Download 1 Chapter 12: The Cell Cycle Cell division results in genetically

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Chapter 12: The Cell Cycle
Cell division results in genetically identical daughter cells.
Importance of the cell cycle
*It makes 2 new identical cells
1) Reproduction
-In unicellular organisms: it makes a whole new organism
-Multicellular organisms: use reproduction to grow
2) Growth/development
3) Repair and replacement
- The cell cycle extends from the creation of a new cell by the division of its parent cell to its own
division into 2 cells
- Mitosis: somatic cells (=normal body cells)
-2 identical diploid cells: same # of chromosomes
-Diploid = 2n
- Meiosis: gametes (=sex cells)
-2 identical haploid cells: half the # of chromosomes
-Haploid = n
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DNA is involved in:
1) Protein synthesis
DNA (only part of the DNA, one gene, is copied) RNA (mRNA, rRNA, or tRNA) 
polypeptide (chain of amino acids)
2) The Cell Cycle
DNA (all is copied)  chromosomes replicate (Interphase in S-phase)  chromosomes
separate (mitosis)  2 nuclei within 2 daughter cells (cytokinsesis)
Cellular Organization of Genetic Material
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Nucleotides: the genetic code/information within the genes of chromosomes
1. Sugar (deoxyribose)
2. Phosphate
3. Nitrogenous base (Adenine --- Thymine ; Guanine ----Cytosine)
DNA: polymers of nucleotides that contain genetic information
-Double helix
-DNA is the chemistry of chromosomes
Chromosomes: structure that contains lots of genetic information
Genes: bits of genetic information; sections of chromosomes
Chromatin: DNA + proteins
-proteins = histones and nucleosomes  they aid in the coiling of DNA so that it can fit inside the
cell
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Distribution of Chromosomes During Eukaryotic Cell Division
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Prior to cell division, a cell copies its DNA and each chromosome densely coils and shortens, forming
sister chromatids.
Sister chromatids: 2 identical replicated chromosomes
-Centromere: where the 2 chromatids are most closely attached
-“Arms:” the side of the chromatid on either side of the centromere
-The 2 chromatids are initially attached by proteins called cohesins, which hold them together in
sister chromatid cohesion.
The 2 sister chromatids separate during mitosis and then the cytoplasm divides during cytokinesis,
producing 2 genetically identical daughter cells with equal numbers of chromosomes
The Phases of the Cell Cycle
2 Main Phases:
-interphase: the period between division where the cell grows and duplicates its chromosomes
**lasts 90% of the cell cycle
-the individual chromosomes are not visible in the nucleus
-3 stages:
1. G1: “first gap” or prereplication
 Cell is assimilating: grows in terms of cytosol, membrane, organelles; takes in
nutrients; gets rid of waste
 Increases in size: the surface area to volume ratio gets smaller
2. S phase: “synthesis”
 Chromosomes replicate
 Doubles the number of genes in the nucleus
3. G2: “second gap” or premitosis
 Organelles within the cell replicate
 Other materials needed for cell division are produced, like RNA, proteins, etc.
-the M phase: the cell divides
-2 stages:
-mitosis: nuclear division
-Major stages: prophase, prometaphase, metaphase, anaphase, telophase
-cytokinesis: cytoplasmic division
The Phases of the Cell Cycle: A Closer Look
Interphase
(G1—S—G2)
Chromosomes appear in the form of chromatin (DNA +proteins)— appears as a dark granular
mass, so chromosomes can’t be seen individually
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Chromosomes have replicated during the S phase
-sister chromatids are long and skinny
A nuclear envelope bounds the nucleus
The cell contains nucleolus (=collection of RNA)
Two centrosomes have formed by replication
-Centrosome = microtubule-organizing center
-In animal cells, each centrosome features 2 centrioles. But centrioles aren’t required for
normal spindle operation.
-Even though plants don’t have centrioles, they still produce spindle fibers which help to pull
the chromosomes apart
The M-phase:
Mitosis
1. Prophase
 The chromatin fibers become more tightly coiled, condensing into visible chromosomes.
 Each duplicated chromosome appears as 2 identical sister chromatids attached at their
centromeres
 The nucleoli “disappear” – it disperses during cell division
 Spindle fibers begin to form
-It is composed of the centrosomes and protein microtubules that extend from
them.
