Biology 1406 Exam 4 Notes Cell Division and Genetics
Ch. 8, 9
Ch. 8
Cell Division
Cells divide to produce new cells – must pass genetic information to new cells
- What process of DNA allows this?
Two types of cellular division to accomplish 2 different goals
- Mitosis - produces cells for growth, replacement and repair, and asexual reproduction
- 2 daughter cells are genetically the same
- Meiosis - produces reproductive cells called gametes (egg and sperm) for sexual
reproduction
- daughter cells have half the number of chromosomes
- produces genetic diversity
Beginning of mitosis and meiosis the same : (before actual division of cell)
- the cell cycle includes interphase and mitotic or meiotic phase
8.4
- interphase – time of normal cell function, metabolically active , cell grows
- synthesis subphase in which cell grows and produces new cell parts, including chromosomes
- DNA replication occurs in the synthesis subphase of the cell cycle
- Each chromosome becomes 2 identical strands
8.3
(chromatids) held together at one spot (centromere)
- in mitosis and meiosis nucleus divides with separation of chromosomes
- cytokinesis is division of cell cytoplasm into two cells, each with a nucleus
What are plasmodial slime molds and how do they form?
A cell has 12 chromosomes at the beginning of the G1 subphase of interphase. How many
chromosomes and how many chromatids will it have at the end of synthesis subphase?
Mitosis
- continuous process divided into 4 recognizable stages
- know stages and key events that occur in mitosis :
8.4 – 8.10
8.5
Prophase
– chromosomes coil into short thick rods
- spindle fibers begin forming and moving to “poles” of cell
Prometaphase (Late Prophase)
- nuclear membrane breaks down
- spindle fibers extend from poles and attach to centromere of chromosomes, one
from each pole to each sister chromatid
- chromosomes moved toward the center of the cell
Metaphase
- chromosomes line up on “equatorial” or metaphase plate
Anaphase
- centromeres divide
- one chromatid (now a chromosome) moves to each side
- each pole receives one of each chromosome
Telophase
- cytokinesis (division of cytoplasm) begins
- chromosomes begin to uncoil
- nuclear membrane begins to reform
Daughter Cells
- cell division is complete
- 2 identical cells (daughter cells) enter interphase
What is the outcome of mitosis?
How genetically similar are the cells produced by mitosis?
At what stage in the cell cycle does DNA replicate?
If a cell begins with 20 chromosomes, after mitosis how many chromosomes will each of the
daughter cells have?
Homologous chromosomes
8.11
- chromosomes from two different parents, brought together by sexual reproduction
(one in egg and one in sperm)
- have same genes at same loci so they affect same traits, but genes may be different alleles
(therefore, they are not identical)
What are alleles and how do they originate?
“ploid” terms tell how many chromosomes are in a homologous set
- haploid (1N) – cells have only one of each chromosome with the same genes – normal sex
cells
- diploid (2N) – cells have two of each chromosome
- normal body cells from fertilized egg
- ex. human body cells have 46 chromosomes, 23 sets of 2 homologous chromosomes
- human sex cells (gametes) have 23 chromosomes
- triploid (3N) – unusual condition in which a cell has three of each chromosome
- tetraploid (4N) “
“
four of each chromosome
If homologous chromosomes have the same genes why are they not identical?
Meiosis
- production of sex cells (gametes = egg and sperm) that are haploid for sexual reproduction
- reduces the number of chromosomes to one half
- 2 cycles of cell division (Meiosis I and Meiosis II)
- first separates homologous chromosomes (2N becomes 1N)
- second separates sister chromatids
- produces genetic variation
8.15-8.17
- crossing over during Prophase I (genetic recombination)
- random independent orientation of homologous chromosomes in Metaphase I
Know the stages of meiosis :
8.13
(as in mitosis the chromosomes are replicated in the synthesis substage, each chromosome has two
identical chromatids)
prophase I – chromosomes coil into short thick rods
- homologous chromosomes come together and join arm-to-arm (this is called
synapsis)
- crossing over – homologous chromosomes exchange identical section
- spindle fibers start forming and move to the poles of the cell
- the nuclear membrane breaks down
- a spindle fiber from one of the poles attaches to the centromere of one of the
chromosomes in a homologous pair, a spindle fiber from the other pole
attaches to the other homologous chromosome
- the chromosome pairs are moved toward the equatorial, or metaphase, plate in the
middle of the cell
After crossing over does a chromosome have the same genes as before? Does it have the same
alleles?
