Transcript CHAPTER 1 Genetics An Introduction

Peter J. Russell
A molecular Approach 2nd Edition
CHAPTER 1
Genetics: An Introduction
edited by Yue-Wen Wang Ph. D.
Dept. of Agronomy,台大農藝系
NTU
遺傳學 601 20000
Chapter 1 slide 1
Classical and Modern Genetics
1. Humans have long understood that offspring tend
to resemble parents, and have selectively bred
animals and plants for many centuries. The
principles of heredity were first explained by
Mendel in the mid nineteenth century, using
defined crosses of pea plants.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 2
Classical and Modern Genetics
2. In the last century, genetics has become an important
biological tool, using mutants to gain an understanding of
specific processes. This work has included:
a. Analyzing heredity in populations.
b. Analyzing evolutionary processes.
c. Identifying genes that control steps in processes.
d. Mapping genes.
e. Determining products of genes.
f. Analyzing molecular features of genes and regulation of gene
expression.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 3
Classical and Modern Genetics
3. Recent important milestones in genetics include:
a. Berg’s construction (1972) of the first recombinant
DNA molecule in vitro.
b. Boyer and Cohen’s first cloning (1973) of a
recombinant DNA molecule.
c. Invention by Mullis (1986) of the polymerase chain
reaction (PCR) to amplify specific DNA sequences
台大農藝系 遺傳學 601 20000
Chapter 1 slide 4
Classical and Modern Genetics
4. Completion of genomic sequencing for an
increasing number of organisms has spawned the
new field of genomics. Knowledge of individual
genes and their regulation will be important to
basic biological research, as well as to specific
applications such as medical genetics.
5. Powerful new techniques in genetics raise
important ethical, legal and social issues that will
need thoughtful solutions.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 5
Basic Concepts of Genetics
The concepts and processes of genetics summarized
here are intended as a review from the
introductory biology course.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 6
DNA, Genes and Chromosomes
1. Genetic material of both eukaryotes and
prokaryotes is DNA (deoxyribonucleic acid).
Many viruses also have DNA, but some have
RNA genomes instead.
2. DNA has two chains, each made of nucleotides
composed of a deoxyribose sugar, a phosphate
group and a base. The chains form a double helix
(Figure 1.1).
台大農藝系 遺傳學 601 20000
Chapter 1 slide 7
Fig. 1.1 DNA
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 8
DNA, Genes and Chromosomes
3. There are four bases in DNA: A (adenine), G
(guanine), C (cytosine) and T (thymine).
a. In RNA, U (uracil) replaces T.
b. The sequence of bases determines the genetic
information.
c. Genes are specific sequences of nucleotides that pass
traits from parents to offspring.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 9
DNA, Genes and Chromosomes
4. Genetic material in cells is organized into
chromosomes (literally “colored body” because it
stains with biological dyes).
a. Prokaryotes generally have one circular chromosome.
b. Eukaryotes generally have:
i. Linear chromosomes in their nuclei, with different
species having different numbers of chromosomes.
ii. DNA in organelles (e.g., mitochondria and
chloroplasts) that is usually a circular molecule.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 10
Transmission of Genetic Information
1. Transmission of traits from parents to offspring was
addressed in Mendel’s work with peas.
a. He selected strains that differed in particular traits (e.g., smooth
or wrinkled seeds, purple or white flowers) (Figure 1.2).
b. After making genetic crosses, he counted the appearance of traits
in the progeny and analyzed the results mathematically.
c. He concluded that each organism contains two copies of each
gene, one from each parent, and that alternative versions of the
genes (alleles) exist (e.g., pea seed color alleles are yellow, Y,
and green, y).
台大農藝系 遺傳學 601 20000
Chapter 1 slide 11
Transmission of Genetic Information
2. An organism that has the same alleles for a trait is
homozygous (e.g., YY or yy). An organism with
two different alleles (e.g., Yy) is heterozygous.
3. The complete genetic makeup of an organism is
its genotype. All observable traits of an organism
are its phenotype. The genotype interacts with
both internal and external environments of the
organism to produce the phenotype.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 12
Transmission of Genetic Information
4. Mendel considered the factors controlling the phenotypes he observed in
peas.
a. He deduced that the factors (now called genes) segregate randomly into
gametes (Mendel’s first law, the Principle of Segregation).
b. The two factors for a particular trait assort independently of factors
controlling other traits (Mendel’s second law, the Principle of
Independent Assortment).
c. An example is seed color in peas:
i. True-breeding plants with yellow seeds (YY) are crossed with truebreeding plants with green seeds (yy).
ii. The progeny (F1) have yellow seeds, and a heterozygous genotype
(Yy).
iii. When the progeny self-pollinate, the F2 contains threeyellow:1
green, with genotypic ratios of 1 YY : 2 Yy : 1 yy.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 13
Transmission of Genetic Information
5. Mendel died in 1884, the material basis of gene
segregation was shown until 1902.
