DNA: The Molecule Of Life

Transcription

1 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 Hershey and Chase showed that DNA is the genetic material in James Watson and Francis crick elucidated the structure of DNA in 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

2 -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

3 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

4 -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?

5 -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 Lisa or Josh