Biology Lab Practical #2 (Topics 8-12)

Meiosis

Used for the creation of gametes

• Creates 4 haploid cells (1n) → 23 chromosomes for humans

• Creates genetic variation and the resulting cells are different from each other and from the starting cell

Mitosis

Used in cell growth and repair

• Creates 2 diploid cells (2n) → 46 chromosomes for humans

• Cells are identical

Homologous chromosomes

chromosomes with same genes at the same loci but different alleles and same morphology (size and shape); one from each parent (mom and dad)

Meiosis I

Homologous chromosomes separate

• Interphase – DNA replication

• Prophase I – creation of tetrads (homologous chromosomes), synapsis, and crossing over occurs

• Metaphase I – independent assortment, Homologous chromosomes line up at the equator of the cell

• Anaphase I – pulls homologous chromosomes apart

• Telophase I – homologous chromosomes are completely separated, uncoil, and Cytokinesis I overlaps with the cleavage furrow formation and complete division of the cytoplasm

Meiosis II

sister chromatids of chromosomes separate

• NO interphase goes straight to prophase

• Prophase II – Chromosomes with sister chromatids are visible

• Metaphase II – Chromosomes line up on the equator of the cell

• Anaphase II– pulls sister chromatids of chromosomes apart

• Telophase II – chromosomes are completely separated

• Cytokinesis II – cleavage furrow pinches in, cytoplasm is divided

End result: 4 haploid daughter cells each genetically different

Spermatogenesis (Male meiosis)

Occurs inside each testis; one diploid germ cell makes four haploid sperm cells

Oogenesis (female meiosis)

one diploid cell makes one haploid ovum (female egg cell) with most of it cytoplasm and two or three polar bodies that are smaller and haploid to get rid of extra chromosomes (degenerate)

Tetrad

group of four sister chromatids that form when two homologous chromosomes pairs come together (synapsis) during prophase I of meiosis

Locus

the position where a gene is found on a chromosome

Synapsis

homologous chromosomes pair up (Meiosis I → Prophase I)

Centromere

region where the cell’s spindle fibers attach

Chiasma

where crossing over takes place

When does crossing over occur?

Prophase I of Meiosis I

Table 8-1.) Combinations of alleles and chiasma positions

New combination of alleles:

1.) A B c d & a b C D

2.) a b C D & A B c D

3.) A b c D & a B C d

See extra problems in the Folder Materials for Lab Practical II

What kind of genetic variation occurs due to crossing over?

unique combinations of alleles along length of chromosome

What kind of genetic variation occurs due to independent assortment?

different combinations of chromosomes in daughter cells

Homozygous

individual has two identical alleles for a specific gene

ex.) AA or aa

Heterozygous

individual has two different alleles for a specific gene

ex.) Aa

Recognize the phases of Meiosis I and II

Meiosis I looks similar to Mitosis with some differences, Meiosis II starts off with two haploid daughters and split into four haploid daughters

Starting with three pair of homologous chromosomes, how many different combinations of chromosomes can be present in the meiotic cells?

8 different possible chromosome combinations

formula: 2n

• n → number of homologous chromosome pairs

• n = 3 → 2(3) = 8

In humans, there are 23 pairs of homologues. 

• How many different combinations of chromosomes are expected to result from independent assortment in humans? Express your answer in exponential form with appropriate base.

• Derive a formula that gives the number of different combinations of chromosomes expected from independent assortment per number of homologous chromosome pairs in the starting cell.

• 2²³

• C = 2ⁿ

What type of genetic variation is generated by independent assortment during meiosis?

new combinations of maternal and paternal chromosomes and alleles

The two homologous chromosomes that come together in Prophase I in Meiosis I are called a(n) _______.

tetrad

Crossing over occurs in _____.

Prophase I of Meiosis I

Homologous chromosomes separate in ______.

Meiosis I

_______ is a type of division that produces genetic variation within the new cells produced.

Meiosis

_________ chromosomes are chromosomes having the same genes at the same loci but different alleles, and one comes from each parent.

Homologous

What stage of Meiosis is this?

Anaphase I

_____ always involves two successive divisions.

Meiosis

Allele Combination Before Crossing Over:

A B C d e F

a b c D E f

Chiasma position between C and D.

What is the new combination of alleles?

Allele 1 recombination/crossing over results: A B C D E f

Allele 2 recombination/crossing over results: a b c d e F

Sister chromatids separate in ____.

Meiosis II

About how many different combinations of chromosomes are expected to result from independent assortment in humans? 

2²³ or 8 million

What is the diploid number of chromosomes for humans?

46

• Diploid → 2n

• 2(23) → 46

Are the cells at the end of Meiosis I haploid or diploid?

Haploid (n)

How many spermatids are formed through spermatogenesis?

Four

How many secondary oocytes are formed through oogenesis?

One ????? and 2-3 polar bodies

Why do spermatogenesis and oogenesis produce a different number of gametes?

Spermatogenesis → equal cytokinesis

Oogenesis → unequal cytokinesis

Describe the relationship between expected and observed outcomes as sample size increases.

As sample size increases, the expected and observed outcome get closer; Observed gets closer to expected, 0.50

What is the expected phenotypic ratio?

3:1

(2 heterozygous parents → 3 express dominant trait, 1 express recessive trait)

What is the expected genotypic ratio?

1:2:1

(1 AA, 2 Aa, 1 aa)

What proportion of the progeny (offspring) is expected to breed true (homozygous) for flower color?

