Structure, Replication & Protein Synthesis

DNA  DNA (deoxyribonucleic acid) is the hereditary material for all living things.  contains the codes for constructing proteins in the body, including the various enzymes.  Made up of repeating units called nucleotides.

DNA  Nucleotides are composed of a phosphate group, a deoxyribose, and a nitrogen base.  The nitrogen bases are Adenine Thymine Cytosine Guanine

Nucleotide

DNA Structure  The structure of DNA was determined in the 1953 by James Watson and Francis Crick.  They proposed that DNA is made from two chains that wrap around each other in the shape of a double helix. Kind of like a winding spiral staircase

DNA Structure  Watson and Crick used findings from other scientists to develop the final version of the DNA model.  Rosalind Franklin and Maurice Wilkins were researchers that had taken X-ray diffraction photographs of DNA crystals.  These photographs helped to reveal the double helix 3- D structure of DNA

DNA Structure  Another scientist that paved the way for Watson and Crick was a biochemist named Erwin Chargaff.  Chargaff set out to see whether there were any differences in DNA among different species.

 Chargaff concluded that almost all DNA no matter what organism it comes from has certain properties.  He also concluded that the amount of adenine (A) approximately equals the amount of thymine (T), and the amount of guanine (G) approximately equals the amount of cytosine (C). In other words  (A + G) = (T + C)

Chargaff’s Rule

DNA Structure  With this information from Rosalind Franklin, Chargaff and other scientists, Watson and Crick were able to develop the model of DNA and win the Nobel Prize in 1962.

DNA is packaged into chromosomes

DNA Replication  The process by which DNA is copied.  Why do you think DNA needs to be copied?

DNA Replication  The process by which DNA is copied.  Why do you think DNA needs to be copied?  Before a cell divides by mitosis, meiosis, or binary fission it must copy its DNA so that each new cell will have the genetic information to survive.

DNA Replication  Steps Unzip String on new nucleotides Linking and Release of two new strands

DNA Replication  The first thing that must happen to copy of double strand of DNA is …

DNA Replication – Step 1

 The Y shaped region that is formed when the two strands are separated is called the REPLICATION FORK.  Remember that the two DNA strands run anti-parallel.

DNA Replication – Step 2  Proteins called DNA Polymerase add complementary nucleotides to each of the parent strands.  A’s bond with T’s and C’s bond with G’s  Each parent strand acts as a template for the two new strands.

DNA Replication – Step 2  Proteins called DNA Polymerase add complementary nucleotides to each of the parent strands.  One of the strands is copied continuously from the 3’ end to the 5’ end.  This is called the LEADING STRAND

DNA Replication – Step 2  The other DNA strand is copied discontinuously from the 5’ to the 3’ direction. Making little unfinished strands.  These are called Okazaki fragments  Short bits of DNA that make up the lagging sttrand

DNA Replication – Step 3  DNA polymerase finishes replicating and fall of off the strands  DNA ligase links the short unfinished strands of DNA to create one long strand.

Semi - Conservative  DNA replication is called semi conservative because each daughter DNA consists of half of the parental DNA and half of the new DNA.

Mutations  What is a mutation?  Mistakes in DNA replication Like if and adenine pairs with a cytosine instead of a thymine. The bases sequence of the newly formed DNA molecule is different from that of the original DNA molecule. DNA polymerase acts as a proofreader to prevent most mutations. It can change the wrong base and correct it with the correct one.

Mutations  What causes mutations? Random Chemicals (like carcinogens in cigarettes) Radiation (from the sun)

Protein Synthesis  Remember this!!! DNA mRNA Protein  DNA is transcribed to mRNA.  mRNA takes the code for a gene out of the nucleus to a ribosome where a protein is made.  This is called the CENTRAL DOGMA

RNA  DNA is a huge molecule. But it holds important information. But cannot leave the nucleus.  RNA is a nucleic acid that can hold a small amount of information.  mRNA is a small molecule that can leave the nucleus!  mRNA is not the only type of RNA.

RNA  What does RNA stand for?  Ribonucleic Acid  It is a nucleic acid just like DNA but…..

RNA  There are some differences.  Contains a phosphate, a sugar (ribose) and a base (A, U, C, G)  U is for uracil. There is no Thymine in RNA

RNA  RNA is single stranded instead of double stranded.  There are three types of RNA mRNA rRNA tRNA

Protein Synthesis  Now we are going to make a protein!  The first step in protein synthesis is… TRANSCRIPTION

Transcription  The process by which the genetic instructions in a specific gene are rewritten into an RNA molecule  Takes place in the nucleus  RNA polymerase is the protein that catalyses this process.

Transcription  A section of DNA serves as a template for an mRNA molecule.  Except instead of Adenine pairing with Thymine Adenine pairs with Uracil. A --- U C --- G

Transcription

Translation  Every three nucleotides on an mRNA molecule is called a codon  Each codon codes for an amino acid  Amino Acids are the building blocks of proteins  Remember 21 amino acids?

Translation  Is the decoding of mRNA into a string of amino acids (protein)  All three types of RNA are involved in translation. mRNA carries the info from DNA out of the nucleus. rRNA makes up the ribosomes where proteins are made tRNA carries the amino acids one by one to the ribosome to be linked together to form a protein.