BIOL 112 Lecture Notes - Lecture 7: Karyotype, Dna Replication, Centromere
16 May 2018
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BIOL 112- Lecture 11- The Cell Cycle
History:
Mystery of heredity is to reconcile/explain the following observations
•!All organisms come from other organisms
•!All organisms resemble their parents
•!Siblings are not identical
Ancient Greeks reasoned parents must share some “essence” that mixes to produce
children o Eventually realized that sperm contains cells, which led to cell theory
Chromosomes:
Chromosome: a single string of DNA
•!Come either circular (bacteria) or linear (most
other organisms)
When the cell is about to divide, chromosome condenses by
associating with histones (proteins)
Chromatin: combination of DNA and protein
DNA winds around histones, which itself gets wound, etc.
Homologs: pairs of chromosomes
•!In eukaryotes, chromosomes normally come this
way
•!Just before cell division, each chromosome is
replicated once and forms two chromatids
Centromeres: hold chromatids together
Karyotype: way of organizing chromosomes with staining
•!Flatten a cell about to divide, stain chromosomes,
take a picture, line up chromosomes according to
banding pattern
o!Cells have to be dividing because only
then are chromosomes visible
•!Two pieces of DNA (two chromatids) bound by a
centromere seen in a karyotype
o!Chromosomes given the name mitotic chromosomes to indicate
they represent special case
o!92 chromosomes represented in a karyotype
•!Number of chromosomes is characteristic of a species of organism
o!E.g. humans have 46
o!E.g. bacteria typically have one
•!Autosome – non-sex chromosome
•!Cells need exactly one of each chromosome
o!Only case where a human with
incorrect number of autosomes
can survive is Down, which comes
from an extra chromosome 21
•!Can’t generally see which alleles
chromosome has
o!Exception: sex chromosomes,
which are different enough that
we can tell
Probability Theory:
•!Little effort to divide cytoplasm; lots to divide chromosomes
•!The probability that both daughter cells will each get exactly one chromatid
by chance is extremely low
•!Extremely high chance that both cells will get at least one mitochondrion,
since there are so many
o!Mitochondria reproduce by binary fission; one is enough
Cell Theory:
•!All organisms consist of cells
•!Cells divide to produce new cells
•!Higher organisms fuse their cells to produce a new
organism (includes sperm and egg or pollen and egg)
Steps of the cell cycle:
1.!Chromosome (DNA) replication: S phase
2.!Either mitosis or meiosis
•!Mitosis (M phase) – the process by which somatic cells make identical
copies (clones) of themselves by creating daughter cells that inherit
one copy of each chromosome
•!Meiosis – the process by which germ cells make non-identical copies
of themselves by creating daughter cells that have one of each
homolog
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BIOL 112 Full Course Notes
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Document Summary
Mystery of heredity is to reconcile/explain the following observations: all organisms come from other organisms, all organisms resemble their parents, siblings are not identical. Ancient greeks reasoned parents must share some essence that mixes to produce children o eventually realized that sperm contains cells, which led to cell theory. Chromosome: a single string of dna: come either circular (bacteria) or linear (most other organisms) When the cell is about to divide, chromosome condenses by associating with histones (proteins) Dna winds around histones, which itself gets wound, etc. In eukaryotes, chromosomes normally come this way: just before cell division, each chromosome is replicated once and forms two chromatids. Karyotype: way of organizing chromosomes with staining: flatten a cell about to divide, stain chromosomes, take a picture, line up chromosomes according to banding pattern o cells have to be dividing because only then are chromosomes visible. Probability theory: little effort to divide cytoplasm; lots to divide chromosomes.