6. In 1902, Sutton and Boveri proposed that genes
are on chromosomes and their movement
explainable by the segregation of chromosomes
during meiosis.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 14
Expression of Genetic Information
1. Gene expression is the process by which a gene produces
its product and the product carries out its function.
2. Beadle and Tatum (1941) showed in the fungus
Neurospora crassa that there is a relationship between a
gene and each enzyme needed in a biochemical pathway,
resulting in the one gene-one enzyme hypothesis (now
modified to one gene-one polypeptide, since not all
proteins are enzymes and some require more than one
polypeptide).
台大農藝系 遺傳學 601 20000
Chapter 1 slide 15
Expression of Genetic Information
3. Production of proteins requires two steps:
a. Transcription involves an enzyme (RNA polymerase) making an
RNA copy of part of one DNA strand. There are four main
classes of RNA:
i. Messenger RNAs (mRNA), which specify the amino acid
sequence of a protein by using codons of the genetic code.
ii. Transfer RNAs (tRNA).
iii. Ribosomal RNAs (rRNA).
iv. Small nuclear RNAs (snRNA), found only in eukaryotes.
b. Translation converts the information in mRNA into the amino
acid sequence of a protein using ribosomes, large complexes of
rRNAs and proteins.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 16
Fig. 1.3 Transcription
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 17
Expression of Genetic Information
台大農藝系 遺傳學 601 20000
Chapter 1 slide 18
Expression of Genetic Information
4. Only some of the genes in a cell are active at any given
time, and activity also varies by tissue type and
developmental stage. Regulation of gene expression is not
completely understood, but it has been shown to involve
an array of controlling signals.
a. Jacob and Monod (1961) proposed the operon model to explain
prokaryotic gene regulation, showing that a genetic switch is
used to control production of the enzymes needed to metabolize
lactose. Similar systems control many genes in bacteria and their
viruses.
b. Genetic switches used in eukaryotes are different and more
complex, with much remaining to be learned about their
function.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 19
Sources of Genetic Variation
Genetic differences between organisms arise from mutations, recombination
and selection. All three are necessary for the process of evolution.
a. Mutations (heritable changes in the genetic material) may be spontaneous
or induced. Only those that escape the cell’s DNA repair mechanisms are
fixed in the genome and passed to the next generation.
b. Recombination (exchange of genetic material) is produced by enzymes
that cut and rejoin DNA molecules.
i. In eukaryotes, recombination via crossing-over is common in meiosis
and occurs more rarely in mitosis.
ii. In prokaryotes, recombination may occur when two DNA molecules
with similar sequences become aligned.
c. Selection (favoring particular combinations of genes in a given
environment) was described by Darwin. Its main consequence is to
change the frequency of genes affecting traits under selection. Different
genotypes contribute alleles to the next generation in proportion to their
selective advantage.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 20
Geneticists and Genetics Research
1. Enormous amounts of genetic research have been
done, typically using the hypothetico-deductive
method of investigation, which consists of:
a. Making observations.
b. Forming hypotheses to explain the observations.
c. Making experimental predictions based on the
hypotheses.
d. Testing the predictions, resulting in new observations
and another cycle of research.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 21
Geneticists and Genetics Research
2. Research is unpredictable, which helps motivate
scientists by making the work exciting. (An
example of unpredictability is McClintock’s work
with corn kernel color, which led to the discovery
of transposons).
台大農藝系 遺傳學 601 20000
Chapter 1 slide 22
The Sub-disciplines of Genetics
1. Genetics is often divided into four subdisciplines:
a. Transmission (classical) genetics deals with movement of genes
and genetic traits from parents to offspring, and with genetic
recombination.
b. Molecular genetics deals with the molecular structure and
function of genes.
c. Population genetics studies heredity in groups for traits
determined by one or a few genes.
d. Quantitative genetics studies group hereditary for traits
determined by many genes simultaneously.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 23
The Sub-disciplines of Genetics
2. Historically, transmission genetics developed
first, followed by population, quantitative and
finally molecular genetics.
3. Genes influence all aspects of an organism’s life,
and are relevant to all fields of biology.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 24
Basic and Applied Research
1. Basic research is done to understand fundamental
phenomena, regardless of usefulness for immediate
applications. Most of the information in this book comes
from basic research. The results of basic research are used
to fuel basic and applied research.