Why is it accurate to say that after prophase I of meiosis there is no longer mother and father
chromosomes
metaphase I
– the homologous chromosomes are on the metaphase plate as pairs, one facing one
pole with a spindle fiber, the other facing the other pole with a spindle fiber
- the random independent orientation of homologous chromosomes determines how
the chromosomes will be divided
anaphase I
– the homologous chromosomes in each pair separate and begin moving towards the
poles
telophase I
- the chromosomes are pulled to the poles and cytokinesis occurs to divide the
cytoplasm into two daughter cells
- in some species the nucleus is reformed, in others it does
not
prophase II
– nuclear membrane breaks down if it reformed, chromosomes move toward
metaphase plate
metaphase II
– individual chromosomes (each with two chromatids) are on the metaphase plate
- a spindle fiber from one pole attaches to the centromere of one chromatid, a spindle
fiber from the other pole attaches to the other chromatid
anaphase II
– chromatids separate and begin moving to each pole
telophase II
– cytokinesis occurs to divide the cytoplasm and produce two daughter cells
- chromosomes uncoil and the nuclear membrane is reformed
Sources of Genetic Variation
- mutations
- crossing over
]
] meiosis to
- random independent
] form
orientation of
] gametes
homologous chromosomes ]
- random mating
]
]
]
sexual
] reproduction
]
]
]
Genetic variation allows genetic change through time = adaptation
Is a cell in metaphase II haploid (1N) or diploid (2N)?
What is the value of crossing over and random independent orientation of homologous chromosomes
during meiosis?
If a cell with 20 chromosomes goes through meiosis, how many chromosomes will it have?
Ch. 9 Genetics
Heredity - passing of traits from parent to offspring
Genetics - scientific study of heredity
9.1
History of genetics:
- Ionian Greeks – Pangenes
- Early biologists (1800) – blending inheritance (continuous)
- Gregor Mendel - 1860’s – particulate inheritance, basic principles of heredity
(discrete)
- Thomas Morgan – early 1900 geneticist, role of chromosomes and genes, molecular
genetics
- Eugenics – 1920-1930’s – science misused
- “pseudoscience”
- “biodeterminism”
- genetically superior groups
- Structure of DNA described 1950’s
- Genetics of 1960’s – nurture (environment) more important than nature (genes)
- Genetics today - outcome of gene and environment interaction
- ex. 9.11B
Gregor Mendel’s Experiments
9.2 – 9.3
- based on breeding garden peas, following 7 true-breeding traits through several generations
- used well designed breeding experiments, large samples and mathematical techniques
- determined basic patterns of inheritance
These experiments led Mendel to several ideas:
- for each trait there are 2 factors (today called alleles of a gene on homologous
chromosomes)
9.4
- one factor inherited from each parent
- each sex cell carries only 1 factor
- one of the two factors determines the trait
From these ideas Mendel proposed two Laws of Heredity:
Law of Segregation – factors of inheritance separate into ½ of the sex cells (separation of
homologous chromosomes in meiosis)
9.3
Law of Independent Assortment – factors determining a trait segregate independently of
factors for other traits (independent orientation of homologous
chromosomes in meiosis)
9.5
- observed by following 2 traits at once in a cross
(called a dihybrid cross)
Probability
9.7
Inheritance is statistical process – follows rules of probability
Random = chance
probability of 0 to probability of 1
0% chance 100% chance
What are the 2 chance events in sexual reproduction?:
- forming sex cells by meiosis (different combinations)
- combining sex cells in fertilization (which sperm and which egg)
Why is probability not used in studying asexual reproduction?
