Science_ The Definitive Visual Guide ( PDFDrive )

1890–1970

E very living organism is made up of Triple-bond link Phosphate Cell
a complex substance called protein. All plant and animal cells
It is central to the structural fabric of GC have a nucleus.
cells and, in the form of enzymes, it
controls the chemical reactions of life. AT Nucleus
This is the cell control center and
By the late 19th century many CG contains the DNA in the form of
scientists already thought that the cell chromosomes.
nucleus held the secret of inheritance. TA
A phosphoric-acid-rich substance had Cytoplasm
been identified in cell nuclei in 1869. Sugar Double-bond link The part of a cell in which
In 1937 the British physicist William protein synthesis takes place.
Astbury produced X-ray diffraction Base pairing
patterns that showed this substance to The bases of nucleotides of DNA can pair up only in two Chromosome
be deoxyribonucleic acid (DNA), configurations because their sizes and shapes fit. Each Tight package of DNA that develops
which had a regular structure, quite pair has one large base and one small, adhered by at cell division, formed to make DNA
unlike the tangled form of proteins. either three bonds—guanine (G) with cytosine (C)—or copies easier to manipulate.
two bonds—A (adenine) with T (thymine).
By 1953 James Watson and Francis Chromatin thread
Crick had confirmed that this structure Consists of DNA coiled around
was a double helix, which enabled the proteins called histones.
fundamental process known as
replication to occur (see pp.346–47).
Scientists knew that protein

The Genetic Code

The characteristics of organisms are established by differences in Packaging of DNA
their chemical makeup. One of these chemicals, DNA, carries the Lengths of DNA double helix are organized into
instructions for building another important substance—protein. threads that solidify to form chromosomes.
It is the remarkable variety of proteins that accounts for the great These unravel between cell divisions at certain
diversity of life processes. points, enabling the DNA molecules to expose
their information for making protein
BEFORE production was influenced by DNA, itself and replicate means that the and to allow replication.
but they did not know how and so genes in all cells of the same body are
The idea of genes as particles of inheritance began an exciting new line of enquiry. identical, but vary from one body to Histone protein
originated with Gregor Mendel in 1866, but another. Likewise the protein encoded This acts as a scaffold for the huge
that the genes were composed of DNA would Determining characteristics differs, so their characteristics vary too. quantity of DNA to coil around.
not be widely appreciated until the 1900s.
DNA provides the instructions for the Cracking the code Sugar-phosphate “backbones”
CONFLICTING IDEAS ABOUT HEREDITY formation of protein (see opposite). A double helix of DNA consists of
In the early 1800s French zoologist Jean-Baptiste As an organism grows, its cells divide DNA and proteins are long-chain two chains of nucleotides. The
Lamarck proposed a theory that characteristics in such a way that every cell has a copy molecules (polymers) made up of “backbones” are made up of units
acquired during the life of an organism could be of the DNA instructions for making repeated building blocks. By 1929 of sugar and phosphates either
passed on. In 1859 Darwin’s evolutionary protein; although some cells use these Phoebus Levene, a Russian–American side of a core of paired bases.
theory ff 198–99 corrected the principle of instructions selectively. In 1902 English biochemist, discovered that DNA was
this view. But it was not until the 20th century physician Archibald Garrod anticipated made up of units called nucleotides,
that Gregor Mendel’s Laws of Inheritance a link between genetic material and which consist of one sugar, one
ff 306–307 were combined with studies of protein. He recognized that some phosphate, and one base. The base can
DNA and protein production to prove that inherited diseases upset the balance of be one of four types: adenine, cytosine,
acquired characteristics could not be inherited. chemical reactions: some proteins were thymine, or guanine. Proteins, on the
not made properly because the other hand, have 20 different types of
DARWIN’S FINCHES I instructions were faulty. He called them building blocks called amino acids.
“inborn errors of metabolism.”
ff SEE ALSO In the aftermath of the double-helix
pp.308–309 C In 1941 American geneticists George breakthrough biologists began to look
Beadle and Edward Tatum found they at how the nucleotide sequence in
could induce similar errors in bread DNA determined the amino-acid chain
mold. Their work confirmed that of a protein. In 1961 Francis Crick and
proteins—specifically enzymes (protein South African biologist Sydney
that speed reactions)—were affected. Brenner confirmed earlier research and
Beadle and Tatum identified that demonstrated that a set of three
sections of DNA—the gene—provided
the codes to make enzymes and was
later modified to include all proteins.

