3. asexual reproduction

8/12/2019 3. Asexual Reproduction

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Audesirk Textbook: Chapter 11 (p.185-198)

Cell and Molecular Biology Textbook: Chapter 14.1

and 14.2

Asexual Reproduction


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Cell Theory: All Cells Arise from

Preexisting Cells

The cell theory states that all cells must arisefrom preexisting cells, disproving spontaneousgeneration

To create new cells, the old cell must go through

cellular reproduction Cell division happens under a variety of

circumstances: Reproduction of an entire unicellular organism

Growth of a fertilized egg into an adult multicellularorganism

Repair and replacement of cells in an adult

Not all cells go through reproduction, afterdifferentiation Blood cells have been specialized so that they do


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Properties of Cell Division

In cell division, the parent cell(old) divides toform two new daughter cells(new)

After reproduction, the parent cell does not exist

anymore

Each daughter cell is genetically identical to the

parent cell from receiving the parent cells

complete set of hereditary information, with the

exception of mutations

Each daughter cell receives about half of the

parent cytoplasm


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Types of Asexual Reproduction Binary fission

Prokaryotes are relatively simple DNA replicates and cell pinches to form daughter cells

Eukaryotic cellular division Eukaryotes are more complex than prokaryotes

The cell cycle goes through a complex series of steps Budding

A part of the cell/organism grows outwards from the generalbody and eventually cuts off from the parent

Common in yeasts and hydra

Vegetative Clones Some plants produce genetically identical reproductive

structures that result in a genetically identical plant

Regeneration Some planarians have the ability for a cut off piece to grow

back into a full organism


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Binary Fission Bidirectional DNAreplication

The cell cycle of prokaryotes is relatively simple: Cell growth and DNA replication Cell division by binary fission

Prokaryotic DNA is circular and attaches to the cellularmembrane using a protein. The attachment site is the

origin DNA replication in prokaryotes is called bidirectional

replication

1. The cell grows and expands its cytoplasm

2. The DNA starts replication from the origin and continuesreplication in opposite directions

3. The replicated DNA folds over to the other side of thecytoplasm

4. The continuing expansion of the cytoplasm pulls the DNAstrands further apart

5. A protein called FtsZ goes into the center of the cell andmarks the location for the cell membrane to pinch in,creatin two dau hter cells
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Eukaryotic DNA and

Chromosomes During Interphase, the time when the cell is not going

through mitosis, the eukaryotic nucleus exists as aloosely organized complex of DNA strands andproteins, called chromatin

After DNA is replicated, the two copies of the sameDNA strand are called sister chromatids

To prepare for mitosis, a variety of proteins compactchromatin to form chromosomes. Chromosomes aresturdier for mitosis than loosely stranded DNA

After full compaction, chromosomes appear as twosister chromatids(identical DNA sequences) next toeach other

The sister chromatids are joined at a narrow regionconsisting of many proteins, called the centromere


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Eukaryotic DNA and

Chromosomes A chromosome consists of four major parts:

Sister chromatid: appear next to each other in

condensed chromosomes THAT HAVE ALREADY

GONE THROUGH DUPLICATION, have identical DNA

sequences, and separate during division to be the DNAof the daughter cells

Gene loci: the space on the chromosome one gene

takes up

Centromere:the complex of proteins in the

chromosome center that help join the two sisterchromatids together

Telomeres:extraneous DNA sequences at the tips of

chromosomes that do not code for any genes; every

time DNA replicates, some of the strand is destroyed or

chopped off in the process; the telomeres act as a


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Homologous Chromosomes Homologous chromosomes: are pairs of chromosomes

that code for the same genes

Homologous chromosomes express the genes they codefor in different ways due to difference in nucleotidesequence

Therefore, every gene in eukaryotes has two DNAsequences coding for it due to the homologouschromosomes

Cells with homologous chromosome pairs are calleddiploid cells (2n), because they contain two chromosomesto code for each gene

Cells without homologous chromosome pairs only haveone chromosome coding for each gene are called haploidcells (n)

Humans have 23 pairs of homologous chromosomes, for atotal of 46 chromosomes

The first 22 pairs in humans are called autosomes The 23 pair is called the sex chromosomesand are


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The Cell Cycle Cell CycleAnimation