Example: 52 cards, 4 suits, 13 different cards per suit
Probability of randomly selecting a card of spades 13/52 = 1/4
Probability of randomly selecting a Queen 4/52 = 1/13
Rule of addition (probability of 1 thing or another thing)
Probability of spade or club = 1/4 + 1/4 = 2/4 = 1/2
Rule of multiplication (probability of 1 thing and another thing together)
Probability of spade and Queen = 1/4 * 1/13 = 1/52
Small sample size – chance can give different results
Large sample size – actual (observed) more likely to be accurate (accurate measure of probability)
Some terminology:
- phenotype – the expressed, or physical, trait
- genotype – the alleles that are present; its genetic makeup
- allele – a variation of a gene due to a mutation
- homozygous – a diploid individual with the same two alleles for a gene (ex. RR )
- heterozygous – a diploid individual with different alleles for a gene (ex. Rr )
- dominant allele – is expressed whenever it is present, determines the individuals trait
- recessive allele – has no effect when in combination, expressed only when homozygous
- Punnett square – graphic method of determining probability distribution of genotypes and
phenotypes of offspring from cross between two individuals;
- hypothesis in experiment
- testcross – using a homozygous recessive individual to cross with an individual with an
unknown genotype
Experimental breeding (crossing individuals) is basis for discovering patterns of inheritance
– experiment to test hypothesis.
problems as examples:
Some complicating details of inheritance
9.11
(these examples follow Mendel’s laws of inheritance - affects phenotype , but not genotype)
Alleles can interact in different ways:
- alleles can interact as dominant and recessive pair = complete dominance;
heterozygous individuals have same phenotype as dominant homozygous
“simple Mendelian”
- alleles can interact to produce an intermediate trait = incomplete dominance;
heterozygous individual’s phenotype between homozygous
9.11
- alleles can both be expressed = codominance ; both alleles expressed independent of the
other
9.12
Multiple alleles - many genes have more than 2 alleles – Where do these alleles originate?
Ex.: ABO blood groups
9.12
Pleiotropy – a single gene may affect more than one trait
Ex.: sickle cell anemia
9.13
Polygenic inheritance - most traits influenced by more than one gene
9.14
Ex. skin color
Epistasis – the expression of one gene depends on another gene
Ex. production of A and B blood factor, agouti fur color, dogs, comb shape in chickens
Some complicating details of inheritance
(genes do not follow Mendel’s laws of inheritance)
Gene linkage – genes on same chromosome
9.17
- inheritance does not follow Mendel’s Law of Independent Assortment. Why?
- linked genes separated by crossing over in meiosis. Which phase?
9.18
- sex cells produced by crossing over cannot be included in Punnett square. Why?
- the farther apart the genes are on the chromosomes, the more frequently they are
separated (recombination frequency)
9.19
- recombination frequency data used to make chromosome maps – where genes are
located on the chromosome
- through evolutionary time alleles of genes that work well together came to be close
together on chromosome
Sex-link genes – if sex is determine by sex chromosomes
Sex determination
9.20
Genetically determined in many organisms
- sex chromosomes (all other chromosomes called autosomal)
X - Y sex chromosomes
XX ♀
XY
♂
X - 0 sex chromosomes
XX ♀
X
♂
Z - W sex chromosomes
ZW ♀
ZZ
number of chromosomes
2N ♀
♂
1N ♂
fertilized egg = 2N
Environmentally determined in some organisms
- temperature of nest
- chemicals produced by other individuals can change from one sex to another
Some organisms are both male and female (hermaphroditic)
Sex-limited genes – control sexual traits (sex organs, sexual characteristics)
– expressed under right conditions
– mostly not on sex chromosomes – on autosomal chromosomes
- inheritance typical "Mendelian"
Inheritance of sex chromosomes and Sex – linked genes
9.21-9.23
Sex chromosomes are not exactly homologous
(in humans ≈ 1100 genes on X only, ≈ 80 genes on Y only, 15 on both)
sex-linked genes on X or Y but not both
SRY
X
Y
Recessive alleles must be
homozygous to be expressed
in female; just one recessive
will be expressed in male
XX
♀
9.22
XY
♂
Example: hemophilia
red-green color blindness
Duchenne’s muscular dystrophy
Female offspring get X from mother and X from father
Male offspring get X from mother and Y from father
Pattern of inheritance of sex-linked genes
9.21
How does this affect phenotype ratios in males and females differently?
