Ms. SastryAP Biology
Leigh High School
Unit 3 – Genetics and Molecular Biology (Chp 12 to 16)
Checked and approved by your instructor – use to compare answers/study for final.
Chapter 12 – Mitosis Objectives:
1)What loses heat faster – an elephant or a mouse? Why?
A mouse will lose heat faster because it has a smaller volume to surface area ratio.
2)Why do cells divide?
Cells divide in order to maintain a small volume to surface area ratio. This ensures that the cells are efficient in transporting nutrients and ridding itself of waste.
3)What is mitosis? When do cells undergo mitosis?
Mitosis is the process by which the cells divide. A cell will undergo mitosis when it is signaled to by its surrounding and has reached the sufficient size to do so.
4)What are somatic cells? How many chromosomes do they contain? Do they undergo mitosis?
Somatic cells are cells in a multicellular organism that are not sex cells. They contain the normal amount of chromosomes in that particular organism. For example, the normal human contains 46 chromosomes. These cells undergo mitosis.
5)What are gametes? How many chromosomes do they contain? Do they undergo mitosis?
Gametes are sex cells- sperm or ova. Each contains half the number of chromosomes that are in the somatic cells. Gametes do not undergo mitosis in humans.
6)What are chromosomes – why are they important for mitosis?
Chromosomes are the condensed DNA strands present during mitosis. The DNA would be unmanageable during mitosis if it did not condense.
7)What are genes? What is the genome?
Genes are the parts of the DNA that code for proteins. The genome is the complete length of genetic material in the cell.
8)Describe the ultra-structure of the chromosome (include definitions for chromatin, chromosome, beads on a string, histone and nonhistones). Make a quick sketch of how DNA is folded to make chromosomes.
chromatin- uncondensed DNA, contained inside the nuclear envelope
chromosome- condensed DNA present during mitosis and meiosis
histone- a protein containing a high proportion of positively charged amino acids which binds to negatively charged DNA to hold it in its chromatin structure
nonhistone- proteins that loop DNA together to create the chromosome structure
9)What are sister chromatids? Draw them. What structure connects the two sister chromatids?
Sister chromatids are identical chromosomes present in the cell after DNA replication. During mitosis, a centromere connects the two sister chromatids.
X
10) What is DNA replication and how does it relate to sister chromatids?
DNA replication copies the DNA so that there are two identical copies in the cell. Sister chromatids are identical copies of the same segment of DNA and are the result of DNA replication.
11) Why should DNA or chromosome replicate/duplicate itself? When does this happen during the cell cycle?
The DNA replicates itself so that the genetic information is present in both daughter cells. This happens during the S phase of interphase.
12) What is cytokinesis – when does it occur?
Cytokinesis is the division of the cytoplasm. It occurs at the end of telophase, the cell cycle phase following mitosis.
13) What are the cell cycle phases? Use this cell cycle website to review it.
The cell phases are interphase, prophase, metaphase, anaphase, telophase, cytokinesis. Interphase is broken into three parts: G1, S, G2.
14) Using the above cell cycle website and the Sci. Am article – ‘How Cancer Arises’ write an essay detailing how regulation of the cell cycle occurs and the consequences of the failure in the checkpoints of the cell cycle.
Enzymes called kinases regulate cell division in conjunction with proteins such as cyclins. These proteins are dependent on the kinase enzyme to become phosphorylated thus, activating/deactivating them. There are several checkpoints during the cell cycle that ensure the proper replication, alignment, distribution of genetic material to the daughter cells. When these checkpoints are transgressed, abnormalities result such as aneuploidy/cancer.
15) How long does a cell spend in the mitosis phase? How long does it spend in interphase?
The cell spends about 90% of its time in interphase, so 10% of its time is in the mitosis phase.
16) What are the three phases of interphase? Describe what happens during each one.
