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 The continuity of life
 The cell cycle
 Mitosis
 Meiosis
 Chromosome sets
 Sexual life cycles
Reading, Campbell Biology, 2Cadia edition
 Chapter 12, pp. 243-253
 Chapter 13
1. During the history of biological science, the continuity of life was first
understood to consist of the continuity between generations of individual
organisms, then of cells, then of chromosomes, and finally of the base
sequence of DNA molecules. (1950s continuity of life is from the continuity
of cellular light….Rudolph Virchow)
2. Life also has generational continuity in terms of things other than DNA
sequences – for example, 1.DNA base modifications and 2.biological
membranes. –both are not continuity
(cytosine -> methylcytosine (a modified DNA base) you can remove these
ethyl tags. These tags can pass across cellular generations- even single
generations (parents to offspring) the tags can be removed. *the expression
pattern of genes can be transported across generations which is a part of
continuity of life.
We cannot build membranes from scratch. Can add to these membranes,
changes to genes, the continuity from cell generation to generation,
epigenetic continuity (off the genome)
3. To refer to genes and DNA as a “blueprint” is a misleading metaphor.
(you can change a blueprint but usu you don’t. It is a blueprint in the sense
of containing info. You need some pre-existing cellular structure to make
another cell, there is always an interaction between the genes and growth in
environment, *if we are naïve about genetic info. All those things are
possible, there has to be some kind of continuity off the genes, if you have
the blueprints and the raw materials they can build a building- but you
cannot build a cell from scratch, you have to have continuity from
generation to generation, especially membrane bound parts. Genes are
always in action. Once the building is finished the blueprints are put awaygenes are used on a daily basis.
1. The cell cycle is a controlled sequence of events that comprises cell growth and
division. (Gap 1 (G1) cell is growing-DNA synthese (s phase ) genes are
duplicated – G2 “Have I duplicated my chromosomes yet? – mitois- division of
nuclear content – cytokinesis- mitotic phase
2. Cells that progress through the cell cycle growing unicellular organisms
embryonic cells in young animals stem cells in mature animals
cells in the meristems (growing points) of plants
cells at the growing points of algae and fungi
cells that divide to repair an injury in animals and plants
3. Cells that do not normally progress through the cell cycle differentiated cells
performing specialized functions
dormant or resting stages of unicellular or multicellular organisms (dormant cellsthat do not divide eg. corn seed, until appropriate growth conditions return and
signals are sensed)
some cells opt out of the cell cycle, most cells in your body are in G zero
pg. controls of the cell cycle have been lifted somehow, gone into wrong place- cancer
cells can turn cancerous when control of the cell cycle fails (fig. 12. 19) the cells keep piling up
Fig. 12.6 What activities does the cell engage in during the Gpat of interphase?
How much DNA is there in the cell’s nucleus during G
from Campbell Biology, 2nd Canadian edition, by Reece et al. 2018
stem cells (in bone marrow)- can produce more cells, they can differentiate
differentiation- same cells take on different jobs
B cells-produce antibodies, can divide, but normally just coursing through the
T cells- kill aids virus, can divide, but normally just coursing through the blood
Skin cells- always dying off
4. The cell cycle is subject to complex control.
Cell growth, division of the cell's contents, and division of the cell's genetic
information must all be coordinated.
1. Mitosis is a division of the nucleus that involves an equational division of the
chromosomes. (“maybe cells are more organized than we thought” it’s an
equational divison of the chromosomes)
What is a chromosome? Special structures made of chromotin. ChromotinConsists of 50% DNA (genes, non-genic sequence), 50% protein.->genes are on
chromosomes, than scientists found out that protein are on chomosomes.
Nucleotides & amino acids.
Chromotin- term given to the material in which the chromosome is made of.
