LIFESCI 7B Chapter ALL: LS 7B Launchpad
30 Apr 2018
School
Department
Course
Professor
14.1 The Rate and Nature of Mutations
I. Mutations = mistakes in DNA replication or from unrepaired damage to DNA
A. Caused by reactive molecules produced in the normal course of metabolism
1. Chemicals in the environment, or by radiation of various types, including
X-rays and ultraviolet light
2. Most genomes also contain DNA sequences that can “jump” from one
position to another in the genome, and their insertion into or near genes is
a source
3. Incorrectly repaired chromosome breaks caused by reactive chemicals or
radiation
B. Most mutations are spontaneous, occurring by chance in the absence of any
assignable cause
II. Most common: substitution of one nucleotide pair for a different nucleotide pair
A. Relatively rare
B. Highest rates of mutation per nucleotide per replication are found among RNA
viruses and retroviruses, including HIV
1. RNA is a less stable molecule than DNA and the replication of these
genomes lacks a proofreading function
C. Lower rates occur in DNA viruses, and even lower in unicellular organisms such
as bacteria and yeast
D. Rates of mutation per nucleotide per DNA replication are nearly the same for all
multicellular animals, including mice and humans
E. Certain nucleotides are especially prone to mutation and can exhibit rates of
mutation that are greater than the average by a factor of 10 or more
1. Sites in the genome that are especially mutable are called hotspots
F. In some multicellular organisms, the rates of mutation differ between the sexes
1. human : the rate of mutation is substantially higher in males
G. The rates for the multicellular animals depend on the type of cell: a distinction
must be made between mutations that occur in germ cells (haploid gametes and
the diploid cells that give rise to them) and mutations that occur in somatic cells
(the other cells of the body)
III. Rate of mutation per nucleotide per replication in most multicellular organisms is low, the
rate of mutation per genome per generation depends on the size of the genome and the
number of cell divisions per generation
A. Humans have the smallest rate of mutation per nucleotide per replication, but
also have the largest rate of mutation per genome per generation
1. Average number of newly arising nucleotide-substitution mutations per
genome in one generation is about 30, or 60 per diploid zygote
a) About 80% of the newly arising mutations in a zygote come from
the father
(1) In a human male at age 30, the diploid germ cells have
gone through about 400 cycles of DNA replication and cell
division
find more resources at oneclass.com
find more resources at oneclass.com
(2) While the number of newly arising mutations in the mother
remains approximately constant with mother’s age, the
number of newly arising mutations from the father
increases with age
b) Only tolerable because more than 90% of the nucleotides in the
genome seem free to vary without deleterious consequences for
the organism (only 2.5% of the genome code for protein)
IV. Mutations that occur in eggs and sperm and the cells that give rise to them are called
germ-line mutations
A. Transmitted to future generations because they occur in reproductive cells
B. Rate of mutation per genome per generation matters more
C. May eventually come to be present in many individuals descended from the
original carrier
V. Mutations in non reproductive cells are called somatic mutations
A. Affect only the individual in which they occur
B. Rate of mutation per nucleotide per replication matters more
C. Transmitter to daughter cells in mitotic cell divisions
D. Affects all the cells that descend from it
E. Most cancers result from mutations in somatic cells
1. Increases the activity of a gene that promotes cell growth and division
2. Decreases the activity of a gene that restrains cell growth and division
3. Usually requires two or more mutations to cause cancer
a) Must occur sequentially in a single cell line
VI. Mutations occur randomly, and without regard to the needs of the organisms
A. The role of the environment is not to create specific mutations, but instead to
select for them
11.3 Meiotic Cell Division
I. A new organism produced by sexual reproduction has the same number of
chromosomes as its parents because the egg and the sperm each contain half the
number of chromosomes as each diploid parent
II. Gametes are produced by meiotic cell division, a form of cell division that includes two
rounds of nuclear division
A. Produces haploid gametes
III. Differences between meiotic cell division and mitotic cell division
A. Meiotic cell division results in four daughter cells instead of two
B. Each of the four daughter cells contains half the number of chromosomes as the
parent cell
C. The four daughter cells are each genetically unique
IV. In multicellular animals, the cells produced by meiosis are the haploid eggs and sperm
that fuse in sexual reproduction
V. In other organisms, such as fungi, the products are spores and in some unicellular
eukaryotes, the products are new organisms
VI. Meiotic cell division consists of two successive cell divisions
find more resources at oneclass.com
find more resources at oneclass.com
A. Called meiosis 1 and meiosis 2
1. Each cell division results in two cells, so that by the end of meiotic cell
division a single parent cell has produced four daughter cells
2. Meiosis 1
a) Homologous chromosomes separate from each other, reducing
the total number of chromosomes by half
3. Meiosis 2
a) Sister chromatids separate
VII. Meiosis 1
A. Prophase 1: the chromosomes are fully condensed and form chiasmata, the
nuclear envelope has begun to disappear, and the meiotic spindle is forming
1. Earliest visible manifestation of chromosome condensation
a) DNA replication has already taken place, so each chromosome
has become 2 sister chromatids held together at the centromere
2. The homologous chromosomes pair with each other, coming together to
lie side by side, gene for gene, in a process known as synapsis
a) Even the X and Y chromosomes pair, but only at the tip where
their DNA sequences are nearly identical
b) Because one of each pair of homologs is maternal in origin and
the other is paternal in origin, chromosome pairing provides an
opportunity for the maternal and paternal chromosomes to
exchange genetic information
c) Because each homologous chromosome is a pair of sister
chromatids attached to a single centromere, a pair of synapsed
chromosomes creates a 4-stranded structure: 2 pairs of sister
chromatids aligned along their length (CALLED A BIVALENT, and
the chromatids attached to different centromeres are called NON
SISTER CHROMATIDS)
(1) Nonsister chromatids result from the replication of
homologous chromosomes (one is maternal and the other
is paternal in origin), so they have the same set of genes in
the same order, but are not genetically identical
(2) Sister chromatids result from replication of a single
chromosome, so are genetically identical
3. Within the bivalents are cross-like structures, each called a
chiasma/chiasmata (visible manifestation of a crossover, the physical
breakage and reunion between nonsister chromatids)
a) Homologous chromosomes of maternal and paternal origin
exchange DNA segments
b) Positions are essentially random, and therefore each chromosome
that emerges is unique, containing some DNA segments from the
maternal chromosome and others from the paternal
c) Very precise: no nucleotides are gained or lost
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Mutations occur randomly, and without regard to the needs of the organisms: the role of the environment is not to create specific mutations, but instead to select for them. A new organism produced by sexual reproduction has the same number of chromosomes as its parents because the egg and the sperm each contain half the number of chromosomes as each diploid parent. Gametes are produced by meiotic cell division, a form of cell division that includes two rounds of nuclear division: produces haploid gametes. In multicellular animals, the cells produced by meiosis are the haploid eggs and sperm that fuse in sexual reproduction. In other organisms, such as fungi, the products are spores and in some unicellular eukaryotes, the products are new organisms. Meiosis 2: sister chromatids separate, creating haploid daughter cells (almost. Similarity of meiosis 2 and mitosis suggests that meiosis likely evolved from mitosis.
