Genetics, Ecology & Evolution summary
Book - Biology: A Global Approach
Chapter 12: Mitosis
Cell division = the continuity of life is based on the reproduction of cells.
Cell cycle = the life of a cell from the time it is first formed during division of a parent cell until its
own division into two daughter cells.
12.1: Most cell division results in genetically identical daughter cells
A dividing cell replicates its DNA, distributes the two copies to opposite ends of the cell, and
then splits into daughter cells.
Cellular Organization of the Genetic Material
Genome = the genetic information (DNA) of a cell.
The replication and distribution of so much DNA are manageable because the DNA molecules
are packaged into structures called chromosomes. Each eukaryotic chromosome consists of
one very long, linear DNA molecule associated with many proteins.
The DNA molecule carries several hundred to a few thousand genes.
Genes = the units of information that specify an organism’s inherited traits.
Chromatin = the entire complex of DNA and proteins that is the building material of
chromosomes.
Somatic cells = all body cells except the reproductive cells.
Gametes = reproductive cells.
Distribution of Chromosomes During Eukaryotic Cell Division
When a cell is not dividing, and even as it replicates its DNA in preparation for cell division, each
chromosome is in the form of a long, thin chromatin fiber. After DNA replication, the
chromosomes condense as a part of cell division: Each chromatin fiber becomes densely coiled
and folded, making the chromosomes much shorter and so thick that we can see them with a
light microscope.
Sister chromatids = joined copies of the original chromosome.
The sister chromatids are attached all along their lengths by protein complexes called cohesins
→ sister chromatid cohesion.
Centromere = a region made up of repetitive sequences in the chromosomal DNA where the
chromatid is attached most closely to its sister chromatid.
Once the sister chromatids separate, they are called individual chromosomes.
Mitosis = the division of the genetic material in the nucleus.
Cytokinesis = the division of the cytoplasm.
,12.2: The mitotic phase alternates with interphase in the cell cycle
Phases of the Cell Cycle
The cell cycle consists of the mitotic phase ( includes both mitosis and cytokinesis) and
interphase, which often account for about 90% of the cycle. Interphase can be divided into three
phases: the G1 phase (first gap), S phase (synthesis) and the G2 phase (second gap).
During all three phases of interphase the cell grows by producing proteins and cytoplasmic
organelles such as mitochondria and endoplasmic reticulum.
Mitosis has five stages:
Interphase → two centrosomes and the chromosomes have been duplicated.
1. Prophase → Each duplicated chromosome appears as two identical sister chromatids
and the mitotic spindle begins to form, the centrosomes move away from each other.
2. Prometaphase → Nuclear envelope fragments, kinetochore has now formed at the
centromere of each chromatid.
3. Metaphase → chromosomes have all arrived at the metaphase plate and the
centrosomes are now at opposite poles of the cell.
4. Anaphase → the cohesin proteins are cleaved, this allows the sister chromatids the let
go of each other. Each chromatid becomes a chromosome and are moving towards the
opposite end of the cell.
5. Telophase and Cytokinesis → two daughter nuclei form in the cell, nuclear envelopes
arise. The division of the cytoplasm is done by cytokinesis.
The Mitotic Spindle: A Closer Look
The mitotic spindle structure consists of fibers made of microtubules and associated proteins.
The assembly of spindle microtubules starts at the centrosome.
Centrosome = a subcellular region containing material that functions throughout the cell cycle
to organize the cell’s microtubules.
During mitosis centrosomes go to opposite ends of the cell and the spindle microtubules grow
out from them.
Aster = a radial array of short microtubules, extends from each centrosome.
Kinetochore = a structure made up of proteins that have assembled on specific sections of
DNA at each centromere
Metaphase plate = an imaginary plate where the kinetochore microtubules pull the chromatids
to.
Microtubules that do not attach to kinetochores have been elongating, and interact with other
nonkinetochore microtubules from the opposite poles of the spindle.
Separase = an enzyme that cleaves the cohesins holding together the sister chromatids of each
chromosome.
The microtubules become shorter, because a motor proteins walks over the chromosomes
along the microtubules and because a motor protein reeles the chromosomes.
,Cytokinesis: A Closer Look
Cytokinesis occurs by a process known as cleavage.
