DNA Replication Review Three main steps: Helicase unzips/unwinds the DNA molecule DNA Polymerase brings in new nucleotides Ligase zips the new DNA back together Why is DNA Replication important? The important idea is that an exact duplication of the DNA message is required, so that each new cell in the body has the same set of genetic instructions as the cells that preceded it. This also insures that every new generation of individuals has the same genetic information as his/her parents.

DNA carries information that can be used to construct the proteins which form structures and regulate the body’s activities. Protein synthesis involves two processes: transcription and translation. In transcription the DNA message is converted into an RNA molecule. In translation the RNA message is used to assemble amino acids into a protein chain.

Central Dogma

First, let’s learn about RNA RNA is also a nucleic acid, called ribonucleic acid It only has one strand (DNA has two) It contains the sugar ribose instead of deoxyribose It has the nitrogenous base URACIL (U) instead of thymine (T)

There are THREE type of RNA (wait for next slide for notes) Messenger RNA (mRNA)  Long strands of RNA nucleotides that are formed complementary to one strand of DNA Ribosomal RNA (rRNA)  Associates with proteins to form ribosomes in the cytoplasm Transfer RNA (tRNA)  Smaller segments of RNA nucleotides that transport amino acids to the ribosome where proteins are made by adding 1 a.a. at a time

Messenger, ribosomal, transfer RNA

TRANSCRIPTION (the first step in protein synthesis)  Through transcription, the DNA code is transferred to mRNA in the nucleus.  DNA is unzipped in the nucleus and RNA polymerase binds to a specific section where a mRNA will be synthesized

Transcription does not happen all the time Operon – the “switch” to turn on/off transcription Promoter – DNA site that promotes RNA polymerase to bind Repressor – molecule that binds to DNA to block transcription Inducer – molecule that takes repressor away

Transcription animation

Changing nucleic acids into amino acids  The three-base code in DNA or mRNA is called a codon.  They are always coded in threes  Each triplet code corresponds with one amino acid  This is where TRANSLATION begins

TRANSLATION (the last step in protein synthesis)  Translation begins when mRNA binds to the RIBOSOME in the cell.  In translation, tRNA molecules act as the interpreters of the mRNA codon sequence.  At the middle of the folded strand, there is a three- base coding sequence in the tRNA called the anticodon.  Each anticodon is complementary to a codon on the mRNA.

Translation animation

Cleaning up the Message When the genetic message is copied to make mRNA, the message contains unwanted base sequences. The ‘junk’ sequences (called introns) are removed from the message and the remaining sequences (exons) are linked together to produce a sequence of codons that will translate into a polypeptide. This process occurs before the message leaves the nucleus.

The role of Ribosomes The third type of RNA is ribosomal RNA (rRNA). Ribosomes are made of RNA and PROTEIN. Ribosomes are the ‘decoding’ units of the cell. (Sites of protein synthesis) Ribosomes consist of two major components — the small ribosomal subunit which reads the RNA, and the large subunit which joins amino acids to form a polypeptide chain. Ribosomes have binding sites for both tRNA and mRNA molecules.

Crash Course Biology (14:08)