Chromosomes

Lecture #18

Honors BiologyMs. Day

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

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).

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)

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))

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

How Does DNA Fit into a Small Cell?

Prokaryotic organisms No nucleus;

instead have a nucleiod region

DNA = 1 chromosome

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”

DNA + histones form nucleosomes

(function = help to “super coil” DNA)

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.

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)

Chromosomes and Cell Division

In preparation for cell division (mitosis)DNA is replicated (single double chromosome)

V shape X shape

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

Double Chromosome Double Chromosome StructureStructure

Kinetochore attaches to

spindle fibers

Sister

Draw and Label the Pieces of a Duplicated Chromosome

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

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)

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

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)

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

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.

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