Honors Biology 298 - Week 2

HOW IS GENETIC INFORMATION STORED IN DNA?

If DNA is composed of linear sequences of nucleotides and proteins are linear sequences of amino acids, then how is the sequence in a gene (DNA) converted into the correct amino acids in a protein?

THE FLOW OF GENETIC INFORMATION IS A MULTISTEP PROCESS

2 Main Steps:

1)      Transcription –

a single-stranded polynucleotide of RNA is made

one strand serves as the template

the molecule of mRNA is directed to the cytoplasm

process of transcription occurs in three stages

a)      initiation – enzyme called RNA polymerase

b)      elongation –

  termination –

            mRNAs are processed/modified to produce a mature mRNA molecule

2)      Translation –

      specialized molecules called ribosomes

ribosomes found free in the cytoplasm or bound to the membranes

            the genetic information contained in mRNA is encoded in a sequence of three

                 nucleotides called a codon

a codon codes for a particular amino acid in a protein

a start or initiator codon

also have stop or terminating codons

tRNA molecules are small, single-stranded molecules

tRNAs act as adaptors to match the codons of mRNA with the proper amino

     acids for incorporation into a polypeptide chain.

translation also occurs in three steps:

1)      initiation –

2)      elongation –

3)      termination –

* once again it is very important that the fidelity of this system is constant

                        ribosomes rarely make mistakes in the amino acids placed in polypeptide

                        chains

Levels of Structure in Proteins

            Mentioned previously that the amino acid sequence determines the 3-D shape

            Have identified 4 levels of protein structure:

1)      Primary structure –

2)      Secondary structure –

3)      Tertiary structure –

4)      Quaternary structure –

Functional Definition of a Gene

DNA does not directly form protein but works through an intermediate

Important to recognize that a gene is a coded sequence in the DNA molecule

From a functional point of view, a gene is a section of the DNA molecule

What is it that genes do?

            a mutation in a gene encoding a specific polypeptide can alter the ability of the

            encoded protein to function

            this idea makes up the premise behind the one gene-one enzyme hypothesis

                        provides a link between the gene and phenotype

            recently, the one gene-one enzyme theory has been changed

So out of the > 3 billion (3 X 10-9) base pairs of DNA in the human genome (total information stored in the chromosomes of an organism), how much is important from a genetic standpoint?

The remaining 10% contains approximately 100,000 protein-coding genes

So why have this extra DNA?

Genes Can Have a Complex Internal Organization

Years ago it was thought that a gene was a continuous coding stretch of DNA

            most genes in humans contain nucleotide sequences that are transcribed into an

            mRNA precursor

These noncoding sequences, called introns

On the other hand, those sequences in a gene that are transcribed and translated into the amino acid content of a protein are called

Introns are spliced out of the primary transcript and exons are spliced together to yield the intact coding sequence in the mature mRNA molecule.

Accurate “cut and paste”

It is of interest to note that bacteria have streamlined their structural genes

Genes Can Have a Complex External Organization

Have introns and exons, and only a fraction of a typical eukaryotic gene is translated

What about the regions on either side of a gene, called flanking regions, are they important?

            5’ flanking region – sequences adjacent to the 5’ end of a gene

            3’ flanking region –