Genetics

How an Organism’s How an Organism’s DNA Genotype DNA Genotype Produces Its Produces Its PhenotypePhenotype

• Genotype-Genotype- genetic make-up; the sequence of nucleotides

• Phenotype-Phenotype- organism’s specific traits

• The chain of command is from DNA in the nucleus of the cell to RNA to protein synthesis in the cytoplasm

Two Major StagesTwo Major Stages Transcription-Transcription- the transfer of genetic

information from the DNA into an RNA molecule

Translation-Translation- the transfer of the information in the RNA into a protein

Gene- Enzyme- Protein- Polypeptide

• One Gene- One EnzymeOne Gene- One Enzyme Archibald Garrod-Archibald Garrod- suggested that genes dictate phenotypes

through enzymes, the proteins that catalyze chemical processes (1909).

George Beadle and Edward Tatum-George Beadle and Edward Tatum- formulated the one gene- one enzyme hypothesis (the function of an individual gene is dictated the production of a specific enzyme)

• One Gene- One ProteinOne Gene- One Protein The one gene- one protein hypothesis was modified and

extended beyond enzymes to include all types of proteins.

• One Gene- One PolypeptideOne Gene- One Polypeptide It was discovered that many proteins have one or more different

polypeptide chains, and each polypeptide is specified by its own gene.

To understand how genetic information passes from genotype to phenotype, we need to see how the chemical language of DNA is translated into the different chemical language of polypeptides.

From Nucleotide Sequence to Amino Acid Sequence: An Overview

• DNA & RNA-DNA & RNA- polymers made of monomers in specific sequences that carry information in English. In DNA the monomers are four

types of nucleotides, which differ in nitrogenous bases (A, T, C and G).

In RNA the same is true although it has U instead of T.

Specific sequences of bases, each with a beginning and an end, make up the genes on a DNA strand. A typical gene consists of thousands of nucleotides, and a DNA molecule may contain thousands of genes.

Transcription

• Process when DNA is transcribed and the results to RNA molecule.

Because the nucleic acid language of DNA has simply been rewritten (transcribed) as a sequence of bases of RNA; the language is still that of nucleic acids.

Translation• Conversion of nucleic acid into the

polypeptide language polypeptides are polymers, but the monomers that make

them up- the letters of the polypeptide alphabet- are 20 amino acids common to all organisms

Triplet Code

• Basis of the flow of information from gene to protein.

• The smallest “words” of uniform length that can specify al the amino acids.

Codons

• The genetic instructions for the amino acid sequence of a polypeptide chain are written in RNA and DNA as a series of three-base words called codons.

• 1st codon was deciphered in 1961 by Marshall Nirenberg

The Dictionary of the Genetic Code (Listed by RNA codons)

• Triplet AUG has a dual function Codes for amino acid

methionine (Met) Provide signal for the start

of polypeptide chain.• Three of the other codons do

not designate amino acids. They are the stop codons that instruct the ribosomes to end the polypeptide (UAA,UAG, UGA)

• Tryptophan (Trp)- the only codon for the amino acid (but most amino acids are specified but two or more codons)

The Genetic Code

• The set of rules relating nucleotide sequence to amino acid sequence.

• Notice in the dictionary that there is redundancy but no ambiguity.

For example, although codons UUU and UUC both specify phenylalanine (redundancy), neither of them ever represents any other amino acid (ambiguity).

Transcription: From DNA to RNA

• An RNA molecule is transcribed from a DNA by a process similar to DNA replication.

• RNA polymerase-RNA polymerase- the transcription enzyme that links the RNA nucleotides.

3 Stages3 StagesInitiation of TranscriptionRNA ElongationTermination of Transcription

Initiation of Transcription

• Promoter-Promoter- nucleotide sequence signaling the start of transcription which is located at the start of the gene– a specific place where RNA polymerase

attaches.– dictates which of the two strands is to be

transcribed.

• Initiation-Initiation- 1st phase; the attachment of RNA polymerase to the promoter and the start of RNA synthesis.

RNA Elongation

• Elongation-Elongation- 2nd phase; RNA grows longer

• The RNA strand peels away from its DNA template, allowing two separated DNA strands to come back together in the region already transcribed.

Termination of Transcription

• Termination-Termination- 3rd phase; RNA polymerase reaches a special sequence of bases in the DNA template.

• Terminator-Terminator- term for the special sequence that the RNA reaches.– Signals the end of the gene

• The polymerase molecules detaches from the RNA molecule and the gene.

The transcription of an entire gene occurs in 3 phases.

As RNA nucleotides base-pair one by one with DNA bases on one DNA strand (called the template strand), the RNA polymerase links the RNA nucleotides into an RNA Chain. The green shape in the background is the RNA polymerase.

The Processing of Eukaryotic RNA

• Messenger RNA (mRNA)- Messenger RNA (mRNA)- the result of transcription in prokaryotic cells, wherein RNA transcribed from the gene immediately functions as the messenger molecule.

• RNA Processing-RNA Processing- or modification on RNA in transcribed in eukaryotic cells before translation

• Thru these processes the “final draft” of the eukaryotic mRNA is ready for translation

RNA Processing

• Capping-Capping- addition of extra nucleotides to the ends of the RNA transcript.

CapCap and and Tail-Tail- the additions; protects the RNA fro attack by cellular enzymes and help ribosomes recognize the RNA as mRNA.

• Tailing- Tailing- made by noncoding stretches of nucleotides that interrupts the nucleotides that actually code for amino acids.

introns-introns- internal noncoding regions extrons-extrons- the coding region; the parts of the gene that are

expressed.

• RNA splicing-RNA splicing- The process before the RNA leaves the nucleus, the introns are removed and the exons are joined to produce an mRNA molecule with a continuous coding sequence.