The three phases of interphase are G1(gap 1), S, G2(gap 2). During gap 1, the cell grows in size and it is also making protein and other factors needed for DNA replication. S phase is all about replicating DNA. G2 phase – organelles are dividing and more growth and protein synthesis before mitosis.
17) How many chromosomes do we have in each of our cells? Why do we have 2 sets of each chromosome even in G1 phase, before replication of DNA? So, how many chromosomes will we have then after S phase?
G1- 23 chromosomes come from the mother, and 23 from the father, which together form the 23 pairs. Because the sister chromatids are attached after replication, we will still have 23 pairs of chromosomes in the cell after the S phase. For a brief time during Anaphase and Telophase we will have double the number of chromosomes – 46 pairs and the original condition – 23 pairs is restored when cytokinesis occurs.
18) Describe what happens during the different phases of Mitosis.
(Website -
Interphase- cell growth, DNA replication
Prophase- condensation of chromatin (forms chromosomes), nuclear membrane disappears
Metaphase- chromosomes line up at center of the cell (metaphase plate)
Anaphase- separates sister chromatids
Telophase- two nuclear membranes form (one for each daughter cell), cytokinesis occurs
19)Compare plant and animal cell mitosis.
Plants form a cell plate, animal cells do not (cleavage furrow and actin filaments). Animal cells contain centrioles, plant cells do not.
20) Describe how the mitotic spindle is formed and used to pull the chromosomes apart.
The mitotic spindle is made of many subunits of microtubules (specifically tubulin) combined. Subunits assemble and disassemble to pull chromosomes apart. The microtubules disassemble on the side towards the centrioles and the spindle fibers appear to reel in the chromatids to the respective poles.
Chapter 13 – Meiosis Cell Division Objectives:
1)Define heredity and variation.
heredity- process of passing on genes
variation- genetic variations that result in different phenotypes, essential for evolution
2)What is a karyotype?
A karyotype is a map of an individual’s chromosomes. It is derived from the mitotic chromosomes.
3)Where did you get your 23 pairs of chromosomes from?
23 chromosomes come from the mother, and 23 from the father, which together form the 23 pairs.
4)How many chromosomes does your gamete/germ cell contain?
The gamete/germ cell contains 23 chromosomes.
5)Why is sexual reproduction a price to pay compared to asexual reproduction? What are the returns/benefits of going SEXUAL?
In sexual reproduction, the individual only passes on half of their chromosomes, while asexual reproduction results in exact clones. The advantage is more variation.
6)Explain the terms – haploid and diploid.
Haploid – set of single chromosome
Diploid – set of duplicate chromosomes
7)What are homologous chromosomes? Are they the same as sister chromatids? Why or why not?
They are pairs of chromosomes from both parents. They are not the same; sister chromatids are exact copies of a single chromosome and homologous chromosomes are a pair that code for the same trait but are not the same.
8)How many sets of homologous chromosomes do you have?
23
9)What happens to the homologous chromosomes during meiosis?
Ripped apart during meiosis 1
10) Review question - What happened to the sister chromatids during mitosis? Will this happen in meiosis at some point? When?
The sister chromatids are taken apart in anaphase in mitosis. They will come apart during meiosis 2
11) What is reduction division – why is meiosis considered as such? In comparison, what is mitosis referred to as?
Gametes end up with less chromosomes than they started with so reduction division. Mitosis is equation division.
12) Will the organism change from diploid to haploid after meiosis? Why or why not?
The gamete cell (not organism) is haploid. They are haploid because they must pair with the mates’ chromosomes when they form the zygote.
13) Describe how Prophase I of meiosis causes the genes from the homologous chromosome pairs to get mixed up. What is this called? (Define tetrad, chiasmata, synapsis)
It is called crossing over
tetrad: homologous chromosome held together
chiasmata: crossing over
synapsis: held together to form tetrads
14) Compare Metaphase I with Metaphase II. Do the same for the remaining phases of Meiosis.
