chapter 5
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ChromosomesChromosomesWhere Are the Genes Found?Where Are the Genes Found?
What Cellular Structure Holds the Genetic Information?
• Chromosomes– Contain the genetic
material: DNA, RNA
• Chromatin– Is the chromosomal
material in its decondensed, threadlike state.
Mitosis
• Form of asexual reproduction.
• Occurs when organism grows or replaces damaged cells.
• Prior to mitosis, cell undergoes replication.– Process in which chromatin
is copied.
• Produces diploid cells.
Prophase
• Start of mitosis• Chromatin condenses
into rod-like chromosomes– Each chromosome
consists of sister chromatids, connected at the centromere
• Nuclear membrane disappears
Metaphase
• Chromosomes align themselves in flat plane at cell equator.
Anaphase
• Centromeres split.• Sister chromatids-
now chromosomes- are pulled to opposite poles of the cell.
Telophase
• Chromosomes unravel, returning the chromatin to its non-dividing threadlike state.
• Nuclear membrane assembles.
Cytokinesis
• Division of the cytoplasm.
• Begins during anaphase and telophase.
Cytokinesis
• Differs in animals and plant cells.
• Plant cells form a cell plate.– membranous vesicles
congregate at center of cell.
– Vesicles contain cell wall material.
Cytokinesis
• Animal cells form a cleavage furrow.– Forms around the
periphery of the dividing cell.
– Furrow becomes deeper and deeper until membrane pinches off forming two cells.
Chromosomes Come in Matched Pairs
• Homologous pairs: chromosomes that are closely matched in size and shape– Determine the same
traits
• Sex chromosomes: Those that determine the gender of the organism.
Chromosomal Theory of Inheritance
• The two members of each pair of a homologous pair of chromosomes carry alleles for the same genes and, therefore, affect the same traits.
• Proposed in 1903.
Mapping genes
• Locus: location of a gene on a chromosome.
• Currently trying to map all human genes to the appropriate chromosome.
• Example: gene responsible for sickle cell anemia is located on chromosome 11.
Sexual reproduction
• Type of reproduction in which genetic information from female combines with male.
• Requires fertilization– Fusion of gametes
(egg and sperm)– These cells need to
have half the amount of genetic information.
Meiosis
• Produces haploid cells (gametes-eggs and sperm)– Posses only one member
of each pair of homologous chromosomes
• Chromosomes replicate before meiosis.
• Requires two rounds of division.
Prophase I
• Chromatin condenses into compact chromosomes.
• Nuclear envelope disappears.
• Synapsis occurs.– Homologous pairs of
chromosomes closely align allowing exchange of chromosome segments
Metaphase I
• Aligned pairs of replicated chromosomes move to the equator of the dividing cell.
Anaphase I
• Members of homologous pairs of chromosomes separate from each other
• They move to opposite poles of the cell.– Chromosomes
experience independent assortment.
Telophase I
• Chromosomes cluster at opposite poles of cell and begin to decondense
• Nuclear envelope may reform.
• Cytokinesis occurs• Interkinesis: phase
between meiosis I and and II.
• Daughter cells now haploid.
Prophase II
• Partially unraveled chromosomes condense again.
Metaphase II
• Chromosomes move to cell equator.
• No longer homologous pairs, so chromosomes line up singly in middle of cell.
Anaphase II
• Centromeres divide and chromosomes separate.
• Move to opposite poles of the cell.
Telophase II
• Clustered chromosomes at cell pole begin to decondense.
• Nuclear membrane develops.
• Cytokinesis occurs.• Produces four non-
identical haploid cells.
Mitosis vs. Meiosis
Cell Cycle
• Repetitive sequence of events that characterizes life of cell.
• Consists of two main phases:– Interphase
• Period that cells are in when not dividing.• 90% of cell cycle
– M phase• Includes mitosis and cytokinesis
Cell Cycle
Cell Cycle
• Interphase has three subphases– G1 (first gap)
• Cell makes copies of organelles and grows larger
– S (synthesis)• Genetic material is copied• End of this phase, cells chromosomes are doubled
– Copies are attached; thus total number of chromosomes remains the same
– G2 (second gap)• Cell prepares upcoming M phase
Regulation of Cell Cycle
• Cell cycle must be regulated or can result in cancer– Uncontrolled cell growth
• Metastasize: cell breaks free from original cancerous mass and resides in new area in the body
Regulation of Cell Cycle
• Two checkpoints– First between G1 and S
– Second between G2 and M
• To pass checkpoints, cell must possess appropriate amount of protein in cytoplasm.– These proteins activate
other proteins necessary for production of genetic material and mitosis
Regulation of Cell Cycle
• To pass checkpoints, cell must possess appropriate amount of regulating protein in cytoplasm.– When regulator concentration is high, cell
cycle progresses.– When low, cell cycle is suspended at that
stage.
• External and internal regulatory agents also influence passage through checkpoints.
Why So Some Genetic Traits Tend to Travel Together?
• Chromosomes contain genes– Chromosomes follow law of independent
assortment, not genes
• If two genes are on the same chromosome, – the two genes are inherited together or are
said to be linked or in linkage groups
Why So Some Genetic Traits Tend to Travel Together?
• Example: sex linkage• Sex chromosomes
– Contain other genes aside from those to determine gender.
• Example: eye color and gender in fruit flies
Chromosomes Can Exchange Segments During Meiosis
• Crossing over– Exchange of genetic
material between chromatids of homologous chromosomes.
– Occurs at the chiasmata.
Chromosomes Can Exchange Segments During Meiosis
• Crossing over– Important mechanism
for creating new combinations of genes.
– Disrupts linkage groups.
What Is the Chemical Nature of the Gene?
• 1860s– Frederich Meisner studied fundamental
constituents of life• Discovered unknown substance contains carbon,
nitrogen, oxygen, and phosphorus.• Found it came from nucleus of cell.• Named it nucelin
– His students renamed substance nucleic acid after finding it was acidic.
What Is the Chemical Nature of the Gene?
• 1881– Discovered nucleic acids were contained in
chromatin.
• Question: Was the genetic material made of proteins or nucleic acids?
DNA is the Genetic Material
• 1928– Fred Griffiths, medical officer for British
Ministry of Health• Studied the bacteria pneumoncoccus
– Two kinds» Smooth: Virulent form that appears smooth and
shiny when grown on agar plate» Rough: harmless form that appears rough when
grown on agar plate.
• Experimented by injecting the two types in mice.
DNA is the Genetic Material
DNA is the Genetic Material
• Griffiths identified the material as the transforming principle
• Avery, MacLeod and McCartney– Study transforming principle for 20 years.– They determined that the transforming agent
was DNA.
DNA is the Genetic Material
• Alfred Hershey and Martha Chase– Studied viruses that infect bacteria
• Viruses are called bacteriophages
DNA is the Genetic Material
• Viruses are made of • Protein coat and nucleic acid
• Viruses mix their genes with host genes– hijack cell machinery and use it to produces
new viruses– Usually kills host cell
DNA is the Genetic Material
• Hershey and Chase labeled protein and DNA differently with isotopes– Variants of elements that share same chemical
properties but differ in number of neutrons
• Label DNA with 32P and protein with 35S
DNA is the Genetic Material