-Aster = the radial arrays of shorter microtubules that extend from centrosomes
 Centrosomes move away from each other to opposite poles of the cell, propelled by the
lengthening microtubules between them
2. Prometaphase
 The nuclear envelope fragments.
 The spindle has formed from the centrosomes and microtubules
 Chromosomes are short and thick –have coiled back on themselves so it’s easier to move
around
 Each of the 2 chromatids of each chromosome now has a kinetochore, a specialized protein
structure located at the centromere.
 Kinetochore microtubules: attach to the kinetochores of chromosomes and jerk
the chromosomes back and forth
 Nonkinetochore microtubules (polar microtubules): don’t attach to
chromosomes and help to lengthen the cell
3. Metaphase
 The longest stage of mitosis, often lasting 20 minutes
 The centrosomes are now at opposite poles of the cell.
 Sister chromatids align at the equator or metaphase plate.
-The chromosomes’ centromeres lie on the metaphase plate.
-Chromatids move to the center due to the alternate tugging by the kinetochores
microtubules.
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 For each chromosome, the kinetochores of the sister chromatids are attached to
kinetechore microtubules coming from different poles of the spindle
4. Anaphase
 The shortest stage of mitosis, lasting only a few minutes
 Begins when centromeres that join the sister chromatids spit and cohesin proteins are
cleaved
 Sister chromatids split to form separate chromosomes
 The 2 new chromosomes move towards opposite ends of the cell as their kinetochore
microtubules shorten.
-Tubules depolymerize (=disassembled by enzymes) at their kinetochore end
-They look like V’s because the centromere is pulled towards the poles first
-The chromosomes continue to move until they have separated into two groups that are
each found near the poles of each of the spindles
 Ends when the chromosomes have stopped moving: now both ends of the cell have
equivalent and complete collections of chromosomes
5. Telophase
 Two daughter nuclei form in the cell.
 Nuclear envelopes reform from the fragments of the parent cell’s nuclear envelope and
other portions of the endomembrane system.
 Nucleoli reappear.
 The spindle breaks apart, forming centrosomes.
 Chromosomes become less condensed and uncoil to form a tangle of chromatin
 This marks the completion of mitosis.
Cytokinesis
 This is when the cytoplasm divides. It often occurs during telophase, so the 2 daughter cells
appear shortly after the end of mitosis.
 This results in the production of 2 identical daughter cells.
 In animal cells:
-cleavage furrow: pinches the cell in two
 In plant cells:
-cell plate: forms from the fusion of membrane vesicles derived from the Golgi
apparatus. The membrane of the enlarging cell plate joins with the plasma membrane,
separating the 2 daughter cells. The cell plate forms from the center of the cell outward.
-A new cell wall develops between the cells from the contents of the cell plate.
All cells don’t undergo cytokinesis. For example, skeletal muscle cells have multiple nuclei in
order to make more proteins and more ATP for movement and support.
Binary Fission
 Single-celled eukaryotes reproduce asexually by a process known as binary fission, which includes
mitosis.
 The binary fission of prokaryotes does NOT include mitosis.
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 The bacterial chromosome, a single circular DNA molecule, beings to replicate at the origin of
replication. One of these duplicated origins moves to the opposite pole of the cell.
 Replication is completed as the cell doubles in size, and the plasma membrane grows inward to
divide the 2 identical daughter cells.
-The mechanism of chromosome movement is not fully understood.
Molecular Control System
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Normal growth, development, and maintenance depend on proper control of the riming and rate of
cell division.
The frequency of cell division varies with the type of cell.
The cell cycle differences result from regulation at a molecular level
The Cell Cycle Control System
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The cell cycle control system coordinates the sequential events of the cell cycle
-Also called the “molecular clock”
-Important internal and external signals are monitored to determine whether or not the cell
cycle will proceed.
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The cell cycle has specific checkpoints where the cell cycle stops until a go-ahead signal is
received
1) G1 checkpoint:
-If a cell receives a go-ahead signal at this checkpoint, the cell continues on in the
cell cycle. Most cells go through this point because they have the necessary
chemicals to receive the signal.
-If a cell doesn’t receive a go-ahead signal, the cell exits the cell cycle and goes into
the G0 stage.
-the nondividing stage: cells don’t reproduce
-Example: brain and nerve cells
-Liver cells go into G0 but exit G0 and reproduce when the liver is damaged
2) G2 checkpoint:
-involves cyclins and Cdks (see below)
3) M checkpoint:
-requires an internal signal to pass through: the cohesins holding sister chromatids
together are not cleaved until all chromosomes are attached at their kinetochores
to spindle microtubules.