The cell cycleis an ordered sequence of eventsfor cell division

The cell cycle consists of four stages

Interphase: duplication and growth of cell

contents

1. G1: cell and cytoplasm growth, organelle

duplication, normal cellular metabolic activities

2. S: duplication of DNA and centrosomes

3. G2: cell growth and preparation of mitosis

Mitotic Phase (M): division of the nucleus into

two

Cytokinesis (C): division of cytoplasm
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Control of the Cell Cycle: Transition

into Different Phases

Transition from one phase into another in the cell cycle isstimulated by a change in cell chemistry

Transition from one phase to another: Depends on an enzyme whose sole activity is to

phosphorylate other proteins The activity of this enzyme is controlled by other protein

subunits

The progression into another phase of the cell cycle iscaused by the phosphorylation of different cellular proteinsby using a phosphate group from ATP; the enzymesresponsible for this are called cyclin-dependent kinases

(Cdks) The activity of Cdks are regulated by protein subunits

called cyclins

Cyclins bind to Cdks, which causes a conformationalchange allowing for the Cdk to phosphorylate other cellularproteins

The activity of Cdks can be excited or inhibited by other


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cdc2 kinase is a type of

Cdk

G1cyclins attach to

Cdks to move cells into

S phase

Mitotic cyclins attach to

Cdks to move cells into

M phase


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Checkpoints

Control of the Cell Cycle

Animation

Checkpointsare mechanisms that halt the progress of thecell cycle if:

Any of the chromosomal DNA is damaged

Certain critical processes, such as DNA replication during Sphase or chromosome alignment during M phase, have not

been properly completed There are three major checkpoints:

G1-S transition

G2-M transition

Metaphase-anaphase transition

Checkpoints ensure that each of the various events thatmake up the cell cycle occurs accurately and in the properorder

Checkpoints arrest the cell in its current cell cycle phase,allowing time for the repair and correction of DNA or other

errors
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Checkpoint Response to Damaged

DNA

If a cell preparing toenter mitosis issubjected to UVirradiation, ATRkinase catalyzes a

cascade of reactionsthat arrests the cellin G2phase

If a cell is exposed toionizing radiation,ATMcauses theproduction of p53,which causes thetranscription of DNAto produce p21, aprotein that arrests

the cell in the G1phase


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Mitosis: Cell Division of

Eukaryotes

Mitosisis the division of the nucleus and itschromosomes into two in eukaryotic cells

Mitosis progresses through a series of stages:

Prophase

Prometaphase (sometimes included within

prophase)

Metaphase

Anaphase

Telophase

Cytokinesisis the division of the cytoplasm into

two in eukaryotic cells

Cytokinesis often overlaps with telophase


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Prophase

Prophase is the first phase of mitosis

In prophase, the following happen:

Duplicated chromosomes condense

Mitotic microtubule spindles form from centrosomes

Nuclear envelope breaks down; nucleolus

dissipates

Golgi complex and ER fragment and cytoskeleton is

disassembled


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Prophase: Chromosome

Condensation

DNA is found in the nucleus as loose strands of chromatinandother proteins

In order for chromosomal division to occur properly, DNA has togo through chromosome compaction, which stabilizes theDNA against damage

The DNA wraps around histoneproteins which compact theDNA partially

For the DNA to coil and completely condense, the use of aprotein called condensinis needed

Condensin binds to DNA in the presence of topoisomerase andATP

The structure of condensin allows for the supercoiling of DNA,transforming the DNA conformation to its most compact form

Condensin is activated by the phosphorylation of its subunits byCdks that transition the cell into the M phase

Cohesinis another protein that is essential for chromosomeformation

Cohesin has a circular structure, which hold the sisterchromatids to ether and is most abundant at the centromere


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Spindle and Dissolution of Nuclear

Envelope

During S phase, DNA duplicates as well as centrosomes When activated by certain proteins, the cytoskeleton of the

cell rapidly disassembles

After the duplication of the centrosomes, disassembledcytoskeleton start rebuilding into microtubules starting from

the centrosomes, creating mitotic spindles Motor proteins aid in moving the centrosomes to the

opposite poles

The disassembly of the nuclear envelope is caused by thephosphorylation of certain membrane proteins, which arepromoted by mitotic Cdks

The fragmented nuclear envelope become vesicles andare dispersed throughout the cell

The Golgi apparatus and the ER fragment into separate

vesicles


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Prometaphase

Prometaphase is sometimes included withinprophase, in which prometaphase is called lateprophase

The primary objective of prometaphase is to:

Attach chromosomes to spindle fibers To start the alignment of chromosomes

The spindle microtubules attach to certainproteins on the chromosome, called

kinetochores, which are located at thechromosomal centromere

The spindle microtubules begin pullchromosomes back and forth, in a process knownas congression, which starts to pull thechromosomes towards the center of the cell


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Metaphase

In metaphase, congression continues until all thechromosomes are aligned in the center

The result of metaphase is that all the

chromosomes are aligned on an imaginary line

called the metaphase plate, which is near thecellular equator

The alignment of chromosomes is very important

to the cell cycle, and there is a checkpoint to

make sure that the chromosomes are aligned


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Metaphase-Anaphase

Checkpoint The checkpoint between metaphase and anaphase iscalled the spindle checkpoint

The spindle checkpoint prevents a cell from going intoanaphase when a chromosome fails to align properly

This is important, because when cells go through withanaphase while having a misaligned chromosomes, thechromosomes are not divided evenly between daughtercells

A daughter cell with an abnormal amount of chromosomesis called an aneuploidy

Unattached kinetochores have a protein complex attached

called Mad2 Mad2 arrests the cell in metaphase, preventing the cell

from entering anaphase

Once the chromosomes are properly aligned, the Mad2complex detaches, and the cell is no longer arrested in

metaphase

Anaphase Acti ation and


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Anaphase: Activation and

PreparationCohesin Animation

Transition into anaphase is regulated by APC(anaphase promoting complex), Cdc20, securin,

and separase

When given certain signals, APC interacts with

Cdc20

The protein complex destroys securin, which is a

major anaphase inhibitor

When securin is destroyed, a protease (anenzyme that lyses proteins) called separase is

released

Separase denatures the cohesin molecules

holding the two sister chromatids together
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Anaphase The objective of anaphase is to separate the twosister chromatids, which will form the DNA of the

daughter cells

Spindle microtubules that are attached to thekinetochores pull the sister chromatids apart in

synchrony The chromatids move towards opposite poles of the

cell as spindle microtubules shorten

The movement process is very slow compared to therate of other cellular activities

Anaphase Arefers to the separation and movementof chromatids to the opposite poles

Anaphase Boccurs simultaneously to anaphase A,and refers to the movement of the spindle polesfurther and further apart, which is caused by

unattached spindle microtubules pressing againstteach other


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Telophase MitosisAnimation

Telophase is the last phase of mitosis They major events of telophase are:

Mitotic spindle disassembles

Chromosomes decondense

Nuclear envelope reforms

The ultimate objective of telophase is to return

daughter cells to the interphase condition

Telophase is NOT the partitioning of thecytoplasm into two separate cells
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Cytokinesis

Cytokinesisis the partitioning of the parentcytoplasm into two separate daughter cells

Cytokinesis is not part of mitosis, and is

considered as its own phase (C)

Cytokinesis is different in animal and plant cells

due to the difference in their outer structures


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Cytokinesis in Animal Cells

Cytokinesis in animal cells is performed through the use ofa cleavage furrow

The formation of a cleavage furrow is made possible byactin and myosin filaments, which interact in similarly tohow muscles contract

1. The cell membrane begins to pinch slightly in, starting acleavage furrow

2. The release of a protein called RhoA assembles actinand myosin and activates the motor activity of myosin

3. The sliding of the actin and myosin filaments pull theplasma membrane deeper into the cell, increasing the

cleavage furrow4. Cytoplasmic vesicles containing materials needed to

build the plasma membrane fuse on the cleavage furrow,generating more membrane

5. This continues until a new membrane has completely

formed in between and the parent cell has now beenpartitioned into two daughter cells


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Cytokinesis in Plant Cells CytokinesisAnimation

Because of the rigidity of the cell wall, the cell wall and cellmembrane cannot pinch in using cleavage furrows likeanimal cells do

Therefore, the cell wall and cell membrane has to forminside the cell and are extended outwards

The imaginary line of where the new cell wall will beformed to partition the cytoplasm is called the cell plate

The formation of a new cell wall comes from Golgi-derivedcarbohydrate-filled vesicles that fuse together

The phragmoplastis a cluster of microtubules, actinfilaments, and vesicles that are the remnants of the spindle

fibers The phragmoplast guides the Golgi-derived carbohydrate

vesicles towards the cell plate

The carbohydrate vesicles fuse to form the cell wall andthe cell membrane
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3. asexual reproduction

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