*find out chromotin vs. chromosome
Light microscopes- uses light, (if using blue light, you can resolve two points that
are about a half a wave length apart) 200nm if you have two things closer
together you would see a dumbbell shape
Electronic microscope- uses electrons, electrons have a wavelength that is a lot
smaller than that of light. Resolving power** is greater. More detailed images b/c
of this
Understanding the size of things help you sort out things in your mind.
Micrographs will be accompanied with a scale bar that you should pay attention to.
After mitosis is finished, each daughter nucleus will be genetically identical to
2. Before mitosis, the chromosomes are duplicated in S phase to give two sister
As a result of chromosome duplication, each allele (version of a gene) is copied.
3. Stages of mitosis
prophase start seeing chromosomes, takes a while, chromosomes start getting
shorter, early mitotic spindle (little motor protein)
prometaphase nuclear envelope starts to disintegrate, each duplicated chromosome
has two kinetochores (adapted complex, cannot react directly with chromatin)
attachment of spindle microtubules to kinetochores
Fig. 12.5 Can light microscopy resolve individual chromosomes at all stages of the
cell cycle?
from Campbell Biology, 2nd Canadian edition, by Reece et al. 2018
metaphase each chromosome is found in the circle in the middle of the cell. Lined
up on the metaphase plate
anaphase two sister chromatids have separated, and are considered separate
Fig. 12.7 How many chromosomes are shown in the diagram of prometaphase?
How many chromatids? from Campbell Biology, 2nd Canadian edition, by Reece
et al. 2018
4. Mitosis is usually (but not always) followed by cytokinesis, division of the cell’s
contents into two separate cells.
*animal cells – actin drawstring, they form a circle and rubbing together to form a
cell plate, when they divide they retain a certain freedom, they can flake off and go
to a different organism. –neurocrest cells –parts of your head, brain
plant cells – don’t have the same freedom as animals to go to dif locations, can get
cancer tumors stay in one place,
A coenocyte is a cell in which one or more rounds of mitosis has occurred without
cytokinesis. How cells get big, you got to have a lot of nuclei (eg. Coconut,
caulerpa taxifolia –marine alga-not a plant but looks like a plant, uni-cellularmillions of nuclei
1. Meiosis is a division of the nucleus that involves a reductional division of the
This is necessary for organisms with sexual life cycles.
2. Each daughter nucleus will get half the chromosomes of the mother nucleus.
Each daughter will be genetically distinct from the mother nucleus and the other
daughter nuclei.
3. Every species has its characteristic chromosome numbers.
chromosome number in a nucleus before meiosis: diploid, or 2n
chromosome number after meiosis: haploid, or 1n
plants produce more chromosomes or less, weird numbers: animals are generally
like humans and have a certain number of chromosomes
4. In a diploid cell there are two similar, but not identical, versions of each kind of
These two versions are called homologous chromosomes.
For each pair of homologous chromosomes in each of your cells, you received one
homologue from your mother and one from your father at fertilization.
5. Before meiosis the chromosomes must be duplicated, as they are before mitosis.
Fig. 13.7 It is important that you understand in what respects homologous
chromosomes are similar, and in what respects they are different. Two members of
a homologous pair will have genes governing the same characters in the same
positions; but for many genes, the two homologues might differ in the traits
specified by their specific versions of the gene (alleles).
You must also understand the distinction between homologous chromosomes and
sister chromatids.
from Campbell Biology, 2nd Canadian edition, by Reece et al. 2018
6. Stages of meiosis Meiosis I
prophase I: homologous chromosomes find each other (homologue pairing) and
might exchange arms. The exchange is called crossing over, and is usually
reciprocal down to the base pair.
metaphase I: paired homologues, still associated, line up on the metaphase plate
anaphase I: homologues separate
Fig. 13.8 The crossover events depicted here imply that a given chromosome that
you inherited from your mother was a mixture of her mother's and her father's
DNA. Crossing over recombined your grandparents’ DNA in your mother’s
meiocyte (meiotic cell). The same thing happened during the meiosis in your
father’s meiocyte that led to the sperm cell. from Campbell Biology, 2nd Canadian
edition, by Reece et al. 2018
Meiosis II
Sister chromatids line up in metaphase II, and then separate in anaphase II.
