• Translation occurs on ribosomes

Protein Synthesis

• During transcription, a DNA strand provides a template for the synthesis of a complementary RNA strand
• Transcription of genes produces messenger RNA (mRNA) molecules
• There is a change of language during translation: Nuc → a.a
• An RNA intermediate provides protection for DNA and its genetic information
• An RNA intermediate allows more copies of a protein to be made simultaneously
• Copies can be made because many RNA transcripts can be made from one gene
• Each gene transcript can be translated repeatedly
• Basic mechanics of transcription and translation are similar in eukaryotes and prokaryotes

Prokaryotes vs Eukaryotes

• Bacteria lack nuclei, their DNA is not segregated from ribosomes and other protein-synthesizing equipment
• Coupling of transcription and translation occurs
• Ribosomes attach to the leading end of an mRNA molecule while transcription is in progress
• Transcription and translation occur together in prokaryotes
• A cell lacking a nucleus, mRNA produced by transcription is immediately translated without additional processing
• Transcription occurs in the nucleus in a eukaryotic cell
• Translation occurs at ribosomes in the cytoplasm in a eukaryotic cell
• A primary transcript (pre-mRNA) is the name for initial RNA transcript of any gene
• RNA processing yields the finished mRNA
• RNA transcripts are modified before becoming true mRNA
• The nucleus provides a compartment for transcription
• Eukaryotic cell original RNA transcript, called pre-mRNA, is processed before leaving the nucleus as mRNA
• Genes program protein synthesis via genetic messages in the form of messenger RNA
• The molecular chain in a cell is DNA → RNA → protein
• 4 Nucleotides lead to 20 amino acids, with a possible 64 combinations

Genetic Code

• It would not be enough combinations to code for all 20 amino acids if the genetic code was not a triplet code
• Triplets of nucleotide bases are the smallest units of uniform length that can code for all the amino acids
• Three consecutive bases specify an amino acid, creating 64 possible code words based on the triplet code
• Genetic information is encoded as a sequence of nonoverlapping base triplets, or codons
• One DNA strand provides a template for ordering the sequence of nucleotides in an RNA transcript during transcription
• The complementary RNA molecule is synthesized according to base-pairing rules
• Uracil is the complementary base to adenine
• mRNA base triplets are called codons
• Codons are written in the 5' → 3' direction
• During translation, the sequence of codons along an mRNA molecule is translated into a sequence of amino acids
• The amino acids create a polypeptide chain
• During translation, the codons are read in the 5' → 3' direction along the mRNA
• The entire code was deciphered by the mid-1960s
• Sixty-one of 64 triplets code for amino acids
• Codon AUG codes for the amino acid methionine and indicates the "start" of translation
• Three codons are "stop" signals marking the termination of translation
• A codon in messenger RNA is either translated into an amino acid or serves as a translational stop signal
• There is redundancy but not ambiguity in the genetic code
• Several codons may specify the same amino acid
• No codon specifies more than one amino acid
• The redundancy in the code is not random
• The genetic code must have evolved very early in the history of life
• Coding is nearly universal and shared by organisms from the simplest bacteria to the most complex plants and animals

Research

• Genes can be transcribed and translated after being transplanted from one species to another
• Bacteria are programmed to synthesize certain human proteins after insertion of the appropriate human genes
• To extract the message from the genetic code requires specifying the correct starting point
• This establishes the reading frame; subsequent codons are read in groups of three nucleotides
• Genetic information is encoded as codons, each of which is translated into a specific amino acid during protein synthesis