Date post: | 14-Dec-2015 |
Category: |
Documents |
Upload: | maribel-lupton |
View: | 217 times |
Download: | 1 times |
Cell Division
Essential for body growth and tissue repair
Interphase G1 phase
Primary cell growth phase S phase
DNA replication G2 phase
Microtubule synthesis
Mitosis
Nuclear division
Cytokinesis Cytoplasmic division
Chromosomes: Carriers of Inherited Traits
Chromosomes were first observed by the German embryologist Walther Fleming in 1882
Chromosomes exist in somatic cells as pairs Homologous chromosomes or homologues
The number of chromosomes varies enormously from species to species The Australian ant Myrmecia spp. has only 1 pair Some ferns have more than 500 pairs
Human Chromosomes
Humans have 46 chromosomes
This display is termed a karyotype
The 23 pairs of homologous chromosomes can be organized by size
Basic Terminology
Diploid cells have two copies of each chromosome Replicated chromosomes consist of two sister chromatids
These are held together at the centromere
Chromosomes
Chromosomes are composed of chromatin Complex of DNA (~ 40%) and proteins (~ 60%)
A typical human chromosome contains about 140 million nucleotides in its DNA This is equivalent to
About 5 cm in stretched length 2,000 printed books of 1,000 pages each!
In the cell, however, the DNA is coiled
Chromosomes: Packaged DNA
The DNA helix is wrapped around positively-charged proteins, called histones 200 nucleotides of DNA coil around a core of eight histones, forming a
nucleosome The nucleosomes coil into solenoids Solenoids are then organized into looped domains The looped domains appear to form rosettes on scaffolds
Mitosis
The phases of mitosis are:
Prophase
Xm
Xf
Xm
Xf
Xm
Xf
Condense
Metaphase
Xm
XfXmXf
Xm
Xf
Line Up
Anaphase
ImIf
ImIfImIfIf
Im
IfImIfIm
Separate
Telophase
Im IfImIfIm
If
If ImIfIm
If Im
Divide
Cytokinesis Cleavage furrow formed in late anaphase by contractile ring Cytoplasm is pinched into two parts after mitosis ends
Early Prophase
Early prophase
Early mitotic spindle
Pair of centrioles
Centromere
Aster
Chromosome, consisting of two sister chromatids
Figure 3.32.2
ProphasePLAYPLAY
Asters are seen as chromatin condenses into chromosomes
Late prophase
Fragments of nuclear envelope
Polar microtubules
Kinetochore
Kinetochore microtubule
Spindle pole
Late Prophase
Figure 3.32.2
PrometaphasePLAYPLAY
Nucleoli disappear Centriole pairs separate
and the mitotic spindle is formed
Metaphase
Metaphase plate
Spindle
Metaphase
Figure 3.32.4
Chromosomes cluster at the middle of the cell with their centromeres aligned at the exact center, or equator, of the cell
This arrangement of chromosomes along a plane midway between the poles is called the metaphase plate
MetaphasePLAYPLAY
Anaphase
Figure 3.32.5
Daughter chromosomes
Anaphase
Centromeres of the chromosomes split
Motor proteins in kinetochores pull chromosomes toward poles
AnaphasePLAYPLAY
Telophase and Cytokinesis
Telophase and cytokinesis
Nucleolus forming
Contractile ring at cleavage furrow
Nuclear envelope forming
New sets of chromosomes extend into chromatin
New nuclear membrane is formed from the rough ER
Nucleoli reappear Generally cytokinesis
completes cell division
TelophasePLAYPLAY
Controlling the Cell Cycle
1. Cell growth is assessed at the G1 checkpoint
2. DNA replication is assessed at the G2 checkpoint
3. Mitosis is assessed at the M checkpoint
G0 is an extended rest period
The eukaryotic cell cycle is controlled by feedback at three checkpoints
Control of the cell cyclePLAYPLAY
During fetal development, many cells are programmed to die
Cell Death
Human cells appear to be programmed to undergo only so many cell divisions About 50 in cell cultures
Only cancer cells can divide endlessly
Programmed cell death
Fingers and toes form from these paddle-like
hands and feet
What is Cancer?
Cancer is unrestrained cell growth and division The result is a cluster of cells termed a tumor
Benign tumors Encapsulated and
noninvasive
Malignant tumors Not encapsulated and
invasive
Leave the tumor and spread throughout the body
Can undergo metastasis
What is Cancer?
