DNA Functions- Expression

•  Expression of Information !  A different subset of the genetic information is

read in each cell type. !  Expression requires two processes.

-  Transcription - With DNA as a template, mRNA, tRNA, and rRNA are made during transcription.

-  Translation " Messenger RNA (mRNA) from transcription

is translated to produce proteins.

Transcription

•  Three different types of RNA (ribonucleic acid) produced: !  Messenger RNA (mRNA) !  Transfer RNA (tRNA) !  Ribosomal RNA (rRNA)

•  RNA is made up of numerous nucleotides assembled in exactly the same way as in DNA except that……. !  1. RNA is mostly single stranded and not a

helix. !  2. the sugar is ribose instead of deoxyribose. !  3. the base thymine is replaced by uracil.

Transcription

Messenger RNA (mRNA) carries copies of instructions for assembling amino acids into proteins

from the nucleus to the ribosomes.

Transcription

Ribosomes are made up of proteins and ribosomal RNA (rRNA).

Transcription

During protein construction, transfer RNA (tRNA) transfers each amino acid to the ribosome. Each form of tRNA has a specific anticodon loop

Transcription

•  In transcription, the DNA molecule unzips, just as in replication.

•  RNA polymerase will attach to assemble the new RNA molecule.

•  Only one side of the DNA molecule is transcribed.

•  This is assured by the fact that RNA polymerase is not attracted to just any stretch of DNA, but only to certain DNA base sequences, called “promoters.”

Transcription

•  Promoters are sequences of bases that do not determine protein structure but serve only to send the message “RNA polymerase, start here.”

•  Transcription is essentially identical to replication. The differences are… ! 1. assembled with ribonucleotides instead

of nucleotides. ! 2. adenine pairs with uracil.

-  AND

Transcription

•  3. daughter RNA strand does not stay attached to the parent DNA strand. Instead it separates from the DNA, and the DNA zips back together. The RNA migrates out of the nucleus of the cell and into the cytoplasm.

Transcription

Transcription

•  RNA Editing -  The DNA of eukaryotic genes contains

sequences of nucleotides, called introns, that are not involved in coding for proteins.

-  The DNA sequences that code for proteins are called exons.

-  When RNA molecules are formed, introns and exons are copied from DNA.

Transcription

The introns are cut out of RNA molecules. The exons are the spliced together to form mRNA.

Translation

•  Translation is the part of the process where the information from the gene is used to create the amino acids and then the protein.

•  The ribosome has to take the nucleotide bases and equate it with codons that specify amino acids.

•  Each amino acid is represented by certain codons.

•  The “language” that bridges the gap between gene and amino acid is the genetic code.

Translation

•  The genetic code is based on codons. •  A codon consists of three consecutive

nucleotides that specify a single amino acid that is to be added to the polypeptide.

-  64 possible combinations -  Code for 20 amino acids.

Translation

Translation

Translation

•  Translation begins when an mRNA molecule in the cytoplasm attaches to a ribosome.

•  As each codon of the mRNA moves through the ribosome, the proper amino acid is brought into the ribosome by tRNA.

•  In the ribosome, the amino acid is transferred to the growing polypeptide chain [protein].

•  Each tRNA molecule carries only one kind of amino acid.

•  In addition to an amino acid, each tRNA molecule has three upaired bases

Translation

•  These bases, called the anticodon, are complementary to one mRNA codon.

•  In the case of the tRNA molecule for methionine, the anticodon bases are UAC, which pair with the methionine codon, AUG.

•  The ribosome has a second binding site for a tRNA molecule for the next codon.

Translation

Translation

•  Like an assembly line worker who attaches one part to another, the ribosome forms a polypeptide bond between the first and second amino acids, methionine and phenylalanine. [see diagram]

•  At the same time, the ribosome breaks the bond that had held the first tRNA molecule to its amino acid and releases the tRNA molecule.

•  The ribosome then moves to the third codon, where a tRNA molecule brings it the amino acid specified by the third codon.

Translation

Translation

•  The polypeptide chain continues to grow until the ribosome reaches a stop codon on the mRNA molecule.

•  When the ribosome reaches a stop codon, it releases the newly formed polypeptide and the mRNA molecule, completing the process.

Translation

Central Dogma of Molecular Dogma