chapters 12 and 13 n objectives f describe binary fission in bacteria f describe the structures that...
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Chapters 12 and 13
Objectives Describe binary fission in bacteria Describe the structures that play roles in the mitotic
phase of the cell cycle: the centrioles, spindle microtubules and chromosomes
Outline the phases of the cell cycle Describe the factors that control cell growth and
how cancer results from a breakdown of this control Outline the general progression and overall results
of meiosis, contrasting them with mitosis
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Explain how meiosis provides possibilities for genetic recombination
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Introduction Ch12/13
Life cycle is sequence of life forms from one generation to next
Sexual reproduction involves passing traits from two parents to next generation
Asexual reproduction involves passing traits from one parent to next generation
Cell division is basis of all processes that link phases of life cycle
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Cellular Basis of Reproduction and Inheritance
Chapter 12 and 13
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Like beget like (more or less)
True only for organisms that reproduce asexually single-celled organisms reproduce asexually by
dividing in two called binary fission daughter cells receive identical copy of parent’s
genes
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offspring of multi-cellular organisms not genetically identical to parents unique combination of parents traits breeders of domestic plants and animals manipulate
sexual reproduction by selecting offspring that exhibit desired traits
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Cells arise from preexisting cells cell reproduction called cell division two roles
enables fertilized egg to develop through various stages to adult organism
ensures continuity from generation to generation
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Binary Fission
Bacterial chromosomes genes carried on single circular DNA molecule
up to 500x cell length
minimal packaging complexed with few proteins and attached to plasma
membrane at one point
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Binary fission prior to cell division, genome copied
copies attached to adjacent parts of membrane
cell elongation and new plasma membrane separates two genomes
plasma membrane pinches through cell
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Eukaryotic Cell Division
Eukaryotes have large, complex, multiple chromosomes human cells contain about 30,000-35,000 genes
organized into separate, linear chromosomes
DNA complexed with proteins Just prior to division, chromosomes become
visible remain visible during division process
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Somatic Cells
Somatic cells are body cells (not sex cells) Ex. Hair cells These cells need to contain the full set of chromosomes so
that all the directions for functions and activities of the cell can be carried out.
Normally you inherit 23 chromosomes from each of your parents
This complete set of chromosomes (46) is known as the Diploid Number in Humans
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Sex Cells (Gametes)
Sex cells are known as gametes
These cells have half of the number of chromosomes that a body cell would have.
In humans this number is 23
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Sooooo..Somatic (body) cells contain the diploid number of chromosomes compared to sex cells (haploid number)
human cells: somatic cells-46 chromosomes (2n=46)
sex cells-23 chromosomes (n=23)
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What is a chromosome?????? Prior to cell division, chromosomes are duplicated
visible chromosomes consist of two identical sister chromatids attached at centromere
sister chromatids are able to be separated…
Once sister chromatids separate they are again called chromosomes
I know you are all thinking: WHAAAAAAATTTTTT????????
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Lets tie it all together
Humans have 23 pairs of chromosomes They get numbers 1-23 from Mom and 1-23
from Dad = 46 These 46 chromosomes are found in somatic
cells Sex cells ( gametes) have only 23 Each species has a specific diploid number
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Cell Cycle
The cell cycle is like a “ alarm clock” that tells the cell when it is time to do some essential activities and when to divide.
It is regulated by many chemicals inside the cell.
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Cell Cycle
Cell cycle results in cell division
many cells in an organism divide on regular basis
dividing cells undergo cycle: sequence of steps repeated during each division
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Cell Cycle Cont. Cell cycle divided into several steps (phases)
interphase represents 90% or more of cycle time
G1-cell increases in size and increases supply of proteins and organelles
S-DNA synthesis occurs G2-cell prepares for division, increases supply of proteins
necessary for division, checks for DNA damage
G0 – cell stops progressing through cycle- will not divide
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Cell Cycle
G0 = This is a very important phase of cellular activity
The cell has the opportunity to stop progressing towards division, or DNA synthesis
Why would this be important for a cell??????