Most cancers result from mutations in growth-regulating genes
There are two general classes of these genes 1. Proto-oncogenes
Encode proteins that stimulate cell division If mutated, they become oncogenes
2. Tumor-suppressor genes Encode proteins that inhibit cell division
Cancer can be caused by chemicals, radiation or even some viruses
Receiving the signal to divide
Passing the signal via a relay switch
Amplifying the signal
Releasing the “brake”
Checking that everything is ready
Stepping on the gas
Stopping tumor growth
New molecular therapies for cancer
Potential cancer therapies are being developed to target seven different stages in the cancer process Stages 1-6
Prevent the start of cancer within cells Focus on the decision-making process to divide
Stage 7 Act outside cancer cells Prevents tumors from growing and spreading
Diversity is what allows gene populations to survive changes in their environment The greater the diversity, the more likely some individuals will have
the traits to survive a major change
Genetic diversity is the raw material that fuels evolution
No genetic process generates diversity more quickly than sexual reproduction
Importance of Generating Diversity
Contains two sets of chromosomes
Two Types of Reproduction
Sexual reproduction Maximizes diversity Involves the alternation of
meiosis and fertilization Meiosis halves the number
of chromosomes from each parent cell
Fertilization restores the number of chromosomes
Asexual reproduction Creates a faithful copy Does not involve
fertilization
Contain one set of chromosomes
The Sexual Life Cycle
The life cycles of all sexually-reproducing organisms follows the same basic pattern Haploid cells or organisms alternate with diploid cells or organisms
There are three basic types of sexual life cycles
Spend most of life in haploid form
Spend most of life in diploid form
Spend half of life in haploid form and half in
diploid form
Discovery of Meiosis
Meiosis was first observed by the Belgian cytologist Pierre-Joseph van Beneden in 1887
Gametes (eggs and sperm) contain half the complement of chromosomes found in other cells
The fusion of gametes is called fertilization or syngamy It creates the zygote, which contains two copies of each
chromosome
Mitosis
Meiosis
Prophase
Xm
Xf
Xm
Xf
Xm
Xf
Condense
Metaphase
Xm
XfXmXf
Xm
Xf
Line Up
Anaphase
ImIf
ImIfImIfIf
Im
IfImIfIm
Separate
Xf
Xm
Xm
Xm
Xf
Xf
If
If
ImIf
If
Im
Im Im
Im
Im
IfIf
Xm Xf
XmXf
XmXf
ImIm
If
Im
If
Im
ImIm
IfIf
If If
Xf
Xm
Xm
Xm
Xf
Xf
XmfX
mXXf
XmfXMeiosis I
mXXfXmfXXmfX
Meiosis IIXf
Xm
Xm
Xm
Xf
Xf
Telophase
Im IfImIfIm
If
If ImIfIm
If Im
Divide
Unique features of meiosisPLAYPLAY
Meiotic Cell Division: Meiosis I
Figure 27.7 Tetrads line up at the spindle equator during metaphase I In anaphase I, homologous chromosomes still composed of joined
sister chromatids are distributed to opposite ends of the cell At the end of meiosis I each daughter cell has:
Two copies of either a maternal or paternal chromosome A 2n amount of DNA and haploid number of chromosomes
In telophase I: The nuclear membranes re-form around the chromosomal masses The spindle breaks down The chromatin reappears, forming two daughter cells
Prophase I, Metaphase I, Anaphase I, Telophase IPLAYPLAY
Meiotic Cell Division: Meiosis II
Mirrors mitosis except that chromosomes are not replicated before it begins
Meiosis accomplishes two tasks: It reduces the chromosome number by half (2n to n) It introduces genetic variability
Meiosis IIPLAYPLAY
Differences Between Mitosis & Meiosis
Purpose
Where it occurs Body cells Special reproductive organs
Number of divisions after one chromatin replication event
One Two
Number of daughter cells
Two Four
Homologous chromosomes in daughter cells
Two (diploid) One (haploid)
Makeup of daughter cells vs. parent cell
Same as parent cell Different from parent cell
Mitosis Meiosis
Individually Homologous pairs in Meiosis I,Individually in Meiosis II
How chromosomes Line up at metaphase
Differences between meiosis &. mitosisPLAYPLAY
Create new cells(faithful copies)
Create new individuals(diversity)
Evolutionary Consequences of Sex
Sexual reproduction increases genetic diversity through three key mechanisms
1. Independent assortment
2. Crossing over
3. Random fertilization
In humans, a gamete receives one homologue of each of the 23 chromosomes Humans have 23 pairs of chromosomes
223 combinations in an egg or sperm 8,388,608 possible kinds of gametes
Independent assortment
Three chromosome pairs23 combinations
DNA exchanges between maternal and paternal chromatid pairs
Crossing over
This adds even more recombination to independent assortment that occurs later