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2 types of regulatory proteins involved in cell cycle control:
1) Cyclins
2) Cyclin-dependent kinases (Cdks)
-The combination of cyclin and Cdk allows the cell to pass through the G2 checkpoint and
into mitosis
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-The activity of cyclin and cdk fluctuates during the cell cycle: it increases then drops off and
is produced in different amounts throughout the cycle
Growth factors are certain nutrients and regulatory proteins that are essential for cells to divide
Density-dependent inhibition: involves the binding of cell-surface proteins of adjacent cells, which
sends a growth-inhibiting signal to both cells
-aka: cells like to be next to each other
Anchorage dependent: cells must attach to a substratum (=lower layer) in order to divide
-aka: cells need to be in layers
Loss of Cell Cycle Controls in Cancer Cells
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Cancer cells don’t respond normally to the body’s control mechanisms
They don’t need growth factors to grow and divide:
- May make their own growth factors
- May convey a growth factor’s signal without the presence of a growth factor
- May have an abnormal cell cycle control system
May be caused by environmental factors (chemicals and carcinogens) or genetic factors
Benign tumor: harmless and noncancerous; can be surgically removed
Malignant tumor: invades surrounding tissues and metastasize, exporting cancer cells to other
parts of the body through the blood and lymph systems, where they may form a secondary
tumor.
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Chapter 13
Offspring acquire genes from parents by inheriting chromosomes
Inheritance of genes
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The inheritance of traits from parents to offspring involves the transmission of discrete units of
information coded in segments of DNA known as genes
- Most genes contain instructions for synthesizing enzymes and other proteins that then
guide the development of inherited traits.
Precise copies of an organism’s genes are packaged into gametes (=sperm and eggs)
Upon fertilization, genes from both parent are passed on to offspring
The DNA of a eukaryotic cell is packaged along with various proteins into a species-specific
number of chromosomes
- The genome is the entire complement of DNA.
- A gene’s locus is its location on a chromosome.
Comparison of Asexual and Sexual Reproduction
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In asexual reproduction, a single parent passes copies of all its genes to its offspring
- A clone is a group of genetically identical offspring
In sexual reproduction, an individual receives a unique combination of genes inherited from 2
parents
Fertilization and meiosis alternate in sexual life cycles
Sets of Chromosomes in Human Cells
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In somatic cells there are 2 chromosomes of each type, known as homologous chromosomes or
homologs.
- A gene controlling a particular character is found at the same locus on each chromosome
or homologous pair
A karyotype is an ordered display of an individual’s condensed chromosomes
- Isolated somatic cells are stimulated to undergo mitosis, arrested in metaphase, and
stained
- A computer uses a digital photograph to arrange chromosomes into homologous pairs by
size and shape
Sex chromosomes determine the sex of a person:
- Females = 2 homologous X chromosomes
- Males = nonhomologous X and Y chromosomes
Autosomes are chromosomes that aren’t sex chromosomes
Somatic cells contain a set of chromosomes from each parent and are diploid cells.
Gamete cells are haploid cells and contain a single set of chromosomes.
Behavior of Chromosome Sets in the Human Life Cycle
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Fertilization is the fusion of sperm and ovum(=egg)
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Fertilization produces a zygote containing both paternal and maternal sets of
chromosomes
- The diploid zygote then divides by mitosis to produce the somatic cells of the body, all of
which contain the diploid number (2n) of chromosomes
Meiosis is a special type of cell division that halves the chromosome number and provides a
haploid set of chromosomes to each gamete.
- Gametes are produced by meiosis from specialized germ cells in the gonads.
An alternation between diploid and haploid numbers of chromosomes, involving fertilization
and meiosis, is characteristic of sexually reproducing organisms
 In sexual reproduction, meiosis and fertilization are complementary processes:
meiosis produces haploid gametes, while fertilization restores the diploid
chromosome number
Meiosis
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Nuclear division that results in gamete formation
Converts diploid somatic cells (2n) to haploid gametes (n)
Involves replication of chromosomes one time (during interphase)
Crossing over may occur
-Homologous pairs may exchange pieces of DNA  mixing of maternal and paternal
genes when homologs are held together by the synaptonemal complex (synapsis)
**Allows unique gametes and increases variation
-Crossing over is visible in regions called the chiasmata where sister chromatids are held
together by sister chromatid cohesion
-Only occurs during prophase I
Crossing-over
paternal
maternal
homologous pair homologous pair
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paternal
maternal
homologous pair homologous pair
Homologous pairs: code for the same gene and same basic information, but the specific
information may be different.
