DNA: The Molecule Of Life Introductory Concepts -One unique set of DNA in an organism is termed its genome (link to fig 1-3) -DNA is the main component of chromosomes -Humans are diploid organisms, with each somatic cell containing 46 chromosomes. What Is DNA Made Of? -DNA stands for deoxyribonucleic acid -DNA is made up of three basic chemical components: Phosphate. Deoxyribose (a sugar). Two Pyrimidines Cytosine and Thymine. Two Purines Guanine and Adenine. Important experiments relating to DNA -Avery et al. discovered that DNA was the transforming principle in bacteria in 1944 -Hershey and Chase showed that DNA is the genetic material in 1952 -James Watson and Francis crick elucidated the structure of DNA in 1953. The Structure of DNA -Is composed of two strands in shape of a double helix. -The strands are arranged in an anti-parallel fashion. -The backbone of DNA is composed of a repeating phosphate deoxyribose polymer - Bases are bonded to the 1' carbon of each molecule of deoxyribose -The strands are held together by hydrogen bonds Terminology -Nucleoside- A base bonded to a sugar -Nucleotide- A nucleoside bonded to a phosphate group
-Nucleotides form phosphodiester bonds in the 5' ---> 3' direction to make a molecule of DNA What is a gene? -a gene is a region of the DNA molecule which is made up of: Regulatory region Coding region (exons) Non-coding region (introns) Transcription termination sequence The Structure of genes The Regulatory region -All genes have a region which is not transcribed and acts to regulate transcription, termed the regulatory region Introns and exons -Introns - regions of a gene which are transcribed but removed before translation -Exon - the coding region of DNA which is translated Structure Reflects Function -Three requirements for heriditary material: Replication Information content Periodic change Structure of Chromosomes -One way of viewing a cell's set of chromosomes is using a karyotype -The region to which the spindle fibres attach during cell division is called the centromere -Chromosomes can be classified as telocentric, acrocentric, or metacentric depending on the position of the centromere Histone Proteins -An animation explaing the packaging of DNA into chromosomes -The material that makes up chromosomes is called chromatin -complexes of DNA coiled around histones are called nucleosomes and look like a "beaded necklace" -when the nucleosome is further coiled it is referred to as a solenoid Gene Function
Recap: -Structure and organization of DNA -Structure determines function of DNA What we are about to learn: -Information exchange: DNA to protein CENTRAL DOGMA OF GENETICS: DNA RNA PROTEIN Life processes Think of the dogma like this: - DNA = blueprint - RNA carries instructions from the blueprint - PROTEINS = machines made from blueprint instructions Look at Chapter overview before proceeding! But what is RNA? - Nucleic acid (like DNA) -Single stranded (not like DNA) -Ribose instead of deoxyribose (not like DNA) -Adenine, Guanine, and Cytosine bases (like DNA) -Uracil instead of Thymine (not like DNA) Why do we need U and T? -Can fold into many shapes
-Bring information from DNA to protein-making machinery -Help build protein from DNA information And what is protein? -Large molecules made of chains of amino acids -3 or 4 levels of structure -The workhorses of the cell!!! Many functions -structure determines function So when the cell needs a job done it: -Gets instructions from DNA -Sends instructions through dispatcher (RNA) -Machinery makes the right protein for the job -Protein performs its function inside or outside of cell Naming the Process! Transcription = information from DNA passed to RNA Translation = information from RNA used to make protein Transcription Translation DNA RNA PROTEIN Life processes What are the instructions in DNA? -How does DNA sequence (4 different nucleotides) = Protein sequence (20 different amino acids)? -How is a gene sequence specific to one protein? -How do protein-making machines read the instructions from DNA? -The answers are found in THE GENETIC CODE!!! What is the Genetic Code?
-Each gene contains a specific sequence of nucleotides -Every three nucleotides (a codon) code for one amino acid -Code is unambiguous -Code is degenerate -Code is (almost) universal -organelles (mitochondria and chloroplast) have separate genomes Why U s instead of T s in the genetic code? - The code is based on the messenger RNA, not DNA - Here s how it works: - DNA gene sequence is transcribed to form messenger RNA -Complementary base pairing means RNA sequence = DNA sequence -Remember both U and T bind to A!! Functionally the same!!! - Protein-making machine (ribosome) reads messenger RNA sequence -Carriers bring the amino acids to the ribosome by base-pairing to each codon -Wobble hypothesis -Special codons start and stop protein making (translation) What we learn next - Mutations - Genotype vs phenotype - Why peas are wrinkly or smooth! Questions E-mail Lisa or Josh