2/4 (PP and pp)

What proportion of the progeny is expected to segregate (heterozygous) for flower color?

2/4 (Pp)

What proportion of the purple flowered progeny is expected to breed true for flower color?

1/3 (homozygous dominant; look only at purple flowers, not all)

What proportion of the purple flowered progeny is expected to segregate for flower color?

2/3 (Heterozygous; look only at purple, not all)

 If two pea plants, one heterozygous for plant height and the other short, are crossed, what is the proportion of the progeny is expected to be tall?

½ or 2/4 → 2 progeny express tall (Tt)

If two pea plants, one heterozygous for plant height and the other short, are crossed, what is the proportion of the progeny is expected to be homozygous dominant for height?

0/4 (none are homozygous dominant or TT)

If two pea plants, one heterozygous for plant height and the other short, are crossed, what is the proportion of the progeny is expected to be breed true for height?

2/4 (homozygous → tt)

If two pea plants, one heterozygous for plant height and the other short, are crossed, what is the proportion of the tall progeny is expected to breed true for plant height?

0/4 (no homozygous tall)

Probability

number of specified events ÷ total number of possible events

-Probability of heads in coin toss:

• heads (1 event) ÷ heads or tails (2 events) = ½ or 50% probability

-Probability of being born on Monday:

• Monday (1 event) ÷ days (7 events) = 1/7

-Probability of being born in February:

• February (1 event) ÷ months (12 events) = 1/12

Phenotype

physical appearance

ex.) purple or white flowers

Genotype

actual genes; expressed by 2 letters

ex.) PP or Pp

Gene

section of DNA, code for products (proteins, metabolite)

Alleles

alternative versions of a gene

Dominant Allele

expressed by capital letter

ex.) P

Recessive Allele

expressed by lower case letter

ex.) p

What are the 5 reasons we use Drosophila melanogaster (fruit fly)
for genetics experiments?

1. Easily grown in lab or cultured

2. They have short generation times (time from birth to reproductively mature, as short as 10 days)

3. One mating or cross can produce lots of offspring or progeny

4. Large number of mutations are well-known

5. True-breeding lines are available to order to understand contrasting traits and genetic crosses

Be able to identify male and female fruit flies

Male: smaller, darker abdomen

Female: larger, lighter abdomen

Autosomes

numbered chromosomes that contain genes for anything that doesn’t relate to sex determinations (somatic or body cells)

• humans have 44 (22 pairs)

• labeled with numbers (1-22)

Sex Chromosomes

lettered chromosome that is involved in sex determination

• humans have 2 (1 pair)

• labeled with letters (X and or Y)

Be able to recognize chromosomes from a male and female fruit fly

Male has a weird hook thingy compared to Female

P generation

Parental generation in genetic cross

F₁ generation

First filial generation, offspring of the P generation

F₂ generation

Second filial generation, offspring of the F1 generation

Know how to use a chi-square table and determine if the observed and expected frequency are statistically significant.

Steps

• Find Degree of Freedom: (Number of choice - 1)

• P-value = 0.05

• Find value that matches degree of freedom and p-value

If Chi Square value is larger > than the table, the data is signficant; if smaller <, then not significant

Chi square formula

X² = ∑ (Observed value - Expected value)² ÷ (Expected value)

What probability or alpha value do we usually use in science when
using the t-test or Chi Square Table?

0.05 (5%)

If we tested fruit flies and observed number of males, females,
winged, and wingless, what is the Degree of Freedom?

3

• Degree of Freedom = # of choices - 1

• 4 choices (female or male + winged or wingless) - 1

Using the figure, if the Chi Square calculated from your data was 8.03, is this
statistically significant?

8.03 > 3.814; significant

Chi square value greater than table

If an organism's DNA contains 35% adenine, how much guanine does it contain?

15%

What is the final product of translation?

protein

What is the start codon?

AUG (Met)

_____ is a mutation the changes a base pair in DNA but codes for the same amino acid.

A silent mutation

Does mRNA copy the coding strand or the template strand of DNA?

It copies the template strand of DNA.

Describe the orientation of the two DNA strands with respect to one another in a double-stranded DNA molecule.

The orientation is antiparallel.

Where does translation take place in the cell?

Ribosome

Give the standard notation for a strand of DNA that is complementary to: 5' - CGGTA - 3'

5' - TACCG - 3'

What nitrogen base is found in RNA but not DNA?

Uracil

The (template strand / coding strand) is complementary to mRNA? (Pick one choice)

template strand

What is the expected phenotypic ratio when two heterozygous parents are crossed?

3:1

What is the probability of being born on October 1st?

1/365

What gender is the fruit fly on the left?

male

The _________ is the actual genes in an organism, and it normally is represented by two letters.

genotype

If we tested pea plants and observed purple flowers, white flowers, tall plants, and short plants, what is the Degree of Freedom?

3

• DF: number of choices - 1

• 4 choices (purple or white + tall or short) - 1 = 3

The ___________ is the physical appearance of an organism, like red eyed fruit flies.

phenotype

What happens to the observed and expected outcomes as sample size increases?

They should get closer.

When individuals are true breeding, they can be ______.

homozygous recessive or homozygous dominant

If yellow peas are dominant to green peas, and one parent is heterozygous and one parent has green peas, how many of the offspring are true breeding for pea color?

½

Which is the female karyotype?

A