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Ch. 8
Biology 1406 Exam 3 Notes
Cell Division
Ch. 8, 9
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
- cells are genetically the same
- Meiosis
- produces reproductive cells called gametes (egg and sperm) for sexual reproduction
- 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: 8.5
8.4 – 8.10
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
Genetic Variation
- mutations
- crossing over
]
]
] meiosis to ]
- random orientation of
] form gametes ]
sexual
homologous chromosomes ]
] reproduction
]
- random mating
]
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 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
History of genetics:
- Ionian Greeks – Pangenes
9.1
- 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
9.19
- 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 (these examples follow Mendel’s laws of inheritance) 9.11
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
Many genes have more than 2 alleles – multiple alleles
Ex.: ABO blood groups
Where do these alleles originate?
9.12
Pleiotropy – a single gene may affect more than one trait
Ex.: sickle cell anemia, Marfan syndrome
Most traits influenced by more than one gene – polygenic inheritance
Ex. skin color
9.14
- 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
9.13
Biology 1406 Exam 3 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.