Every organism produces thousands
of proteins, each encoded by thousands
of different genes packaged in every
cell of the body. DNA’s ability to copy

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GENETIC CODE

AFTER

nucleotide bases (or triplet) encodes Molecule of mRNA Triplet codon on Amino acid brought to Ribosome straddles two base
one amino acid. Crick and Brenner assembled from nucleotides RNA strand the ribosome by transfer triplets to accommodate two
found that a virus stopped making amino acids
protein if they inserted (or deleted) one RNA (tRNA) molecule With the understanding of the chemical
or two nucleotides from its DNA; but if basis of inheritance came the controversial
three nucleotides were inserted, the DNA strands possibility of manipulating genetic material.
virus could still make protein. separate

In the same year, American geneticist Free nucleotides Gene Amino acid MAPPING GENES
Marshall Nirenberg and German link to form the “template” is specific to codon A genome is a complete catalogue of the genes
biochemist J. Heinrich Matthaei mRNA copied onto bonds to tRNA of a cell or organism. The first draft of the human
cracked the first part of the code. They the mRNA genome 410–11 gg, published in 2001, provides
made a chain of a single nucleotide, 2The second stage of a greater understanding of genetic diseases.
which produced a protein composed of protein production is
identical amino acids. Over the next 10 1The first stage in making proteins is known as known as “translation”. A tRNA leaves DNA-CUTTING ENZYMES
years all 20 amino acids were decoded. “transcription”. Part of the DNA double helix ribosome passes along the the ribosome Scientists developed tools to isolate genes for
unwinds to expose a length of one strand called a mRNA strand and catalyzes to be used further study. Bacterial DNA exists free in the
Making proteins gene. A molecule of mRNA is assembled along the the reaction that joins again cytoplasm. Bacteria contain chemicals called
other strand, building a complementary “copy” of amino acids together. The restriction enzymes, which the cells use to “cut”
Protein is made on granules called the gene sequence or gene code. mRNA provides the Chain of amino DNA and exchange genetic material; a
ribosomes in a cell’s cytoplasm. So information and the tRNA acids develops process that can be replicated in the laboratory.
somehow, instructions from the genes brings amino acids to build
must “move out” of the nucleus. Building proteins up the chain. Once More amino acids SEE ALSO gg G
Spanish biochemist Severo Ochoa Protein synthesis occurs within all cells and has complete, the chain folds added till code pp.386–87 G T
focused on a substance related to DNA two distinct stages. Transcription takes place into a protein. fully translated pp.410–11 T H
called ribonucleic acid (RNA). First inside the nucleus and translation outside it.
identified in 1909, RNA is found in both
the nucleus and the cytoplasm of cells, that brought the amino-acid building Protein folds up into a
and transpired to be the “messenger” blocks to the ribosome for assembly into shape determined by its
molecule (mRNA). The final link came proteins. By understanding protein sequence of amino acids
in 1964 when American biochemist synthesis, molecular biologists had
Robert Holley cracked the structure of finally unveiled the blueprint of
smaller transfer RNA (tRNA) molecules life itself.

Correct base

Correct amino acid Triplet codon Lack of pigment
Substituted base A pigment called melanin normally occurs in skin, hair,
and the iris of the eye. Lack of pigment, known
Adenine–thymine link Incorrect amino acid as albinism, is caused by a faulty variant of a gene that
Guanine–cytosine link carries the pigment-making code for melanin.
# Gene mutation
DNA replicates when cells divide, and normally makes MICROBIOLOGIST (1927–)
an exact copy of itself. Errors in the replication process
can result in alterations in the base-pair sequence of a SYDNEY BRENNER
gene, so that a different amino acid is substituted,
producing a different variety of protein. Brenner grew up in South Africa, but was
educated at Oxford University. In 1953
Complementary base pairs he saw Watson and Crick’s model of the
Pairs of nucleotides are arranged in a specific order DNA double helix, which inspired his
that is replicated before cell division. The sequences work on the genetic code. He made
of three bases (the gene code or triplet codon) groundbreaking discoveries in animal
determine the manufacture of specific proteins. development using a roundworm as his
model species. He shared the Nobel
Prize for Medicine in 2002 and also
founded the Molecular Sciences Institute
in California.

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