2. OVERVIEW: LIFE’S OPERATING INSTRUCTIONS
• In 1953, James Watson and Francis Crick introduced an elegant double-helical
model for the structure of deoxyribonucleic acid, or DNA
• DNA, the substance of inheritance, is the most celebrated molecule of our time
• Hereditary information is encoded in DNA and reproduced in all cells of the
body
• This DNA program directs the development of biochemical, anatomical,
physiological, and (to some extent) behavioral traits
4. The Discovery of the Molecular Basis Of
Inheritance
In the early 1900s, scientists knew that
chromosomes, made up of DNA (deoxyribonucleic
acid) and proteins, contained genetic information.
However, they did not know whether the DNA or
the proteins was the actual genetic material.
In the 1940s, various researchers showed that
DNA was the genetic material
In the 1950s, the structure of DNA was
determined.
5. The DNA (Deoxyribonucleic Acid)
The structure of DNA was determined by James
Watson and Francis Crick in the early 1950s.
DNA is a polynucleotide; nucleotides are
composed of a phosphate, a sugar, and a
nitrogen-containing base.
The sugar in DNA is deoxyribose
The four different bases in DNA are:
adenine (A), thymine (T), guanine (G), and
cytosine (C).
45. Watson and Crick showed that DNA is a double helix in which
A is paired with T
G is paired with C
This is called complementary base pairing because a purine is
always paired with a pyrimidine.
Structure of DNA
55. Replication of DNA
DNA replication occurs during chromosome duplication.
An exact copy of the DNA is produced with the aid of
DNA polymerase.
Hydrogen bonds between bases break and enzymes “unzip”
the molecule.
Each old strand of nucleotides serves as a template for
each new strand.
New nucleotides move into complementary positions are
joined by DNA polymerase.
58. Gene Expression
A gene is a segment of DNA that specifies
the amino acid sequence of a protein.
Gene expression occurs when gene activity
leads to a protein product in the cell.
A gene does not directly control protein
synthesis; instead, it passes its genetic
information on to RNA, which is more
directly involved in protein synthesis.
59. RNA (ribonucleic acid) is a single-stranded nucleic acid in
which
A pairs with U (uracil)
G pairs with C.
Three types of RNA are involved in gene expression:
messenger RNA (mRNA) carries genetic information
to the ribosomes,
ribosomal RNA (rRNA) is found in the ribosomes,
transfer RNA (tRNA) transfers amino acids to the
ribosomes, where the protein product is synthesized.
The RNA (Ribonucleic Acid)
61. Two processes are involved in the synthesis
of proteins in the cell:
Transcription makes an RNA molecule
complementary to a portion of DNA.
Translation occurs when the sequence of
bases of mRNA directs the sequence of
amino acids in a polypeptide.
Structure of RNA
62. The Genetic Code
DNA specifies the synthesis of proteins because it
contains a triplet code: every three bases stand for one
amino acid.
Each three-letter unit of an mRNA molecule is called a
codon.
Most amino acids have more than one codon; there are
20 amino acids with a possible 64 different triplets.
The code is nearly universal among living organisms.
64. The Amino Acids
Amino acids are the building blocks of proteins.
A protein forms via the condensation of amino acids to form a chain of amino
acid "residues" linked by peptide bonds.
There are 20 standard amino acids in a genetic code. These are:
1. Phenylalanine
2. Serine
3. Leucine
4. Tyrosine
5. Cysteine
6. Tryptophan
7. Proline
8. Histidine
9. Glutamine
10. Arginine
11. Isoleucine
12. Methionine
13. Threonine
14. Asparagine
15. Lysine
16. Valine
17. Alanine
18. Aspartate
19. Glutamate
20.Glycine
70. Central Concept or " The Central Dogma"
The central concept of genetics involves the
DNA-to-protein sequence involving
transcription and translation.
DNA has a sequence of bases that is
transcribed into a sequence of bases in mRNA.
Every three bases is a codon that stands for a
particular amino acid.
73. Transcription
During transcription in the nucleus, a
segment of DNA unwinds and unzips,
and the DNA serves as a template for
mRNA formation.
RNA polymerase joins the RNA
nucleotides so that the codons in
mRNA are complementary to the
triplet code in DNA.
78. Translation
Translation is the second step by which gene
expression leads to protein synthesis.
During translation, the sequence of codons in
mRNA specifies the order of amino acids in a
protein.
Translation requires several enzymes and two
other types of RNA: transfer RNA and
ribosomal RNA.
79.
80.
81. Review of Gene Expression
DNA in the nucleus contains a triplet code; each
group of three bases stands for one amino acid.
During transcription, an mRNA copy of the DNA
template is made.
The mRNA is processed before leaving the
nucleus.
The mRNA joins with a ribosome, where tRNA
carries the amino acids into position during
translation.
