chapter 5

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