Transcript 15.1.1 Chemical Nature of Chromosomes and Genes

Blueprint of Life
Topic 15: Chemical Nature of Chromosomes and Genes
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT POINT(s)
 describe the chemical nature of chromosomes and genes
 identify that DNA is a double-stranded molecule
twisted into a helix with each strand comprised of a
sugar-phosphate backbone and attached bases – adenine
(A), thymine (T), cytosine (C) and guanine (G) –
connected to a complementary strand by pairing the
bases, A-T and G-C
Introduction
Although the work of Sutton and
Boveri and Thomas Hunt Morgan
showed that chromosomes are the
physical basis of inheritance (that
is, they carry the hereditary factors
and these genes are arranged in a
linear fashion, like beads on a string),
the actual chemical nature and
chemical structure of the
hereditary material and genes
remained unsolved until the
1940s.
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Introduction
At that stage the common
expectation among scientists was
that the secret of heredity would
be found in proteins. In 1944,
Oswald Avery published his
findings, which contested this
idea and suggested that DNA (not
proteins) encodes the hereditary
information. Many biologists
were not convinced, as they
thought that Avery’s DNA had
been ‘contaminated’ by protein.
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Introduction
The race was on in laboratories
around the world to try to solve
the puzzle—both protein and
DNA structures were being
investigated. In America, Linus
Pauling discovered the structure
of the alpha helix in the protein
haemoglobin in 1948, ahead of
several teams doing similar
studies in Europe.
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Introduction
There were two leading teams in England that were also working
on the molecular structure of biological molecules at the time—
one team at Cambridge University in the Cavendish laboratory
under the leadership of Lawrence Bragg, and one team at King’s
College in London under the leadership of John Randall.
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Introduction
In 1951, both of these
laboratories as well as that of
Linus Pauling in America had
teams doing similar research
into the structure of DNA. In
1953, one of these teams, after
building a detailed model, won
the race—they discovered that
DNA (deoxyribose nucleic acid)
is the molecule that meets all
the requirements of the
hereditary material.
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Introduction
According to the model, DNA:
 Can carry, in coded form, all the
instructions for the formation and
functioning of cells, despite the fact
that its ‘alphabet’ consists of only
four nitrogenous bases.
 Its structure allows self-replication.
 It can be transferred (packaged in
the form of chromosomes) by
gametes from one generation to the
next.
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Introduction
As a result of their innovation,
their attention to sound
scientific detail and their
collaborative approach, Watson
and Crick revealed that DNA is
a double helix or ‘twisted
ladder’.
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General Structure of DNA
Chromosome is made up of two chemicals:
1. DNA, a long, thin thread-like macromolecule, which is the
information-carrying part of the chromosome
2. Proteins around which the DNA is coiled, to keep it neatly
‘packaged’.
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General Structure of DNA
A DNA molecule is made up of two chains or strands of small
building blocks or monomers called nucleotides.
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General Structure of DNA
Each nucleotide consists of three
parts—a phosphate, a sugar
(deoxyribose sugar) and a
nitrogenous base.
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General Structure of DNA
There are four types of bases and each nucleotide is named after
the base that it carries—adenine, thymine, guanine or cytosine.
These are often simply referred to by their first letters—A, T, G
and C. The bases are arranged in a sequence along each strand of
DNA— e.g. GGTCAGGCTTGAACGA—and so each DNA
molecule is thousands of bases long.
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General Structure of DNA
The whole ‘ladder’ molecule, instead of
being flat, spirals and is therefore known
as the ‘double helix’.
■ X-ray crystallography suggested a
helix measuring 3.4 nm for every turn
and this fitted the model where exactly
10 base pairs would measure 3.4 nm in
length and make up one twist of the
helix.
■ The ratio of adenine to guanine and
cytosine to thymine could be explained
by their complementary base pairing.
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General Structure of DNA
The two complementary chains of DNA could ‘unzip’ or open up
along the line of hydrogen bonds between the base pairs, allowing
them to replicate.
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Chemical Structure of DNA
The DNA molecule is a long
chain molecule consisting of two
complementary strands. Each
strand is made up of a sequence
of many nucleotides and the
strands are held together by
weak hydrogen bonds in the
centre. The two strands in the
double-helix model have an
‘antiparallel’ arrangement—that
is, they run in opposite
directions.
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Chemical Structure of DNA
The vertical sides of the
ladder are made up of
alternating sugar and
phosphate molecules (a
‘sugar–phosphate
backbone’) and the ‘rungs’
of the ladder are pairs of
nitrogenous bases (adenine,
thymine, guanine and
cytosine, or A, T, C and G
respectively).
www.britannica.com
Chemical Structure of DNA
Each nucleotide is made up of a phosphate group, a sugar
(deoxyribose sugar) and a nitrogenous base attached to the sugar.
There are four types of nitrogenous bases: adenine, guanine,
cytosine and thymine.
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Chemical Structure of DNA
Chemically, these bases
have to pair in a particular
manner: adenine with
thymine and guanine with
cytosine (that is, A-T and
G-C), held together in the
centre by hydrogen bonds,
forming two
complementary strands.
Genes and Chromosomes
A gene is considered to be the smallest unit of heredity (that is,
what Mendel called a ‘factor’). Chemically, each gene is a portion
of DNA with a specific sequence of bases that encodes for a
particular trait that can be passed from parent to offspring.
www.bio.miami.edu
Genes and Chromosomes
A locus is the position of a
gene on a chromosome. The
coded information within
genes determines how living
things look, behave and
function—that is, it
influences particular
characteristics (phenotypes).
A chromosome can therefore
be described as a linear
sequence of genes.
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Genes and Chromosomes
The total amount of genetic material that an organism has in each
of its cells is called its genome.
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Genes and Chromosomes
Specific staining techniques are used to
show up banding patterns on
chromosomes and these bands
correspond on homologous pairs of
chromosomes. The banding patterns can
also be used to identify the positions of
particular genes on chromosomes. With
modern technology, particular genes can
be marked with fluorescent tags that
show up on the chromosome, assisting
gene mapping.
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Genes and Chromosomes
Task: List the following structures in order of size, from smallest
to largest: chromosome, gene, DNA, nucleotide, base, genome.
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Activity/Homework
-Students to complete Structure of Nucleic Acids worksheet
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