2. Applied research has the goal of an immediate
application, and is important in agriculture and medicine,
producing improved livestock and crop plants, as well as
diagnostic tests and treatments for diseases.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 25
Basic and Applied Research
3. Basic and applied research are closely related, using similar techniques.
Both rely on the accumulated body of information. Recombinant DNA
technology is an example of basic research that has led to many
applications, including:
a. Plant breeding to improve disease resistance, shelf life and flavor.
b. Animal breeding to develop livestock that produce leaner meat, and
more milk or eggs.
c. Medicines including antibiotics, hormones, clotting factors and human
insulin.
d. Diagnostic tests for many human diseases.
e. Forensics techniques that are used in paternity testing, criminal cases
and anthropological studies.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 26
Genetic Databases and Maps
1. Genetic databases have become more sophisticated as computer
analysis tools have been developed. The National Center for
Biotechnology Information (NCBI) is an important website for genetics
(http://www.ncbi.nlm.nih.gov), which includes the following search
tools:
a. BLAST, a tool to compare nucleotide or protein sequences.
b. GenBank, an annotated DNA sequence database.
c. PubMed, which searches literature citations and abstracts and links to
electronic versions of journals.
d. Online Mendelian Inheritance in Man (OMIM), a database of human
genes and genetic disorders.
e. Entrez is a system for searching linked databases.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 27
Genetic Databases and Maps
2. Genetic maps have been constructed since 1902.
They show the sites of genes (loci) on
chromosomes, and genetic distances between
them calculated from recombination in
experimental crosses. Genetic maps can show
whether genes with related functions are on the
same chromosome, and are useful in cloning and
genome sequencing.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 28
Fig. 1.6 Example of a genetic map, here some of the genes on chromosome 2 of the
fruit fly, Drosophila melanogaster
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 29
Model Organisms
1. Many organisms are used in genetic research.
Desirable qualities for an experimental organism
include:
a. A well-known genetic history.
b. A short life cycle so generations can be studied in a
relatively short time.
c. A large number of offspring from each mating.
d. Ease of growing and handling the organism.
e. Marked genetic variation within the population.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 30
2. Eukaryotes keep their DNA in the nucleus, a discrete
structure bounded by a nuclear envelope (absent in
prokaryotes).
3. Eukaryotes can be unicellular or multicellular.
4. These eukaryotes are used in much of current genetic
research:
a. Saccharomyces cerevisiae (a unicellular baking yeast).
b. Drosophila melanogaster (fruit fly).
c. Caenorhabditis elegans (a nematode worm).
d. Arabidopsis thaliana (a small weed in the mustard family).
e. Mus musculus (mouse).
f. Homo sapiens (human).
台大農藝系 遺傳學 601 20000
Chapter 1 slide 31
5. Additional eukaryotes that have made important
contributions in genetics include:
g. Neurospora crassa (orange bread mold).
h. Tetrahymena (unicellular protozoa).
i. Paramecium (unicellular protozoa).
j. Chlamydomonas reinhardtii (unicellular green alga).
k. Pisum sativum (garden pea).
l. Zea mays (corn).
m. Gallus (chicken).
Figure 1.7
台大農藝系 遺傳學 601 20000
Chapter 1 slide 32
台大農藝系 遺傳學 601 20000
Chapter 1 slide 33
Fig. 1.8 Eukaryotic cells
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 34
6. Generalized features of higher plant and animal
cells are (Figure 1.8):
a. A plasma membrane encloses the cytoplasm in both.
b. Plant cells have a rigid cell wall.
c. In both, the nucleus contains DNA complexed with
proteins and organized into chromosomes.
d. The nuclear envelope is two layers of semipermeable
membrane with pores that allow movement of
materials (e.g., ribosomes) between nucleoplasm and
cytoplasm.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 35
e. The cytoplasm contains many materials and organelles. Important in
genetics are:
i. Centrioles (basal bodies) are in cytoplasm of nearly all animals, but not
in most plants. In animals, a pair of centrioles is associated with the
centrosome region of the cytoplasm where spindle fibers are organized
in mitosis or meiosis.
ii. The endoplasmic reticulum (ER) is a double membrane system that
runs through the cell. ER with ribosomes attached collects proteins that
will be secreted from the cell or localized to an organelle.
iii. Ribosomes synthesize proteins, either free in the cytoplasm or attached
to the cytoplasmic side of the ER.
iv. Mitochondria are large organelles surrounded by double membrane that
play a key role in energy processing for the cell. They contain their own
DNA encoding some mitochondrial proteins, rRNAs and tRNAs.
v. Chloroplasts are photosynthetic structures that occur in plants. The
organelle has a triple membrane layer, and includes a genome encoding
some of the genes needed for organelle functions.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 36
7. Prokaryotes have no nuclear envelope. All bacteria are
prokaryotes, and most are single-celled, with their shape
maintained by a rigid cell wall outside the cell membrane
(Figure 1.9).
a. Bacteria are divided into two distantly related groups:
i. Eubacteria, common organisms found in other
organisms and in the environment, and the type most
often studied. E. coli is in this group.
ii. Archaebacteria, normally found in extreme
environments (e.g., hot springs, salt or methane
marshes, deep ocean).
b. Bacteria generally range in size from 100 nm to 10X60
mm. One species, Epulopiscium fishelsoni, is 60X800 mm,
a million times larger than E. coli.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 37
Fig. 1.9 Cutaway diagram of a generalized prokaryotic cell
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 1 slide 38