Chapter 13 Study Guide
Vocabulary:
Word Roots:
a- = not or without
auto- = self
chiasm- = marked crosswise
di- = two
fertil- = fruitful
haplo- = single
homo- = like
karyo- = nucleus
meio- = less
soma- = body
tetra- = four
Key Terms:
Asexual reproduction: A type of reproduction involving only one parent that produces genetically
identical offspring by budding or by the division of a single cell or the entire organism into two or
more parts.
Autosome: A chromosome that is not directly involved in determining sex, as opposed to a sex
chromosome.
Chiasma: (plural, chiasmata) The X-shaped, microscopically visible region representing homologous
chromatids that have exchanged genetic material through crossing over during meiosis.
Crossing over: The reciprocal exchange of genetic material between nonsister chromatids during
prophase I of meiosis.
Diploid cell: A cell containing two sets of chromosomes (2n), one set inherited from each parent.
Fertilization: The union of haploid gametes to produce a diploid zygote.
Gene: A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA
(or RNA, in some viruses).
Haploid cell: A cell containing only one set of chromosomes (n).
Heredity: The transmission of traits from one generation to the next.
Homologous chromosomes: Chromosome pairs of the same length, centromere position, and
staining pattern that possess genes for the same characters at corresponding loci.
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Karyotype: A display of the chromosome pairs of a cell arranged by size and shape.
Life cycle: The generation-to-generation sequence of stages in the reproductive history of an
organism.
Locus: (plural, loci) A specific place along the length of a chromosome where a given gene is located.
Meiosis: A two-stage type of cell division in sexually reproducing organisms that results in cells with
half the chromosome number of the original cell.
Recombinant chromosome: A chromosome created when crossing over combines the DNA from two
parents into a single chromosome.
Sex chromosome: One of the pair of chromosomes responsible for determining the sex of an
individual.
Sexual reproduction: A type of reproduction in which two parents give rise to offspring that have
unique combinations of genes inherited from the gametes of the two parents.
Synapsis: The pairing of replicated homologous chromosomes during prophase I of meiosis.
Tetrad: A paired set of homologous chromosomes, each composed of two sister chromatids.
Zygote: The diploid product of the union of haploid gametes in conception; a fertilized egg.
Chapter 13 Important Points:
 Genes are units of heredity which means they are the molecules that pass genetic
information from parent to offspring
o Genes are located on chromosomes and are linear sequences of DNA nucleotides
o Each gene encodes the instructions for ultimately making a protein or RNA molecule
o Genes are located in specific locations along the chromosomes (locus, plural: loci)
 Sexual reproduction requires the genetic contribution from 2 parents
 In the life cycle of an organism, one haploid gamete formed by meiosis comes from each
parent and when the egg is fertilized by the sperm, a fertilized egg or zygote is formed
o The zygote is diploid and will undergo mitosis to grow and development into a
multi-cellular adult
o Fertilization and meiosis must alternate to maintain the proper chromosome
number
 Somatic cells are always diploid which means they have 2 copies of all the chromosomes
(and therefore 2 copies of every gene located on those chromosomes)
o The diploid number is represented by 2n
o Ex. Fruit flies have 8 chromosomes in their somatic cells so their diploid number, 2n
=8
 Gametes are haploid which means they only have 1 copy of each chromosome and therefore
only one copy of each gene
o The haploid number is represented by n
o In the fruit fly example above, the haploid number, n = 4
o Haploid number is always ½ the diploid
 Pairs of chromosomes that contain the same gene loci are called homologous chromosomes
or homologs
o Somatic cells will have both members of the homologous pair
o Gametes will have only one member
 The sex chromosomes are the X and Y chromosomes and contain genes that determine sex of
the offspring
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o Human females have 2 XX chromosomes
o Human males have 1 X and 1 Y chromosome
All of the other chromosomes are called autosomes
Normal human females therefore have 23 pairs of homologous chromosomes, 22 autosomal
pairs and 1 pair of X (sex) chromosomes
Normal human males have 22 pairs of homologous, autosomes and then 1 X and 1 Y
chromosome that are not homologous
o This means normal human males only have one copy of every gene on the X and Y
chromosome
o Normal human females never have a copy of a Y chromosome gene
When a chromosome duplicates in S phase, it forms a pair of identical sister chromatids,
joined at a centromere
o Chromatids in the other member of a homologous pair are called non-sister
chromatids and while they contain the same gene loci, may not have identical
versions of all genes as the other homologue’s sister chromatids
Meiosis is the form of cell division that makes gametes
o Involves one duplication of DNA and 2 cell divisions
o First division, Meiosis I, forms haploid cells with replicated chromosomes
o Second division, Meiosis II, forms 4 haploid daughter cells with unreplicated
chromosomes.
Meiosis I has 4 stages: Duplicating centrosomes occurs in G2
1. Prophase I: Synapsis occurs creating tetrads (homologous chromosome pairs come
together) so that crossing over between non-sister chromatids of homologues pairs
occurs; microtubule spindle forms
2. Metaphase I: Tetrads move to metaphase plate independently from other tetrads by
action of microtubules in spindle
3. Anaphase I: Homologous chromosomes are pulled to separate ends of cell, still pairs
of sister chromatids
4. Telophase I: Nuclei form around pairs of chromatids at each end of cell, now
haploid
Meiosis II: After cytokinesis of MI, both daughter cells go through MII
1. Prophase II: New spindle forms and attaches to individual chromatids
2. Metaphase II: Pairs of sister chromatids line up at metaphase plate, one chromatid
on each side
3. Anaphase II: Sister chromatids are pulled apart and to opposite sides of cell; called
chromosomes again
4. Telophase II: Re-forming of nuclei around chromosomes; Cytokinesis occurs
forming a total of 4 haploid daughter cells
Sources of Genetic Variation
o Random mutation: Permanent changes in DNA nucleotide sequence
o Random fertilization between sperm and egg
o Crossing over:
 Occurs only during tetrad formation during Prophase I in Meiosis I
 Switching of genes sequences between non-sister chromatids in
homologous pairs
 Creates new combination of maternal and paternal genes in gametes:
Recombinant gametes
o Independent Assortment
 Each pair of homologues comes to the metaphase plate during
Metaphase I on its own
 Allows for variation in maternal and paternal genes in gametes
 The more pairs of chromosomes, the more possible variation: 2n
possible combinations
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Following chromosome/chromatid number in meiosis
o The final number of chromosomes in 4 daughter cells will always be haploid
o Ex. Somatic cell has 4 chromosomes
 G 1:
4 chromosomes
 S:
4 chromosomes; 8 total chromatids, 4 pairs
 G 2:
4 chromosomes; 8 total chromatids, 4 pairs
 PI:
4 chromosomes; 8 total chromatids, 4 pairs
 MI:
4 chromosomes; 8 total chromatids, 4 pairs
 AI:
4 chromosomes; 8 total chromatids, 4 pairs
but 2 chromosomes moving to one end, 2
chromosomes moving to other end
 TI:
2 chromosomes in each daughter nucleus
(4 total chromatids in each daughter nucleus)
 After CI:
2 chromosomes in each daughter cell, each
chromosome is still a pair of chromatids
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 Check:
Cells are haploid now
PII (each cell):
2 chromosomes; 4 chromatids, 2 pairs
MII (each cell):
2 chromosomes; 4 chromatids, 2 pairs
AII (each cell):
4 chromosomes
TII (each cell):
2 chromosomes at either end in daughter nucleus
After CII:
2 chromosomes in each of 4 daughter cells
Somatic (2n) was 4 so haploid (n) should be 2
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