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Homologous pairs = similar
-Homologous pairs carry the same genes, though their DNA sequences may be
slightly different
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Sister chromatids = identical (must be attached at centromere)
The number of chromosomes varies in eukaryotes, but they all have an even number
because the set of chromosomes is divided in half: an individual gets a set of haploid
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chromosomes from each parent which makes a diploid set of chromosomes for the
individual
The cells of most organisms that reproduce sexually have pairs of similar chromosomes
called homologous pairs
Each parent provides one member of each homologous pair: they have 1 chromosome from
each set
In humans, 22 homologous pairs are autosomes and there is 1 pair of sex chromosomes
-Sex chromosomes in males are NOT similar and are called nonhomologous  males
technically have 22 homologous pairs and 1 nonhomologous pair of chromosomes
 Sexual reproduction is when two different parent cells come together to produce one cell
and each parent cell gives half of their DNA
- most organisms are produced this way
- In asexual reproduction, the cells of parent and offspring carry identical sets of
chromosomes, but in sexual reproduction, two parents contribute chromosomes to
offspring.
-For this reason, the gametes that fuse during sexual reproduction are haploid and
carry half the normal number of chromosomes. If gametes were diploid, the number
of chromosomes would double in each generation
- Provides variation which is important because it helps a species survive. It allows at least
some of the species to survive if the environment changes
-Variation = the differences of individuals of the same species
-Traits that are beneficial to a species pass to offspring through fertilization
-Over time, it is possible for characteristics of a population to change over time: this
process is called evolution
 Meiosis occurs in 2 stages:
1) Meiosis I:
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Often called reductional division because it reduces the chromosome sets from two
(diploid) to one (haploid).
**Homologous pairs separate**
Results in 2 HAPLOID cells
Phases: prophase I, metaphase I, anaphase I, telophase I/cytokinesis
2) Meiosis II:
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Often called equational division because the chromosome number stays the same
**Sister chromatids separate**
Results in 4 HAPLOID cells
prophase II, metaphase II, anaphase II, telophase II/cytokinesis
The Stages of Meiosis
Meiosis I
1. Interphase
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 Chromosomes replicate  produces identical sister chromatids that remain attached at the
centromere.
- Sister chromatid cohesion is when sister chromatids are attached along their length
2. Prophase I
 Chromosomes begin to condense for the 1st time and homologous pairs form: they loosely
pair along their lengths, aligned gene by gene
 Tetrads may form – tetrads are sister chromatids that are homologous
 Crossing over may occur (=the exchange of corresponding segments of DNA molecules by
homologous pairs)
- Is completed while homologs are in synapsis, held together by proteins along their length
- Forms chiasmata = points where crossing over has occurred
 Synaptonemal complex: “mesh” of spindle fibers to exactly align the chromosomes
-Synapsis ends mid-prophase and chromosomes in each pair move apart slightly
 Centrosomes move apart, the spindle begins to form, and nuclear envelope breaks down
 Each homologous pair has 1 or more chiasmata and the homologs are still associated due to
cohesion between sister chromatids (=sister chromatid cohesion)
 In late prophase I, microtubules attach to the 2 kinetochores (=protein structures at the
centromeres of the 2 homologs). The homologous pairs then move toward the metaphase
plate
3. Metaphase I
 Pairs of homologous chromosomes line up on the metaphase plate, with their kinetochores
attached to spindle fibers from opposite poles.