1: homologous line up; in 2 it’s the sister chromatids. (metaphase)
in 1 homologous break up; in 2 sisters break up. (anaphase)
15) How do crossing over, independent assortment and random fertilization contribute to the diversity and variation among the offspring produced?
Crossing over occurs in Prophase of meiosis I and it leads to the mixing of alleles on homologous chromosomes. Independent assortment occurs due to there being multiple ways of aligning different sets of chromosomes along the metaphase plate during Meiosis I and II – leading to a mixing of traits inherited on separate chromosomes. Random fertilization causes the union of any sperm with any egg that may e produced leading to more variation. They introduce variations by mixing and matching – more details in powerpoint (important question).
16) Do the G1, S, and G2 phase occur before meiosis?
Yes. But S phase occurs only once and 2 divisions take place therefore chromosome number is halved at the end of meiosis.
Chapter 14 – Mendelian Genetics and beyond - Objectives:
1)What are traits?
Traits are the physical features that can be seen on an organism.
2)Define the following:
a)Phenotype – trait/feature seen on an organism.
b)Genotype – genetic make up of an individual for a given phenotype.
c)Alleles – 2 alleles = a trait; describe the different genes.
d)Homozygous – have two of the same alleles, either recessive or dominant.
e)Heterozygous – having two different alleles, one dominant and one recessive trait.
f)Dominant – allele that is fully expressed in a heterozygote.
g)Recessive – allele that is masked in a heterozygote.
h)True breeding – always producing offspring with the same traits as the parents, when the parent plants are self-fertilized.
3) Draw Punnett squares and illustrate the following crosses:
a)Monohybrid Cross:
A a
AA / AaAa / aa
b)Dihybrid Cross:
AB Ab aB ab
AA BB / AA Bb / Aa BB / AaBbAA Bb / AA bb / Aa Bb / Aa bb
Aa BB / Aa Bb / aa BB / aa Bb
Aa Bb / Aa bb / aa Bb / aa bb
c)Test or Back Cross: one individual has to be homozygous recessive.
D d
Dd / dddd / dd
4)State and prove Mendel’s Law of Segregation using an example
Alleles (A and a) separate in meiosis (gamete formation) since homologous chromosomes are separated. They separate in Meiosis I. The alleles separate in this “law”. An example would be a flower (Aa) whose chromosomes separated in meiosis to become the gametes A, A, a, and a.
5)State and prove Mendel’s Law of Independent Assortment using an example
This is when there are 2 or more allele pairs, and each pair of alleles segregates into gametes independently. There are at least two traits being looked at, as in a dihybrid cross but the alleles are on different chromosomes with no crossing over. An example would be a dihybrid cross between color (Aa) and height (Bb) in mice. The resulting gametes would be AB, ab, Ab, and Ba.
6)Illustrate rule of multiplication and rule of addition with examples
The rule of multiplication – the chance that two or more independent events will occur together. First you compute the probability that it will occur, then multiply them together. For example, the probability of a heterozygous pea plant (Aa) of producing a recessive offspring (aa) is 1/2 x 1/2 = 1/4. The rule of addition - the probability of an event that can occur two or more different ways is the sum of the separate probabilities of those ways. For example, the probability of a heterozygote in a monohybrid is 1/4 + 1/4 = ½ because you can get the allele from mom or dad (2 ways for same event and only one occurs).
7)Explain the following exceptions to Mendelian rules with examples:
a)Incomplete Dominance – heterozygotes show a distinct intermediate phenotype. I.e. – Purple (AA), Blue (Aa), Green (aa).
b)Codominance – two alleles affect the phenotype in separate, distinguishable ways. I.e. – a flower is both purple and white.
c)Pleiotropy – affecting more than one phenotypic character. I.e. – missing protein in blood cells results in sickle cell anemia with many other complications.
d)Epistasis – one gene alters another genes phenotypic expression. I.e. – B = dark hair, b = light hair; BB, Bb = dark hair; bb = light hair.
e)Quantitative characters – traits depending on how many genes are expressed. I.e. – Skin color has three genes to express color; from darkest to lightest: AABBCC, AaBBCC, AABbCC, AABBCc, AaBbCc, aaBBCC, AabbCC, AABBcc.
f)Environmental effects on phenotype – Nature versus nurture. I.e. – the surrounding environment affects how organisms look, along with their genetics.
