Unit 3 – Molecular Genetics Lesson 1 – Intro to Genetics and DNA Structure

Central Themes  Some central themes that apply to this unit:  # 1 Living organisms are extremely diverse, but possess an underlying unity. For example, many biochemical pathways found in bacteria are also present in human beings. Many other aspects of cellular structure and function are universal among living organisms.  # 2 All organisms are capable of self reproduction based on a set of instructions known as the genome. The genome in all cellular organisms is composed of DNA-a striking example of the unity in diversity mentioned above.  # 3 Life as it exists today is the product of evolution: the change in the genomic composition of populations of organisms over time. The course of evolutionary change is directed by natural selection. Evolution by means of natural selection is the most important unifying theme in biology. It explains the unity in diversity, the complementarities of structure and function, and the adaptations to varying niches exhibited by life.

Learning Goals  Identify the monomer units of nucleic acids  Identify the two primary nucleic acid polymers, DNA and RNA, and particulars about their structure.

DNA and You  Discuss the following questions with your table:  What are the potential benefits to knowing more about your genetic predisposition (your chance of developing) to a disease?  What are the possible downsides to knowing?  Aside from health information, what else might you be curious to learn about from your DNA?

Nucleic Acid  Nucleic acids are named as such because they were originally found in the nucleus of the cell and contained phosphate groups (related to phosphoric acid).

Nucleic Acid Monomers  Nucleic Acids are polymers. They are made up of monomer units called nucleotides. A nucleotide consists of a phosphate, a pentose sugar, and a nitrogenous base.

Nitrogenous Bases  There are (generally speaking) 5 different bases found in nucleic acids.  Two double ringed purines: Adenine and Guanine.  Three single ringed pyrimidines: Thymine, Cytosine and Uracil.

Phosphodiester Bonds  Nucleotide monomers can be linked together through a dehydration synthesis that create phosphodiester bonds.  These bonds occur between the 3’ Carbon OH group of one nucleotide and the 5’ Carbon Phosphate group.  How does this compare to other polymerization reactions?

Orientation of Strand  Nucleic acid strands can be described by the orientation of the 5’ end (free phosphate) and the 3’ end (free OH).  We can talk about an order of the bases using this standard. Order is typically denoted in a 5’ to 3’ direction.

Deoxyribosenucleic Acid  DNA is the most well known nucleic acid. It stands for deoxyribonucleic acid.  DNA is double stranded with bases hydrogen bonded to each other at the centre.  Strands run antiparallel to each other (in opposite directions).  Bases are always bound to their complementary base.Adenine forms two hydrogen bonds with Thymine, and Cytosine forms three hydrogen bonds with Guanine.

DNA Double Helix  The strands are wound around each other into a double helix.

Ribonucleic Acid  RNA is another very important type of nucleic acid with a multitude of functions. It stands for ribonucleic acid.  It is a single stranded molecule, and has the sugar ribose rather than the deoxyribose of DNA.  Its bases can temporarily form hydrogen bonds with complementary bases.

Types of RNA  RNA is more versatile than DNA, and there are many types of it that exist. Three main forms it can be found in are messenger RNA (mRNA), Ribosomal RNA (rRNA), and Transfer RNA (tRNA).

Differences Between DNA and RNA  DNA and RNA have a few primary differences in structure:  DNA has deoxyribose and RNA has ribose  DNA has Thymine and RNA has Uracil  DNA is double stranded and RNA is single stranded

Nucleoside vs Nucleotide  You may also see the word nucleoside, which refers to the sugar and the base without the phosphate group. ATP is an example of a nucleoside triphosphate (adenosine triphosphate). GTP, UTP and CTP also exist, as does dATP (deoxyadenosine triphosphate) dGTP, dTTP and dCTP.

Ubiquity of NTPs  The nucleus, cytoplasm, and organelles of a cell are full of NTPs and dNTPs. They are used for energy transfer, cell signalling and can be polymerized into DNA and RNA.

DNA vs Chromatin vs Chromosomes  DNA is found in the nucleus, but it is entwined with proteins, and coiled up again and again into a form called chromatin.  During cell division, chromatin condenses even further into a form known as a chromosome, which can be seen under an optical microscope.

Crash Course 

Review  Define the following terms: nucleotide, complementary base pairing, phosphodiester bond, NTP.  In a DNA molecule, a purine pairs with a pyrimidine. If this is the case, then why can’t A-C and G-T pairs form?  The following is a segment taken from a strand of DNA: 5’- ATGCCTTA-3’. What would the complementary strand be? Be sure to include directionality.  A molecule of DNA was analyzed and found to contain 20% thymine. How much adenine would you expect there to be? What about the other two bases?  What are the differences between DNA and RNA?  As building DNA is an endergonic process, where do you expect the energy to come from for the dehydration synthesis necessary for polymerizing a strand?