UNIT ____: Genes and Genomes Name: _____________________
Essential Idea(s):
Chromosomes carry genes in a linear sequence that is shared by members of a species.
IB Assessment Statements
3.1.U1 | A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic.
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3.1.U2 | A gene occupies a specific position on a chromosome.
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3.1.U6 | The genome is the whole of the genetic information of an organism
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3.1.U3 | The various specific forms of a gene are alleles.
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3.1.U4 | Alleles differ from each other by one or only a few bases.
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3.1.U5 | New alleles are formed by mutation.
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3.4.U10 | Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer.
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3.4.A4 | Consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl.
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3.1.U7 | The entire base sequence of human genes was sequenced in the Human Genome Project
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7.1.A3 | Use of nucleotides containing dideoxyrubonucleic acid to stop DNA replication in preparation of samples for base sequencing
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3.1.NOS | Developments in scientific research follow improvements in technology-gene sequencers are used for the sequencing of genes.
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3.2.S1 | Use of databases to identify the focus of a human gene and its polypeptide product.
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3.1.A2 | Comparison of the number of genes in humans with other species.
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3.1.S1 | Use of a database to determine differences in the base sequence of a gene in two species.
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7.3.NOS | Developments in scientific research follow improvements in computing- the use of commuters has enabled scientists to make advances in bioinformatics applications such as locating genes within genomes and identifying conserved sequences.
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Genome | Gene | Locus | Allele |
Alleles
Examples:
CFTR gene provides instructions for making a protein called the cystic fibrosis transmembrane conductance regulator. Locus: | F (dominant) | f (recessive) | ||
The HTT gene provides instructions for making a protein called huntingtin. Locus: | h (recessive) | H (dominant) | ||
Sickle cell disease is caused by a mutation in the HBB gene (hemoglobin-Beta). Locus: |
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The ABO blood type is controlled by the “I” gene (isoaglutinogen). Locus: | IA (dominant) | IB (dominant) | i (recessive) | |
Note: even though there can be more than two alleles in the population, there are a maxiumum of two alleles in any single individual. Why?
New alleles form through MUTATION:
Because so many diseases are associated with mutations, it is common for mutations to have a negative connotation. However, while many mutations are indeed deleterious, others are "silent"; that is, they have no discernible effect on the phenotype of an individual. In addition, some mutations are actually beneficial. For example, the very same mutation that causes sickle-cell anemia in affected individuals (i.e., those people who have inherited two mutant copies of the beta globin gene) can confer a survival advantage to unaffected carriers (i.e., those people who have inherited one mutant copy and one normal copy of the gene, and who generally do not show symptoms of the disease) when these people are challenged with the malaria pathogen. As a result, the sickle-cell mutation persists in populations where malaria is endemic.
Beyond the individual level, perhaps the most dramatic effect of mutation relates to its role in evolution; indeed, without mutation, evolution would not be possible. This is because mutations provide the "raw material" upon which the mechanisms of natural selection can act. By way of this process, those mutations that furnish individual organisms with characteristics better adapted to changing environmental conditions are passed on to offspring at an increased rate, thereby influencing the future of the species.
The DNA in any cell can be altered through environmental exposure to certain chemicals, ultraviolet radiation, other genetic insults, or even errors that occur during the process of replication. If a mutation occurs in a germ-line cell (one that will give rise to gametes, i.e., egg or sperm cells), then this mutation can be passed to an organism's offspring. This means that every cell in the developing embryo will carry the mutation. As opposed to germ-line mutations, somatic mutations occur in cells found elsewhere in an organism's body. Such mutations are passed to daughter cells during the process of mitosis (Figure 2), but they are not passed to offspring conceived via sexual reproduction.
Spontaneous | Physical | Chemical | Biological |
(i.e. radiation) | (i.e. heavy metals) |
Effects of Radiation
Case Study | What? | When? | Effect? |
Hiroshima | |||
Chernobyl |
Wild type | Nonsense |
Silent | Frameshift |
Missense | Frameshift |
The Sequencing and Mapping of Entire Genomes
AIMS OF THE HUMAN GENOME PROJECT: Visit the website http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml. Record the bulleted summaries of the goals of the Human Genome Project within this box. |
METHOD OF THE HUMAN GENOME PROJECT: Dr. Fred Sanger developed the most commonly used sequencing technique, called chain termination sequencing, in 1977. View the animation at http://www.dnalc.org/ddnalc/resources/cycseq.html and answer the following questions:
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DNA Sequencing
Bioinformatics and DNA Sequence Alignment:
What?
Bioinformatics | Sequence Alignment |
Why?
One reason we would do this is to determine what parts of the sequences are conserved from one species to the next. Another reason would be to see how much an organism has diverged from other organisms simply by comparing their DNA sequences. The more similar two gene sequences are to one another, the more closely the organisms are related. And the more dissimilar the two sequences, the farther the two genes are in evolutionary relationship.
Comparison of Genomes
Organism | Sketch | Genome Size | # of Genes |
T2 phage | |||
Escherichia coli | |||
Drosophila melanogaster | |||
Homo sapiens | |||
Paris japonica |
What is the relationship between organism “complexity,” genome size and the total number of genes?