Fig. 13.8 You should inspect the chromosomes in each of the daughter nuclei to
confirm that each nucleus is haploid, and is genetically distinct from the other
from Campbell Biology, 2nd Canadian edition, by Reece et al. 2018
7. Sources of genetic variability among products of a meiotic division
uncorrected errors (mutations) resulting from DNA replication
crossing over of homologue arms during prophase I
independent assortment of homologous chromosomes at metaphase I: maternal and
paternal homologues of different homologous pairs line up independently at
metaphase I (and therefore segregate independently at anaphase I)
8. Fate of the products of meiosis
in animals
they don’t divide,
in plants
they do divide, haploid body (Fern: hosts meiosis)
1. one set of chromosomes
one version (homologue) for each chromosome in the genome & one version
(allele) of each gene
2. haploid nucleus (n or 1n)
one chromosome set
one homologue per chromosome one allele per gene
3. diploid nucleus (2n)
two chromosome sets
two homologues per chromosome two alleles per gene
1. Sexually reproducing organisms have both 1n and 2n phases of their life cycle.
2. Life cycles of sexual organisms can be represented in terms of chromosome
Fig. 13.6 The sexual life cycles shown here are sometimes given these names: a)
diploid life cycle
b) alternation of generations life cycle
c) haploid life cycle
from Campbell Biology, 2nd Canadian edition, by Reece et al. 2018
Answers are given on the next page.
1. What is the result, genetically, of mitosis? Meiosis?
2. What would happen if a cell underwent mitosis but no cytokinesis?
3. If there are 16 chromosomes in an animal cell in the G1 stage of the cell cycle,
what is the diploid number of chromosomes for this species? The haploid number?
4. How much DNA is present in a G2 nucleus, compared to a G1 nucleus?
5. A nucleus containing 88 chromatids at the start of mitosis would produce nuclei
containing how many chromosomes after mitosis was over? If the same nucleus
were at the beginning of meiosis, how many chromosomes would each of the four
daughter nuclei have at the end?
6. What are the two phases of meiosis that are the source of genetic variation in the
four resulting daughter cells, and why?
7. What are some of the activities a cell engages in during the G1 part of
8. Draw a picture of a diploid cell with eight chromosomes (four pairs of
homologues) at metaphase I in meiosis. Your diagram should include different
shading for maternal and paternal homologues, different centromere positions and
chromosome sizes among the homologous pairs, and crossing over in two of the
four pairs of homologues. Next, draw four products of this meiosis.
Answers to Study Questions
1. Mitosis: the result is two cells, each with the same chromosome number as and
genetically identical to the mother cell. Meiosis: the result is four cells, each with
half the chromosomes as the mother cell and each genetically distinct from the
2. A cell with two or more nuclei would result. This is a reasonably common
occurrence, by the way. There are green algae with millions of nuclei in a common
cytoplasm. Individuals in a fungal phylum, the Zygomycota, also have many nuclei
in one cytoplasmic compartment; coconut milk is a third example. The term
referring to this multinucleate condition is coenocytic.
The Cell Cycle and Sexual Life Cycles - 13
3. 16; 8
4. Since G2 follows S phase, when the chromosomes are duplicated, a G2 nucleus
would contain twice as much DNA as a G1 nucleus.
5. 44 chromosomes in each nucleus; 22 chromosomes in each nucleus
6. Prophase I (because of crossing over in this phase) and metaphase I (because of
the independent assortment of maternal and paternal homologues in this phase).
7. gene expression, acquisition of nutrients, growth
Questions from your text, Campbell Biology, second Canadian edition  Scientific
Skills Exercise, p. 259
 pp. 261-262, Questions 1, 5 – 9, 11
 p.280, Questions 1-8, 11
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Figure Citations
All figures used with permission from Campbell, by Jane B. Reece et al., Pearson, 20