chromosomes lecture #18 honors biology ms. day. why is cell division important? unicellular...
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Chromosomes
Lecture #18
Honors BiologyMs. Day
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Why is Cell Division Important?
Unicellular organismsReproduce by cell division
increasing the population. 100 µm
(a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM).
Figure 12.2 A
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Why Do Multicellular Organisms Depend on Cell Division?
Development & Growth Repair (ex: tissue renewal)
Maintenance 20 µm200 µm
(b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM).
(c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).
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Cell Division (called Mitosis)
Makes 2 genetically identicalidentical daughter cells from 1 parent cell (think= mi “two” sis)
Before cells divide They duplicate their DNA so
each daughter cell receives an exact copy of the genetic material (DNA)
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Organization of the Organization of the Genetic MaterialGenetic Material
All of a cell’s DNA (genetic All of a cell’s DNA (genetic information) is called its information) is called its genomegenomeIn prokaryotesIn prokaryotes
Genome=single, long DNA Genome=single, long DNA molecule in a circlemolecule in a circle
In eukaryotesIn eukaryotesGenome = several DNA Genome = several DNA molecules grouped in clumps molecules grouped in clumps (called (called chromosomeschromosomes))
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DNA Molecules DNA can be in 2 forms
Chromatin : DNA is nnot tightly packed together (loosely coiled; decondensed)Occurs during Occurs during iinterphasenterphaseThink = chromatThink = chromatiinn
Chromosomes : tightly packed together (TIGHTLY coiled; condensed)Occurs during Occurs during mmitosisitosisThink = chromosoThink = chromosommeses
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How Does DNA Fit into a Small Cell?
Prokaryotic organisms No nucleus;
instead have a nucleiod region
DNA = 1 chromosome
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How Does DNA Fit into a Small Cell?
Eukaryotic organisms split their DNA into several pieces (called chromosomes) stored within the cell's nucleus
DNA is coiled around proteins known as histones histones Uses nucleosomesnucleosomes to form a
“supercoil”
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DNA + histones form nucleosomes
(function = help to “super coil” DNA)
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Making ChromosomesMaking Chromosomes
**Occurs right before before cell division (mitosismitosis)
Steps:
1. DNA copies itself through DNA DNA ReplicationReplication
2. The DNA coils (wraps) around proteins called histonehistone proteins.
3. Chromosomes are made.
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Chromosome Chromosome StructureStructure
Two shapes of chromosomes
1.Single chromosome (1 copy of DNA); “V” shaped
2.Double Chromosome (2 copies of DNA); “X” shaped; a.k.a-duplicated chromosome
Occurs right Occurs right before cell before cell
division division (mitosis)(mitosis)
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Chromosomes and Cell Division
In preparation for cell division (mitosis)DNA is replicated (single double chromosome)
V shape X shape
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Each duplicated chromosomehas two sister chromatids, which separate
during cell division0.5 µm
Chromosomeduplication(including DNA synthesis)
Centromere
Separation of sister
chromatids
Sisterchromatids
Centromeres Sister chromatids
A eukaryotic cell has multiplechromosomes, one of which is
represented here. Before duplication, each chromosome
has a single DNA molecule.
Once duplicated, a chromosomeconsists of two sister chromatids
connected at the centromere. Eachchromatid contains a copy of the
DNA molecule.
Mechanical processes separate the sister chromatids into two chromosomes and distribute
them to two daughter cells.
Figure 12.4
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Double Chromosome Double Chromosome StructureStructure
Kinetochore attaches to
spindle fibers
Sister
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Draw and Label the Pieces of a Duplicated Chromosome
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Chromosomes Chromosomes Every eukaryotic species has
a characteristic, uniquecharacteristic, unique # of chromosomes in EACH cell nucleusEx: Humans = 46 chromosomes
Adler Fern = over 2000 C’s # of chromosomes does NOT
necessarily equal complexity
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Different Types of Cells There are 2 types of cells in an
organism
1.1. Somatic cellsSomatic cells: all body cells
2. GametesGametes: reproductive cells (sperm and egg)
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Different cell types Different cell types different different cell divisionscell divisions
Eukaryotic Eukaryotic cell division cell division consists ofconsists ofMitosisMitosis = division of the nucleus = division of the nucleus
Called Called karyokinesiskaryokinesisCytokinesisCytokinesis = division of cytoplasm = division of cytoplasmOccurs in somatic cellsOccurs in somatic cells
In In meiosismeiosisGamete cells are produced after Gamete cells are produced after
chromosome number is cut in ½ chromosome number is cut in ½
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Chromosome NumberChromosome NumberGametes (sperm/egg) have 1
copy of each chromosome called haploid (1n)haploid (1n)
SomaticSomatic (body) have 2 copies of each chromosomescalled diploid (2n)diploid (2n)
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Haploid vs. DiploidHaploid vs. Diploid Haploid= when a cell has 1 copy of
each chromosome. Expressed as n = 23
Diploid= when a cell has 2 copy of each chromosome. (a.k.a TOTAL # of C’s in organisms)
Expressed as 2n = 46
Haploid egg cell Haploid sperm celln n
2nDiploid Diploid ZygoteZygote cell cell
FertilizationFertilization
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Pairs of ChromosomesPairs of Chromosomes Chromosomes have a “twin” (matching
pair)Ex: Human somatic cells have 23 PAIRSPAIRS of
chromosomes (total 46 C’s)All pairs of chromosomes differ in size,
shapes, and set of genes.Homologous Homologous
Chromosomes Chromosomes = chromosomes
that are similar in shape, size,
and genes.
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Set of ChromosomesSet of Chromosomes Each chromosome set contain 2
“homologues” or “twins” Each homologue is also called an autosome
A chromosome that does NOT determine gender Each homologue in a pair comes from each
parent!
Also have a “sex chromosome” pair Chromosomes that DO determine gender
fertilization
ZYGOTE = 1st diploid body cell of organism