DNA, RNA, & Protein Synthesis The foundation of the study of Genetics

Discovery of DNA & its structure DNA structure was not discovered until the 1950’s After looking at an X-ray diffraction photo stolen from Rosalind Franklin by her co- worker (Wilkins), James Watson and Francis Crick were able to create a model of DNA The model depicted a double helix of even width In 1953, James Watson, Francis Crick, and Maurice Wilkins received the Nobel Prize (no mention of Rosalind Franklin)

Rosalind Franklin Franklin’s x-ray images suggested that DNA was a double helix of even width.

James Watson & Francis Crick

Other Evidence Erwin Chargaff, in his biochemical studies, discovered: The amount of Adenine in a DNA sample always equaled the amount of Thymine The amount of Cytosine, likewise, was equal to the amount of Guanine This became known as Chargaff’s rule This information further served to confirm the model created by James Watson and Francis Crick Adenine pairs with Thymine Cytosine pairs with Guanine

Structure of DNA DNA (deoxyribonucleic acid) is made up of long chains of nucleotides Nucleotide structure Phosphate Group 5 carbon sugar (deoxyribose) Nitrogen Base (4 different bases) Adenine Thymine Guanine Cytosine

Nucleotide phosphate group nitrogen-containing base deoxyribose (sugar) nitrogen-containing base

DNA structure Continued DNA is double stranded The two strands are equally spaced & twist This is why the structure of DNA is referred to as a double helix

Bonding in DNA The backbone of each side of DNA is held together by covalent bonds and is made up of the phosphate groups and deoxyribose sugars

Bonding in DNA The two complementary strands (the rungs of the DNA ladder) of DNA are held together by weak hydrogen bonds between the complementary nitrogen base pairs These attractions are easily broken and reformed Important for DNA replication and transcription

Bonding in DNA hydrogen bond covalent bond

DNA Replication Making copies of DNA

DNA Replication Occurs during Interphase (S phase) of the cell cycle DNA replication is required such that the two new daughter cells created will have a complete set of DNA Enzymes break the hydrogen bonds, separating the two strands Other enzymes bring free nucleotides in the nucleus to build the new strands of DNA

Semiconservative DNA replication is called a semiconservative process as the two double helices created consists of: One old strand of DNA One new strand of DNA The “old” sides of the DNA strands are used as the template (building instructions) for building the “new” strands of DNA Two double helices are created in the end

The DNA molecule unzips in both directions. DNA Replication nucleotide The DNA molecule unzips in both directions.

End Result DNA Replication original strand new strand Two molecules of DNA

The basis of gene expression Protein Synthesis The basis of gene expression

Central Dogma States that information stored in DNA flows in one direction from DNA, to RNA, to proteins

Protein Synthesis Phases Transcription The 1st phase of protein synthesis DNA can’t leave the nucleus, so a smaller molecule that can leave has to be made Occurs in the nucleus Translation The 2nd and final phase of protein synthesis The molecule made in transcription is used to build a protein Occurs at the site of a ribosome

Organelles Review Nucleus- where DNA is located; where transcription occurs Ribosome- where proteins are built (site of protein synthesis); where translation occurs Rough ER- Newly made proteins enter this transport organelle Golgi Apparatus/Body-The protein is checked, modified, and re-packaged for transport to its final destination Vesicles-Transport vehicles for the protein around the cell

Transcription A segment of DNA called a gene with the information for building a protein is used as a template (building instructions) to build a messenger RNA (mRNA) molecule Enzymes open the double helix Other enzymes build the mRNA molecule from the gene of DNA Messenger RNA is a copy of the DNA instructions Like a secretary or a transcriptionist that records information

Transcription DNA

DNA vs RNA Double helix Nucleotide sugar: deoxyribose Nitrogen bases Adenine Thymine Cytosine Guanine Base Pair Rule A-T C-G Single stranded Nucleotide sugar: ribose Nitrogen bases Adenine Uracil Cytosine Guanine Base Pair Rule A-U C-G DNA RNA

Translation

RNA molecules involved in Transcription & Translation DNA Messenger RNA (mRNA) Messenger RNA (mRNA) Ribosomal RNA (rRNA) Transfer RNA (tRNA) Transcription Translation

Ribosome Structure

Translation The mRNA molecule leaves the nucleus and moves to a ribosome The mRNA “sticks” to the small subunit of the ribosome, which is made up of a ribosomal RNA (rRNA) molecule The mRNA is fed between the small subunit of the ribosome and the large subunit The mRNA message is “read” by the ribosome 3 nucleotides at a time Each set of 3 nucleotides on mRNA is called a codon

Messenger RNA & codons codon for methionine (Met) leucine (Leu)

Translation Continued For each codon of mRNA, there is a transfer RNA (tRNA) molecule that has the anticodon An anticodon is a set of three nucleotides that is complementary to the mRNA codon Needed for bonding to the mRNA to make a “drop off” The transfer RNA also carries ONE amino acid The transfer RNA bonds temporarily to its mRNA complement codon A chemical reaction occurs that causes the tRNA to let go of its amino acid

Transfer RNA

Translation Continued The amino acids that are dropped off by the tRNA are linked together by a peptide bond This continues until a polypeptide chain or a protein is made based on the instructions originally provided by the DNA.

Transfer RNA at Work

A closer Look at Translation

How protein synthesis is altered when there is a change in the DNA DNA Mutations How protein synthesis is altered when there is a change in the DNA

Mutations Any change in the DNA nucleotide sequence The DNA is checked twice for this during Interphase of the cell cycle After G1: The existing DNA is checked After G2: The replicated DNA is checked to make sure no mutations occurred in the replication of DNA Causes of mutations DNA Replication errors Caused by mutagens: UV light, chemicals, pollutants, changes in the environment etc.

Protein Synthesis & Mutations How mutations can effect protein synthesis No effect: The same amino acid sequence is created regardless Good result: A new adaptive trait comes about as a result (evolution) Bad result: Controls over the cell cycle are lost and rapid cell division (cancer) results.

Two Types of Mutations The overall length of the DNA does NOT change Substitution: Another nucleotide replaces an existing one Inversion: Two, or more, nucleotides switch locations The overall length of the DNA is changed Deletion: One or more nucleotides are removed Insertion: One or more extra nucleotides are added Translocation: A part of one DNA moves to another strand of DNA Point Mutations Frameshift Mutations

Point Mutations mutated base

Frameshift Mutations

Translocation

Can mutations be passed on? Yes If the mutation occurs in the germ cells that divide through meiosis to create a sperm or an egg These remain “hidden” until an offspring is created using either the sperm or the egg with the mutation No If the mutation occurs in the somatic cells that divide through mitosis These will only affect the individual, not their offspring