examples:
Use of pedigrees in studying inheritance of human traits. Why is this necessary?
Determine the inheritance pattern in some sample problems.
9.9, 9.22
Some really different patterns of inheritance:
Genomic imprinting - either paternal or maternal allele "silenced"
unknown inheritance patterns
-segregation distorter gene ex. tt in mice is lethal, but if Tt then t gets into 90% of sperm instead of 50% as
expected
Cytoplasmic inheritance - genes in mitochondria
- mitochondria only from mother (in egg)
Mutations
- changes in DNA copied in replication
- in humans 175 new mutation in 7 billion letters each generation
- inheritable mutations
- only in gametes
- passed to next generation
- produce alleles of a gene or a new gene (genetic variation)
- nonfunctional alleles often recessive
- lethal alleles 9.9
- most lethal alleles are recessive. Why?
- we all have lethal recessive alleles
Inbreeding increases the probability of these recessive lethal alleles being expressed. Why?
Examples:
1. autosomal recessive “loss – of – function”:
cystic fibrosis
phenylketonuria (PKU)
PP
Pp
pp
2.
9.9-9.10
- normal
- “
- PKU
sex-linked recessive:
hemophilia
9.22
Duchenne muscular dystrophy - most frequently occurring fatal genetic disease among
children
females : XD XD – normal
XD Xd - “
Xd Xd - Dmd
males : XD Y - normal
Xd Y - Dmd
1/3,500 in US males
1/3,500 * 1/3,500 = 1/12,250,000 in US females
Why is this different for males and females?
Gene codes for protein called dystrophin which connects cytoskeleton to connective tissue (3685
amino acids)
3. autosomal dominant
9.9
Huntington’s disease - causes deterioration of nerve cells and is always lethal
- usually expressed later in life after reproductive age
AH AH
AH A
A A
- Huntington’s
“
- normal
normal gene produces huntingtin - protein that regulates about 50 other genes including growth factor
for some types of nerve cells
abnormal protein has 3 - 215 repeats of GTC (glutamin) which causes clumps and no growth factor
4. autosomal dominant (incompletely dominant allele - lethal and beneficial)
9.13
Sickle-cell allele - has beneficial effects and lethal effects
H - normal allele describing hemoglobin
HS - sickling allele – result of point mutation which changed one codon and one amino acid
in hemoglobin
Sickling hemoglobin folds together in low oxygen environment; red blood cells then fold or
“sickle”
Sickled cells are destroyed, block capillaries and reduce blood volume
Genotypes
HH
Phenotypes
normal hemoglobin only
H HS
some normal hemoglobin, some sickling hemoglobin – sickle cell trait
HS HS
sickling hemoglobin only – sickle cell disease
Sickle cell disease is lethal; normal hemoglobin gives no protection from malaria; sickle cell trait
(heterozygous individuals) are resistant to malaria and survive best.
Biology 1406 Exam 4 Review Cell Division and Genetics Ch. 8 & 9
Describe the process of DNA replication. What happens and what is produced?
During which phase (subphase) of the cell cycle does this occur?
Describe a chromosome after this stage but before prophase (what are sister chromatids).
Why is DNA replication important to cell division?