Cells can phase into and out of G0 from several other cell cycle phases, its like an escape hatch
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Cell Cycle Cont.
Different cells are in various phases of cycle even in same tissue
Also Different Tissues May Regulate Cycle Differently
Ex. Hair Divides Constantly
Nerve Tissue Never Divides In Adults
Adult Liver Tissue Does Not Divide, Except For Repair
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Cell Cycle cont.
How does a cell progress through the cell cycle?
Many biochemicals stimulate the transition
One of them is a Kinase
A Kinase is an enzyme that catalyzes the transfer of a phosphate group from ATP to another molecule.
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How does a Kinase work ? It works a bit like turning on a light switch….
A PO4 is taken off ATP:
AT-PO4 - PO4 - PO4 AT-PO4 - PO4 + PO4
The PO4 is placed onto an enzyme, which activates the enzyme
The enzyme ( and many other chemicals) now tell the cell to move to the next phase of its cell cycle
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Soooooooooooooooooooooooooooo
If you are thinking…. Who cares????
How is this relevant to my life?????
Get ready to write down the ways!
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Cyclins
Cyclins are special chemicals that make the cell cycle go around
There are many different types
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Cell Cycle cont.
A Cdk is a cyclin dependent kinase
MPF is a co- chemical that is attached to Cdk
These chemicals stimulate the transition to cell division. When they are HIGH, the cell will divide
Why do we care about this?
BECAUSE CYCLIN AND CDK LEVELS ARE ALTERED IN CANCER CELLS……..
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Mitosis: Somatic Cell Division
mitotic (division) phase divided into two steps:
mitosis-nuclear division
cytokinesis-cytoplasmic division
result is two daughter cells with identical
chromosmes
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Mitosis
Somatic cells in humans have 46 chromosomes At the end of mitosis will they be diploid or
haploid and why?????
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Mitosis
Interphase: not part of division; Cell does other work
Prophase (division beginning): mitotic spindle forms from MTOC’s; ends when chromatin coiled into chromosomes; nucleoli and nuclear membrane dissolved
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Metaphase: spindle formed; chromosomes aligned single file with centromeres on metaphase plate; MAD
Anaphase: sister chromatids separate; migrate to poles
Telophase: reverse of prophase
Cytokinesis: division of cytoplasm
movement of chromosomes driven by addition or subtraction of protein subunits to kinetochore end of spindle microtubules
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Cytokinesis differs in plants and animals in animals, ring of microfilaments contracts
around periphery of cell forms cleavage furrow that eventually divides
cytoplasm
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in plants, vesicles containing cell wall material collect on spindle equator vesicles fuse from inside out forming cell plate cell plate gradually develops into new cell wall
between new cells membranes surrounding vesicles fuse to form new
parts of plasma membranes
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In Normal Cells
In mitotic normal mammal cells division only occurs 20-50 times prior to cell death.
