Biology 457 & 557 -  Week 3

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 at an unwound section of DNA

one strand serves as the template

the other strand is called the coding strand

the molecule of mRNA is directed to the cytoplasm

process of transcription occurs in three stages

initiation – enzyme called RNA polymerase

promoter

                              The final step in initiation occurs as RNA polymerase

a)      elongation – once the first base is added in mRNA, RNA polymerase

joins the RNA nucleotides sequentially

In humans, 30-50 nucleotides/sec are added

In bacteria, upwards of 500 nucleotides/sec

b)      termination – RNA polymerization ends at a specific nucleotide sequence

* The length of an average mRNA molecule is

            mRNAs are processed/modified to produce a mature mRNA molecule

In general, both prokaryotes and eukaryotes follow the same basic stages of transcription

            bacteria (for example E. coli) use

            this holoenzyme is large (mole. wt. is ~465 kD) and is composed of 4 different

            types of subunits

core enzyme (a₂bb’)

sigma factor (the s polypeptide)

            eukaryotes have three different RNA polymerase molecules:

1)       

2)       

3)       

all eukaryotic polymerases are large proteins

these polymerases can’t initiate transcription alone, need transcription factors generically called

do have an important sequence in promoter called the TATA box

TATA-binding protein (TBP)

initiation requires the transcription factors to act in a defined manner

Translation –

      specialized molecules called ribosomes

and tRNA (transfer RNA)

ribosomes found free in the cytoplasm or bound to the membranes of endoplasmic

reticulum are the sites of

      : complex of individual ribosomal RNA (rRNA) molecules and more than 50 proteins

bacterial ribosomes:

higher eukaryotic (mammalian) ribosomes:

: ribosomes from all species have great structural and functional

  similarities

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

a codon codes for a particular amino acid in a protein

            : the code is degenerate (contains redundancies)

a start or initiator codon is present in mRNA

also have stop or terminating codons

the sequence of codons that run from a specific start site to termination is called a

tRNA molecules are small, single-stranded molecules that fold back on themselves (double-stranded regions)

tRNAs act as adaptors to match the codons of mRNA with the proper amino acids for incorporation into a polypeptide chain.

anticodon

aminoacyl-tRNA synthetase

charged

why put the anticodon on one end and the amino acid on the other?

translation also occurs in three steps:

initiation –

Shine-Dalgarno sequence

Kozak sequence (-ACCAUGG-)

Initiation factors (IFs) help the small subunit recognize and bind to the initiation site.

To complete initiation, a large ribosomal subunit binds to the small subunit.

GTP hydrolysis provides the energy

1) elongation –

Peptidyltransferase

2) translocation

A site –

P site –

E site –

Elongation factors (EF-Tu and EF-G) –

30-35 amino acids added to growing polypeptide are protected from environment by

The 23S rRNA (bacterial system) and 28 rRNA (eukaryotes) in large subunit may carry out

The antibiotic puromycin inhibits protein synthesis because it

3) termination –

termination or release factors

* 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