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ChromosomesPART 1
Honors Genetics
Ms. Gaynor
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Why is Cell Division Important?Unicellular organisms
Reproduce 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 (Mitosis) Makes 2 genetically identicalidentical
daughter cells from 1 parent cell
Before cells divide They duplicate their genetic
material ensures that 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 (in nucleus of eukaryotes)
can be in 2 formsChromatin : DNA is nnot tightly
packed together (loosely coiled)Occurs during interphaseOccurs during interphase
Chromosomes : tightly packed together (TIGHTLY coiled)Occurs during mitosis (cell Occurs during mitosis (cell division)division)
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Tightly coiled?
Not tightly coiled?
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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
(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
ReplicationReplication2. The DNA coils (wraps) around proteins
called histonehistone proteins. 3. Chromosomes are made.
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The DNA molecules in a cellAre packaged into chromosomes
50 µm
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Chromosome Chromosome StructureStructure
Two shapes of chromosomes1.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 division
0.5 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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Another view…
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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 have 46
chromosomes # of chromosomes does NOT
necessarily equal complexity
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Different Types of Cells There are 2 types of cells in the
human body1.1. 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 nucleusCytokinesisCytokinesis,division of cytoplasm,division of cytoplasmOccurs in somatic cellsOccurs in somatic cells
In In meiosismeiosisGamete cells are produced after a Gamete cells are produced after a
reduction in chromosome numberreduction in chromosome number
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Chromosome NumberChromosome Number Gametes (sperm/egg) have 1 copy of each
chromosome and are called haploid. haploid. SomaticSomatic (body) human cells have 2
copies of each chromosomes and are called diploid.diploid.Human somatic cells have 23 PAIRSPAIRS of
chromosomes (total 46 chromosomes). All pairs of chromosomes differ in size,
shapes, and set of genes.
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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 = 46Haploid egg cell Haploid sperm celln n
2nDiploid Diploid ZygoteZygote cell cell
FertilizationFertilization
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Set of ChromosomesSet of Chromosomes Each chromosome set contain 2
“homologues.” Chromosomes have a “twin” or
matching pair. Homologous ChromosomesHomologous Chromosomes=
chromosomes that are similar in shape, size, and genes.
Each homologue in a pair comes from each parent!