• W. Flemming, 1880s found
clues to physical nature of gene
– Had
fine scope lenses
– Used chemical dyes that cling to certain cellular structures
– Published drawings of dividing cells from salamander embryos
• Chromosomes are apportioned to
daughter cells during mitosis
–
Flemming named mitosis sequence of events during cell division
– Occurs in eukaryotic cells & is used for growth, repair, cell replacement
– Involves vast reorganization of cell interior
• Chromosomes come in matched
pairs
• Their number is characteristic of species
–
Humans – 46
– Chimpanzee – 48
– Fruit flies – 8
• Human cells have 23
homologous pairs
• After mitosis
– 2
daughter cells
– Each have 46 chromosomes with 23 homologous pairs
• In males of some species
– 1
pair of homologous chromosomes is not exactly homologous
– Y vs. X – sex chromosomes
• Walter Sutton:
– “The
two members of each homologous pair of chromosomes carry alleles for the same
genes & therefore affect the same traits.”
• Different alleles are called
heterozygous
• The same alleles are called homozygous
• Gene
–
Segment of DNA that determines a particular trait
• Locus (loci, plural)
– Where
that gene is located on a particular chromosome
• Allele
–
Variation of the gene’s trait that is expressed
• Sexual reproduction where
– Male
and female gametes
– Fuse to form zygote (fertilization)
• Number of chromosomes
donated by male and female
– Are
equal, but
– Chromosome number within species stays constant
• Meiosis is the prelude to
sexual reproduction
–
Multiplies number of cells but
– Also reduces chromosome number in each daughter cell to
• Exactly half the number
present before meiosis
•
Meiosis is the prelude to sexual reproduction
– Daughter cells get
• 1 member of each homologous
pair, i.e. 1 allele for each gene
–
Mitosis produces 2 daughter cells
– Meiosis produces 4 daughter cells
• Cells with both members of
each homologous pair are diploid
– All
body cells in humans are diploid, except gametes
– Cells with 1 member of each homologous pair are haploid
– Meiosis produces 4 daughter cells
• Mitosis, meiosis, and
cytokinesis are single steps in cell cycle
– Cells
not in process of dividing are in interphase
– Chromosomes are duplicated in preparation for the next round of division
during interphase
• M phase
– Chromosomes are
• Separated into 2 nuclei
(mitosis) and
• Nuclei are apportioned into 2 separate cells (cytokinesis)
• Interphase
– Period of
• Active metabolism growth,
• Processing of nutrients and
• Energy
• The cell cycle is highly
regulated
• If a cell breaks free of its parent organ it starts to grow uncontrollably –
metastasize
• Cell cycle control is focused at 2 places
–
Before genetic material synthesis
– At transition between G2 and M phase
• Regulating agents are
proteins whose concentrations rise & fall in a controlled manner
• If agent concentration is high
– Cycle
progresses; if it is low,
– Cycle is suspended
• Cell cycle control – focused
2 places
• Regulating agents can be
controlled by proximity of other cells
• Regulating agents can be controlled by factors inside the cell itself
• Internal & external
stimuli activate or inhibit regulating agents
• These, in turn activate enzymes to help cells divide
• Internal checkpoints & guardians monitor cell health
• Errors in this process can lead to uncontrollable growth and cancer
• Meiosis lead to the
chromosomal theory of inheritance
• But there was a lack of correlation between the number of traits and the
number of chromosomes
• Grouping of many genes, on 1 chromosome explains why 7 traits studied by
Mendel assorted independently
• Results from Morgan’s lab
suggested all traits in a linkage group are not inherited together
• Phenomenon called crossing-over, Parts of one member of pair cross over
to homologue
• In 1940s, B. McClintock found evidence that certain genes jump from place to
place in chromosomes
• In 1960s, others found evidence of jumping genes, calling them transposons
• Chromosomes are both protein
and nucleic acid
• In 1860s, nucleic acid was found to have properties in common with chromatin
• By 1881, nucleic acid was localized to chromosomes and found in every
organism
• Protein seemed at the time to be a better candidate for genetic material
• Protein is the most complex, varied, and ubiquitous substance in living cells
• Protein plays many roles in cells and is present in the chromosome
• Inheritance of
polysaccharide capsule in Pneumococcus
• Pneumococcus occurs in 2 forms:
– S
form – highly virulent
– R form – harmless
• Difference between R & S
forms: polysaccharide capsule in S form
• In
1928, F. Griffith studied S & R pneumococcus on mice and found
–
Harmless R bacteria transformed into virulent S pneumococci
– When it was mixed with heat-killed S bacteria
• Further studies (1944) found
that DNA was the transforming agent
• Nucleic acids
–
Deoxyribonucleic acid (DNA) and
– Ribonucleic Acid (RNA)
• Each made from nucleotides
composed of 3 building blocks:
– A
phosphate group
– Five-carbon sugar
– Nitrogen-containing base
• Nucleotides are hooked
together by condensation-dehydration reactions
• Both DNA and RNA have phosphate groups
• Sugar in RNA - ribose, sugar in DNA - deoxyribose
• Both DNA and RNA use the base cytosine, adenine, guanine, but
• RNA has the pyrimidine uracil, while DNA contains the pyrimidine thymine
instead