Describe the process of mitosis. Make drawings showing how the chromosomes are positioned in
prophase (late prophase = prometaphase) , metaphase, anaphase and telophase.
List the key events that occur in each phase.
In which cells (body or sex cells) does mitosis occur and how similar are the cells produced?
What is the goal of mitosis (what are the cells that are produced used for)?
If a diploid cell with 18 chromosomes completes mitosis how many chromosomes will each of the
daughter cells have?
Define the term homologous chromosomes. Where do they come from?
How similar are they and what are the differences between them called?
How do the terms diploid and haploid relate to homologous chromosomes? Define each term.
Describe the process of meiosis. Make drawings showing how the chromosomes are positioned in
each stage (prophase I and II, metaphase I and II, anaphase I and II and telophase I and II).
List the key events that occur in each stage.
What is produced by this process?
What are the two major goals of meiosis in producing sex cells?
What is meant by the term genetic variation?
Describe two processes in meiosis that produce genetic variation. In which stages do these occur?
Define the terms heredity and genetics.
How does “blending” inheritance differ from “particulate” inheritance?
Who first proposed particulate inheritance?
State Gregor Mendel’s two laws of heredity and explain how they relate to meiosis.
Define the term allele and give an example. What is the source of new alleles in a species?
Define the terms homozygous and heterozygous.
Define the terms genotype and phenotype.
How do dominant and recessive alleles differ from incompletely dominant alleles and codominant
alleles?
What does the phenotype look like in each case?
How does probability relate to the study of inheritance? (Why is it necessary?)
What is a Punnett square and how is it related to probability and predicting offspring?
How is meiosis related to the use of the Punnett square?
What do we mean when we say that the Punnett square is the hypothesis in an experiment?
How are Punnett squares used in the study of inheritance?
Predict the genotypic and phenotypic ratios expected in the offspring from the cross between a
TtDdBB female and a TtDdbb male (T=tall, t=short; D=curly leaf, d=flat leaf; B=brown leaf,
b= white leaf).
Describe some variations that complicate simple “Mendelian” genetics. Give an example of each:
multiple allele genes; pleiotropy; polygenic traits, epistasis.
Describe some variations that complicate simple “Mendelian” genetics and change expected
genotype ratios.
What are linked genes? Why do they not follow Mendel’s law of independent assortment?
What event in meiosis causes linked genes to somewhat follow Mendel’s law of independent
assortment?
What are recombinant phenotypes (recombinants)?
Why are the frequencies of recombinant phenotypes different for different pairs of genes?
How are gene maps drawn from recombinant frequencies?
What is a possible advantage of gene linkage?
Describe how sex is determined in mammals. How is this different from environmentally determined
sex?
What are sex-limited genes and how do they differ from sex-linked genes?
Explain how the inheritance of sex-linked genes differs from sex-limited (and other autosomal)
genes.
What are autosomal genes?
Why are the X and Y chromosomes not homologous, even though they pair in meiosis? Do they cross
over?
Why are sex-linked diseases more common in males than in females?
Sex-linked traits frequently have a very distinct phenotype ratio pattern. What is this and why does it
happen?
What are pedigrees and why are they necessary in the study of human traits?
Complete pedigree analyses assigned and be able to distinguish between autosomal dominant,
autosomal recessive, sex-linked dominant and sex-linked recessive traits.
Define the term inheritable mutations. Where do these mutations first occur?
What are lethal alleles and why are most lethal alleles that we see recessive and not dominant?
What is inbreeding? How, and why, does inbreeding affect the expression of recessive lethal alleles?
Describe the inheritance of the following genetic diseases in humans: phenylketonuria (PKU - a
recessive lethal), sickle-cell anemia (an incomplete dominant lethal), Duchenne muscular dystrophy
(recessive sex linked) and Huntington’s disease (a dominant lethal).
How can a dominant lethal allele survive in a species? How can a lethal allele be both harmful and
beneficial?