Cleavage furrow = a shallow groove in the cell surface near the old metaphase plate.
The actin microfilaments interacts with the myosin molecules, cause the contractile ring of actin
microfilaments to contract. The contraction of the dividing cell’s ring furrow deepens until the
parent cell is pinched in two.
By plants, during telophase, vesicles derived from the Golgi apparatus move along microtubules
to the middle of the cell, where they coalesce, producing a cell plate. The cell plate enlarges
until its surrounding membrane fuses with the plasma membrane along the perimeter of the cell.
That is how a plant gets a new cell wall.
Chapter 13: Sexual Life Cycles and Meiosis
Variations on a Theme
Heredity = the transmission of traits from one generation to the next.
Genetics = the scientific study of heredity and inherited variation.
13.1: Offspring acquire genes from parents by inheriting chromosomes
Inheritance of Genes
Parents endow their offspring with coded information in the form of hereditary units called
genes. Most genes program cells to synthesize specific enzymes and other proteins, whose
cumulative action produces an organism’s inherited traits.
Gametes = reproductive cells that are the vehicles that transmit genes from one generation to
the next.
Somatic cells = all cells of the body, except the gametes and their precursors.
Locus = a gene’s specific location along the length of a chromosome.
Comparison of Asexual and Sexual Reproduction
Asexual reproduction = a single individual is the sole parent and passes copies of all its genes
to its offspring without the fusion of gametes.
Clone = a group of genetically identical individuals, asexually reproducing gives rise to a clone.
Sexual reproduction = two parents give rise to offspring that have unique combinations of
genes inherited from the two parents.
13.2: Fertilization and meiosis alternate in sexual life cycles
Life cycle = the generation-to-generation sequence of stages in the reproductive history of an
organism, from conception to production of its own offspring.
Sets of Chromosomes in Human Cells
During mitosis, the chromosomes become condensed enough to be visible under a light
microscope. At this point, they can be distinguished from one another by their size, the position
, of their centromeres, and the pattern of colored bands produced by certain chromatin-binding
stains.
Karyotype = images of the chromosomes in pairs, starting with the longest chromosomes → 23
pairs of chromosomes.
Homologous chromosomes (homologs) = the two chromosomes of a pair have the same
length, centromere position, and staining pattern. Both chromosomes of each pair carry genes
controlling the same inherited characters.
Sex chromosomes = X and Y chromosomes, are autosomes.
The 46 chromosomes in our somatic cells are actually two sets of 23 chromosomes - a maternal
set and a paternal set.
Diploid cell = 2n, diploid number of chromosomes.
Gametes contain a single set of chromosomes, such cells are called haploid cells (n).
Behaviour of Chromosome Sets in the Human Life Cycle
Fertilization = the union of gametes, culminating in fusion of their nuclei.
Zygote = the resulting fertilized egg, is diploid because its contains two haploid sets of
chromosomes bearing genes representing the maternal and paternal family lines.
Mitosis of the zygote and its descendant cell generates all the somatic cells of the body. The
only cells of the human body not produced by mitosis are the gametes, which develop from
specialized cells called germ cells in the gonads.
Meiosis = this type of cell division reduces the number of sets of chromosomes from two to one
in the gametes, counterbalancing the doubling that occurs at fertilization.
The Variety of Sexual Life Cycles
Alternation of generations = this type includes both diploid and haploid stages that are
multicellular. The multicellular diploid stage is called the sporophyte. Meiosis in the sporophyte
produces haploid cells called spores. A haploid spore divides mitotically, generating a
multicellular haploid stage, called the gametophyte. Cells of the gametophyte gives rise to
gametes by mitosis.
Third type of life cycle = After gametes fuse and form a diploid zygote, meiosis occurs without
a multicellular diploid offspring developing. Meiosis produces not gametes but haploid cells that
then divide by mitosis and give rise to either unicellular descendants or a haploid multicellular
adult organism. The haploid organisms carries out further mitoses, producing the cells that
develop into gametes.
13.3: Meiosis reduces the number of chromosome sets from diploid to haploid
A single duplication by mitosis is followed by two consecutive cell divisions, called meiosis 1 and
meiosis 2. These two divisions results in four daughter cells, each with only half as many
chromosomes as the parent cell.