- one chromosome in each pair faces each pole
 Both chromatids of one homolog are attached to kinetochore microtubules  those of the
other homolog are attached to microtubules from the opposite pole
4. Anaphase I
 HOMOLOGOUS PAIRS SEPARATE
 Breakdown of proteins responsible for sister chromatid cohesion along chromatid arms
allows homologs to separate
 The homologs move toward opposite poles, guided by the spindle apparatus
 Sister chromatid cohesion persists at the centromere causing chromatids to move as a unit
toward the same pole
5. Telophase I and Cytokinesis
 RESULTS IN 2 HAPLOID CELLS
 Each chromosome is a sister chromatid
 Cytokinesis usually occurs simultaneously with telophase 1
 In some species, chromosomes de-condense and the nuclear envelopes re-forms
 No replication occurs between meiosis I and meiosis II
Meiosis II
1. Prophase I
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 A spindle apparatus forms
 In late prophase II, chromosomes (=2 sister chromatids attached at the centromere) move
toward the metaphase II plate
2. Metaphase II
 The chromosomes are positioned on the metaphase plate as in mitosis
 Because of crossing over in meiosis I, the 2 sister chromatids of each chromosome are NOT
genetically identical
 The kinetochores of sister chromatids are attached to microtubules extending from opposite
poles
3. Anaphase II
 SISTER CHROMATIDS SEPARATE
- These sister chromatids aren’t genetically identical because of crossing over
 Breakdown of proteins holding the sister chromatids together at the centromere allows the
chromatids to separate
 The chromatids move toward opposite poles as individual chromosomes
4. Telophase II and Cytokinesis
 Nuclei form, the chromosomes begin decondensing, and cytokinesis occurs
 The meiotic division of 1 parent cell produces 4 haploid daughter cells
 The 4 daughter are not identical cells – they’re genetically distinct
A Comparison of Mitosis and Meiosis
Meiosis
 Produces HAPLOID daughter cells that
differ identically from their parent cell
and from each other
 Involves 2 nuclear divisions
 Crossing over may occur during
prophase I
 In metaphase I, chromosomes line up in
pairs on the metaphase plate
 During anaphase I, homologous pairs
separate
 Sister chromatids are held together by
proteins called cohesions. Enzymes
cleave the cohesins along the arms of
sister chromatids so homologs separate
in anaphase I. A protein called
shugoshin protects the cohesins at the
centromere from cleavage until
anaphase II.
Mitosis
 Produces DIPLOID daughter cells that
are genetically identical to the parent
cell
 Involves 1 nuclear division
 Crossing over does not occur, so
chromosomes are IDENTICAL
 In metaphase I, chromosomes line up as
individuals on the metaphase plate
 During anaphase, sister chromatids
separate
 Sister chromatids are held together by
proteins called cohesins. Enzymes
cleave the cohesins during anaphase I.
The protein shugoshin isn’t utilized
because it will prevent sister
chromatids from separating.
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Genetic Variation
Meiosis introduces variation in 3 ways:
1. Crossing over – allows genes from the mother to be exchanged with genes from the father
-Forms recombinant chromosomes with new genetic combinations of maternal and
paternal genes on the same chromosome  **chromosomes are no longer identical
2. Independent assortment – the random distribution of maternal and paternal chromosomes
from each homologous pair
-Chromosomes can separate in different combinations because each parent provides
one member from each homologous pair, but we don’t know which one will be used.
-Each homologous pair lines up independently at the metaphase plate – but the
orientation of the maternal and paternal chromosomes is random.
-The number of possible combinations of maternal and paternal chromosomes in
gametes is 2n (where n is the haploid number)
3. Random fertilization – the random pairing of one out of many sperm to one egg
4. Mutations – any change in the sequence of nitrogenous bases in DNA
*This is not always from meiosis – it is the original source of genetic variation
Evolutionary Significance of Genetic Variation within Populations
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In Darwin’s theory of evolution by natural selection, genetic variations present in a population
result in adaptation
The individuals with the best variations best suited to an environment produce the most
offspring (=survival of the fittest)
The process of sexual reproduction and mutation are the sources of this variation
Human Gametogenesis
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Gametogenesis is the production of gametes. It differs in males and females.
Human Spermatogenesis
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Results in 4 equal haploid spermatids  cell division is equal and continuous
Sperm cells are haploid
Occurs continuously in the seminiferous tubules of the testes as spermatogonia.
- The primary spermatocyte divides in MEIOSIS I into 2 secondary spermatocytes which
divide in MEIOSIS II to form 4 spermatids.
Human Oogenesis
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Meiotic cytokinesis in unequal: results in 1 large ovum and up to 3 small polar bodies
- The primary oocyte divides into 2 unequal cells in MEIOSIS I: the 1st polar body and the
secondary oocyte
- The secondary oocyte divides into 2 unequal cells in MEIOSIS II: the 2nd polar body and
the ovum or egg
- The ova or egg cell is haploid and survives; the haploid polar bodies will disintegrate
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