Telomeres are the “cell clocks” that govern cell longevity
Telomeres shorten with each division; after about fifty times they reach a critical length and a division cessation signal is given
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Factors Affecting Cell Division
Control of cell division important for proper growth, development and repair of organisms growth factors regulate cell division
product of dividing cell
most plant and animal cells will not divide unless in contact with solid surface-anchorage dependence
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Density Dependent Inhibition
division usually stops when single layer of cells formed and cells touch= density-dependent inhibition
due to depletion of growth factor proteins in cell mass
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Three Cell Cycle Checkpoints
Three major check points in cell cycle G1 of interphase
G2 of interphase
M phase Release of growth factor/ chemical signals at each
of these checkpoints allows cell cycle to continue
The cell will ultimately divide if not halted at a checkpoint
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Cancer
Cancer cells not affected by growth factors that regulate density-dependent inhibition malignant tumor-metastasize benign-no metastasis named for organ or tissue of origin some cancer cells produce factors that keep them
dividing
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Benign tumor becomes malignant when cancerous cells from tumor mass spread to new sites and continue to proliferate movement mediated by either blood or lymph
systems
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Cancer cells and telomerase
Keeps telomeres lengthened
Cells keep dividing; cells with short telomeres should stop manufacturing this enzyme
Not so simple cells in mice lacking telomerase also became cancerous
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Common treatments for cancer: radiation-disrupts normal processes of cell division;
cancer cells more susceptible
chemotherapy-disrupt cell division
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Cell Death
Cells die two ways: Necrosis- from damage, poisons,starvation, hypoxia,
ATP depletion
Apoptosis- genetically programmed cell death; often normal in developmental pathways
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Apoptosis
sunburned cells
Also extends damage after a stroke
Cancer cells loose ability to carry out apoptosis become a problem
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Meiosis CH 13
Chromosomes are matched in homologous pairs share shape, genetic loci; carry genes controlling same
traits- alleles each homolog inherited from separate parent in humans, 22 pairs are autosomes, remaining pair sex
chromosomes female-two X chromosomes male-one X and one Y chromosome
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Question
Are X AND Y Homologous?
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Gametes
Normal Gametes have single set of chromosomes- No Pairs somatic cells have two sets of homologues
diploid (2n) sex cells(gametes) have one set of homologues
haploid (n) produced by meiosis
sexual life cycle involves alternation between diploid and haploid
fusion of haploid gametes at fertilization results in diploid zygote ( embryo)
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Meiosis
Meiosis reduces chromosome number from diploid to haploid occurs only in diploid cells destined to become
gametes preceded by single duplication of chromosomes results in four haploid daughter cells consists of two consecutive phases:
meiosis I-halving of chromosome number meiosis II-separation of sister chromatids
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PROPHASE I
2n- diploid
nuc. memberane breakdown homologs pair; synapsis,
chiasmataDNA condenses
Spindle app. forms
METAPHASE I
2n- diploid
Homologs aligned in cell center =equatorial plate
(MAD genes)
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ANAPHASE I
2n- diploid
Homologs pulled apart
TELOPHASE I
n- haploid (end)
Each cell new haploid
Short interphase no S phase-No DNA synthesis
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PROPHASE II
n- haploid
nuc. membrane breakdown DNA condenses
Spindle app. forms
METAPHASE II
n- haploid
Chromosomes aligned in cell center = equatorial plate
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ANAPHASE II
n- haploid
TELOPHASE II
n- haploid
The other cell from Telophase I also divides into 2 cells so 4 cells
total :each haploid
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Comparison of mitosis and meiosis
all unique events in meiosis occur in meiosis I crossing over during prophase I separation of homologous pairs during anaphase I meiosis II virtually identical to mitosis Except starting cells are haploid mitosis results in two daughter cells with same
number of chromosomes as parent cells but meiosis results in 4 haploid cells
can occur in either diploid or haploid cells
:
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meiosis results in four daughter cells with half number of chromosomes as parent cells
only occurs in diploid cells that will become gametes
Cells only run thru meiosis I and II ONCE Why?
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Independent assortment of chromosomes in meiosis and random fertilization lead to varied offspring
during prophase I each homologue pairs up with its “partner of the same number”
during anaphase I maternally and paternally inherited homologues move to one pole or other independently of other pairs
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for n chromosomes, there are 2n different combinations of haploid pairs for humans, 223 different combinations there are 223x223 combinations possible at
fertilization (64 billion)
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Homologous chromosomes carry different versions of genes
Crossing over increases genetic variability exchange of corresponding segments between
two homologues site of crossing over called chiasma
occurs between chromatids within tetrads as homologues pair up during synapsis
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produces new combinations of genes-genetic recombination
can occur several times in variable locations variability much greater than calculated two individual parents can never produce identical
offspring from separate fertilizations
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Visual Comparison of Mitosis and Meiosis