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
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 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
• 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
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
• 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
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)
• 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?