4.1 Structure and Replication of the Genetic Material Big Ideas The genetic material is deoxyribonucleic acid (DNA), DNA is packaged into chromatin, which condenses to form chromosomes During interphase of the cell, the cell lives, grows and replicates its DNA and in mitotic phase (M phase), the chromosomes distribute to daughter cells A duplicated chromosome consists of two sister chromatids Introduction to genetics video http://www.youtube.com/watch?v=B_PQ8qYtUL0#aid=P9DA8Tgi14A 3 min

Inherited Characteristics In sexually reproducing species, offspring obtain a unique combination of genetic information from their parents results in a unique combination of characteristics A trait is a characteristic of an organism Some traits, such as sun-bleached hair, we acquire as we live Other traits, such as naturally curly hair, we inherit The resemblances found among family members are due to shared characteristics inherited from their biological parents

The Genetic Material Unknown until mid-1900’s It was demonstrated that the genetic material was composed of a molecule called deoxyribonucleic acid (DNA) DNA was transferred between generations of organisms and had the ability to transform the properties of the cell

Every cell’s DNA is organized into genes, which are units of inherited information that carry a code for specific traits or functions Many genes code for proteins The information contained in genes is responsible for inheritance, which is the passing down of traits from parent organisms to their offspring Eye colour gene inherited from parents, results in blue eye colour trait ->

Packaging of DNA A eukaryotic cell has a nucleus which contains the cell’s DNA For most of a cell’s life, its DNA exists as a mass of very long fibres called chromatin, which is a combination of DNA and protein As a cell is preparing to divide, its chromatin fibres condense, becoming visible compact structures called chromosomes Chromatin is very thin and not usually visible with a light microscope

The number of chromosomes varies among different species A chromosome consists of one long, condensed DNA molecule containing hundreds or thousands of genes The number of chromosomes varies among different species The DNA in chromatin is tightly coiled around proteins called histones The DNA and histone packages form structures called nucleosomes A human body cell contains enough DNA to stretch 2m in length

The organization of DNA prevents it from tangling and becoming damaged (think of thread on a spool)

DNA Structure, Components, Sequencing DNA’s structure is a double helix, contains sugar-phosphate backbone on the outside of the helix subunits: nitrogenous bases on the inside of the helix DNA consists of a long chain of subunits called nucleotides, a nucleotide is composed of: a ring-shaped sugar called deoxyribose a phosphate group a nitrogenous base: a single or double ring of C and N

= sugar-phosphate backbone There are four types of nucleotides in DNA which differ only in their nitrogenous bases single ring pyrimidines: Thymine (T) and Cytosine (C) double ring purines: Adenine (A) and Guanine (G) Nucleotides are joined to one another by covalent bonds that connect the sugar of one nucleotide to the phosphate group of the next = sugar-phosphate backbone There are bonds also between specific base pairs that hold the two strands in the helix together = twists molecule into spiral

The bases are complementary to one another Adenine (purine) pairs with Thymine (pyrimidine) Guanine (purine) pairs with Cytosine (pyrimidine) Therefore, if the sequence of one strand is known, then the sequence of the other strand can be determined Nucleotides can combine in various sequences to store information (code for proteins) just like the letters of the alphabet A-T and C-G

The Cell Cycle DNA contains all the instructions a cell needs to function throughout its life Passing on this genetic information to new cells is an essential part of cell division Eukaryotic cells go through a sequence of events called the cell cycle, which consists of two main stages: a growing stage a cell division stage

The growing stage of the cell cycle is called interphase During interphase the cell carries out its metabolic processes and performs its regular cellular activities Cell may spend 90% of cell cycle in interphase Cell prepares for cell division S phase (DNA Synthesis phase) of interphase: DNA of the cell’s chromosomes is duplicated The interphase periods before and after S phase are G1 and G2 phases (Gap phases) Cell prepares for cell division by increasing protein supply, number of organelles, increasing in size

M phase (Mitotic phase): cell division, which includes mitosis and cytokinesis Mitosis: the nucleus and the duplicated chromosomes divide and are evenly distributed between two daughter nuclei Cytokinesis: the cytoplasm is divided, results in two genetically identical daughter cells

Preparing and Dividing DNA Before cell division, the genetic information stored in DNA must be duplicated In order for each cell to have a copy, the cell undergoes DNA replication Once DNA replication is complete and the chromatin fibres have condensed into chromatids, the cell is ready for division

Each chromosome now consists of two identical joined copies called sister chromatids The region where the two chromatids are joined tightly together is called the centromere A dividing human cell starts with 46 duplicated chromosomes, each made up of two sister chromatids Once they separate from each other, each chromatid is a full-fledged chromosome

Every time a eukaryotic cell’s DNA replicates, the ends of each chromosome lose a small amount of DNA Chromosomes have protective end caps called telomeres (telomere DNA does not contain information for making proteins) When a cell’s DNA replicates, the telomeres shorten but the chromosome will not lose essential information The ability to replace telomere DNA allows some cells to divide quickly, what body cells are continually replaced? Blood, epithelial cells (skin, intestinal lining) Limit to how many times most adult cells can divide Homework: page 104 #1-4,13-15,18-20