Central Dogma of Biology

DNA→RNA→Protein

• DNA info is in the form of specific sequences of bases along the DNA strands.

• The DNA leads to specific traits by dictating the synthesis of proteins.

• Proteins are the links between genotype and phenotype.– For example, Mendel’s dwarf pea plants lack a

functioning copy of the gene that specifies the synthesis of a key protein, gibberellins.

– Gibberellins stimulate the normal elongation of stems.

Introduction

One Gene - One Polypeptide

• George Beadle and Edward Tatum were to establish the link between genes and enzymes in their exploration of the metabolism of a bread mold, Neurospora crassa.• Their results provided strong evidence for the

one gene - one enzyme hypothesis.

• Later research refined the one gene - one enzyme hypothesis.

• First, it became clear that not all proteins are enzymes and yet their synthesis depends on specific genes.– This tweaked the hypothesis to one gene - one

protein.• Later research demonstrated that many proteins

are composed of several polypeptides, each of which has its own gene.

• Therefore, Beadle and Tatum’s idea has been restated as the one gene - one polypeptide hypothesis.

DNA vs RNA

• Components of DNA– Sugar (deoxyribose)– Base (A,G,C,T)– Phosphate group

• Components of RNA– Sugar (ribose)– Base (A,G,C,Uracil)

• RNA does not contain thymine– Phosphate group

DNA vs RNA continued

• Structural Characteristics of DNA– Double stranded– Base-pairing rules apply (A:T & G:C)

• Structural Characteristics of RNA– Primarily single stranded– Limited base-pairing (G:C & A:U)

Types of RNA

• Messenger RNA (mRNA)– Complementary to info in DNA strand– Variable in length– Contains specific structural info for the

sequence of amino acids– Processed before using

Types of RNA continued

• Transfer RNA (tRNA)– Multiple varieties, each specific for a specific

amino acid– Relatively small, with a consistent 3-d shape– Specificity for each amino acid is

accomplished by a triplet base-pairing relationship between codon on mRNA and anti-codon on tRNA

Transcription and translation are the two main processes linking gene to protein: an overview• Genes provide the instructions for making

specific proteins.• The bridge between DNA and protein synthesis

is RNA.• RNA is chemically similar to DNA, except that it

contains ribose as its sugar and substitutes the nitrogenous base uracil for thymine.– An RNA molecules almost always consists of a single

strand.

DNA→RNA→Protein

• DNA is TRANSCRIBED to messenger RNA (mRNA)

• mRNA carries the message to tranfer RNA (tRNA)

• tRNA is TRANSLATED to an amino acid chain, which makes up proteins

• In DNA or RNA, the four nucleotide monomers act like the letters of the alphabet to communicate information.

• The specific sequence of hundreds or thousands of nucleotides in each gene carries the information for the primary structure of a protein (the linear order of the 20 possible amino acids)

• To get from DNA, written in one chemical language, to protein, written in another, requires two major stages, transcription and translation.

• During transcription, a DNA strand provides a template for the synthesis of a complementary RNA strand.– This process is used to synthesize any type of RNA

from a DNA template.– Transcription is from the 3’→5’ strand (template

strand)• Transcription of a gene produces a messenger

RNA (mRNA) molecule.– mRNA carries the message from the nucleus to the

ribosomes• During translation, the information contained in

the order of nucleotides in mRNA is used to determine the amino acid sequence of a polypeptide.– Translation occurs at ribosomes.

• The basic mechanics of transcription and translation are similar in eukaryotes and prokaryotes.

• Because bacteria lack nuclei, transcription and translation are coupled.

• Ribosomes attach to the leading end of a mRNA molecule while transcription is still in progress.

Fig. 17.2a

• In a eukaryotic cell, almost all transcription occurs in the nucleus and translation occurs mainly at ribosomes in the cytoplasm.

• In addition, before the primary transcript can leave the nucleus it is modified in various ways during RNA processing before the finished mRNA is exported to the cytoplasm. – Introns are removed

Fig. 17.2b

RNA Packaging

• Introns are removed (only exons contain genetic info)

• Addition of a 5’ cap on mRNA (for orientation purposes)

• Addition of a 3’ tail on mRNA (allows it to last longer)

• To summarize, genes program protein synthesis via genetic messenger RNA.

• The molecular chain of command in a cell is

DNA → RNA → protein.

This is referred to as the Central Dogma of Biology