8)How does a genotype determine the phenotype?
The genotype is the make up of genes, so depending on how the recessive and dominant genes are assorted and combined shows through in the phenotype (physically). I.e. – Genotype: BB, Bb, bb; Phenotype: Dark, dark, light. The genes code for proteins that may be directly expressed as the phenotype ( example blood groups), OR it codes for an enzyme that regulates a pathway leading up to a phenotype – example - skin color and melanin production/deposition.
9)What does DOMINANCE mean in the above context?
Dominance is the allele that is expressed fully, even in a heterozygous genotype. So it allow for that protein to be synthesized. Even if there is one copy of that gene, the protein is still made and expressed normally in most cases except in codominance/incomplete dominance.
10) How are human blood groups inherited? Do they follow Mendelian rules?
Human blood groups are inherited with half from mom and half from dad. It does follow Mendelian rules for the I and i alleles, but not for the A and B alleles (codominance).
IAIA or IAi = A blood group
IBIB or IBi = B blood group
ii = O blood group
IAIB = AB blood group
11) What is a pedigree? How is it used?
A pedigree is a map out of family traits and it predicts phenotypes. It shows the genes of family members and that information is used to guess which phenotype a child will inherit.
12) What are some methods of genetic screening to determine abnormalities in the unborn fetus?
Amniocentesis is where some of the amniotic fluid is extracted from the mother’s abdomen to test it for genetic abnormalities. Chorionic villus sampling (CVS) is when a catheter is used to extract part of the placenta inside the mother. Ultra sound and fetoscopy are visual tests that are used.
Chapter 15 Chromosomes and Heredity – Objectives:
1)How do segregation, crossing over, and independent assortment lead to different genes ending up in different combinations in the offspring?
Crossing over occurs in Prophase of meiosis I and it leads to the mixing of alleles on homologous chromosomes. Independent assortment occurs due to there being multiple ways of aligning different sets of chromosomes along the metaphase plate during Meiosis I and II – leading to a mixing of traits inherited on separate chromosomes. Random fertilization causes the union of any sperm with any egg that may e produced leading to more variation. They introduce variations by mixing and matching – more details in powerpoint (important question).
2) How did Thomas Hunt Morgan show that traits/genes are carried on chromosomes using Drosophila? Show the Punnett squares for the F1 and F2 generations. (website 1)
Morgan showed that genes are carried on chromosomes because there was a three to one ratio but all the white eyes were on the males, which shows that the eye color trait is carried on the X chromosome.
P1F1
XRXR / XRXrXRY / XrY
XRXr / XRXr
XR Y / XRY
3)What else did Morgan’s fruit fly experiment suggest other than the fact that chromosomes carry genes?
Not only was this gene on a chromosome, but more specifically it’s on the X sex chromosome, which disagrees with Mendelian Genetics.
4)What are some common characteristics of X linked traits?
When the gene is recessive, the “disease” always shows up in the males. Also, women need two recessive to have it appear in them, and only one makes them a carrier.
5)What are linked genes? How are they inherited? (website 2)
Linked genes are non-Mendelian and are genes that are inherited together because the whole chromosome is passed on as a unit. They are inherited through meiosis and gamete formation when there is no crossing over to separate these genes – so all genes that are linke will be passed on from one parent only.
6)What are recombinants? How do they arise? (website 3)
Recombinants are genes that cross over and are passed on. This occurs in meiosis I.
7)Show the parental and recombinant phenotypes in a comparison of meiosis end products from linked and unlinked genes (website 2- last image):