General Biology Exam 2

Cell Theory

All Cellular life is made of one of more cells

• Basic organizational and functional units

Cell Division =

Cell reproduction

Unicellular

Reproduction only

Multicellular

Growth and development, repair

Basic Definition of what happens during cell division and what’s its main function

All parts of cell divided into new cells

Main Function: Distribute genetic material into daughter cells

Genome

A cells total genetic material

• Prokaryotes - Usually 1 circular DNA molecule

• Eukaryotes - Usually > 1 linear DNA molecule

What are the two definitions for chromosome

1. Molecule of DNA

2. “Colored Body”
   - Structure of DNA and proteins highly condenses, densely packed
   - Only present during cell division

Chromatin

DNA/Protein complex in dispersed state

DNA in this state when the cell is not actively dividing

Haploid(n)

Having one complete set of chromosomes

• Haploid cells have 1 of each chromosome

• Gametes are haploid

• Go under mitosis

Diploid (2n)

Having 2 complete sets of chromosomes

• 2 of each chromosome (Homologous chromosomes or homologous pair)

  • Homologous chromosomes have length, structural features, same genes

• Human Somaric cells

• Undergo mitosis

Prokaryotes divide through

Binary Fission

• Simpler than mitosis

Interphase

• Time between Cell Divisions

• Cell highly active (Growth, synthesis, metabolic activity

• Long - At least 90% of cell cycle

• DNA is chromatin

• There are 3 phases

  • G1 (Gap 1)

  • S (Synthesis)

  • G2 (Gap 2)

G1(Gap 1)

Growth, development, normal functions

Many cells spend most of their lives in G1

S(Synthesis)

DNA replication

DOES NOT CHANGE PLOIDY

Chromosomes after replication

• 2 sister chromatids - identical copies

• Connect centromere

G2 (Gap 2)

High metabolic activities, preparations for mitosis

DNA still chromatin

Usually shorter than G1 or S

M Phase

Mitosis and Cytokinesis

Shorter part of cell cycle, <10%

Mitosis: Nuclear division of somatic cells

• divided into 4 stages: prophase, metaphase, anaphase, telophase

Prophase

Chromatin Condenses into chromosomes

Nucleus breaks down

Mitotic spindle forms

Mitotic spindle

fibers within cell oriented from pole to pole

guide chromosomes movement during mitosis

Metaphase

Chromosomes align at metaphase plate

Anaphase

Sister chromatids separate, move to opposite polls

Pulled by kinetochores - proteins attached to centromeres

After separation, each chromatid is considered an individual chromosome

Cytokinesis

Cytoplasmic/cellular division - 1 cell divides into 2 cells

Distinct process from mitosis but generally overlaps with telophase

Cytokinesis in “animal” cells (no cell wall)

Cleavage furrow: a region where parent cell pinches inwards

• contracts until parent cell completely divides into two daughter cells

Cytokinesis in “plant” cells (Cells with cell walls)

Cell Plate: new membrane at location of metaphase plate

• Fuse with existing plasma membrane, separates daughter cells

Heredity

Transmission of traits for one generation to the next

Variation

Differences between individuals

Genetics

The study of heredity and heredity variation

Gametes

Reproductive cells that transmit genes for one generation to the next

Asexual Reproduction

Single parent produces offspring

In Eukaryotes:

• 1 diploid parent → 2 diploid offspring

• 1 haploid parent → 2 haploid offspring

Produces clones

Clones

Genetic identical to parent and each other

Unicellular

divide

Multicellular

Buddy or fragmentation

Advantages of Asexual Reproduction

• Fast

• less energy required

• safe

• lots of offspring

• keep adaptations (no genome dilution)

Sexual Reproduction

Fusion of 2 gametes from a zygote

Gametes usually from different parents

Offspring NOT genetically identical to parents or each other

zygote

diploid cell resulting from fertilization

Fertilization

fusion of two gametes

Costs of sexual reproduction

• Slower

• Higher energy requirement

• Dangerous

• Fewer offspring

• Genome dilution

Advantage of sexual reproduction

Genetic variation

• Offspring represent novel combination of parents’ genes

• More likely able to survive environmental change or stress

Why is genetic variation good

Illustration of low and high variation populations exposed to new predator

A problem and the solution about sexual reproduction

Problem: If gametes have same number of chromosomes as parents, chromosomes double very generation

Solution: Meiosis

Meiosis

Reproductive division - Cells divide twice, chromosome number halved

1 diploid cell → 4 haploid cells (gametes)

4 stages, including 2 cell divisions

Humans entering Meiosis

2n=46

46 chromosomes

92 chromatids

Meiosis I

First meiotic division

• Homologous chromosomes separate, ploidy reduced

• 4 stages each named with roman numerals ( I )

Prophase I

Same 3 prophase things as mitosis plus:

Synapsis

Crossing over

• At gene level:

  • The point: new combinations, more diversity

Synapsis

Homologous chromosomes pair up

• Results in Tetras: 2 homologous chromosomes (4 chromatids) held together

Crossing over (homologous recombination)

Exchanges (swapping) between non-sister chromatids in tetrad

Result’s in new combinations of genes

Important source of genetic diversity

How many chromosomes/ tetrads/chromatids at the end of prophase I in humans

Chromosomes: 46

Tetrads: 23

Sister Chromatids: 92

Metaphase I

Tetrads align at metaphase plate

Homologous chromosomes in tetrads oriented towards opposite poles

Anaphase I

Homologous chromosomes separate (disjunction)

Sister chromatids still connected

chromosomes act independently

Telophase I and cytokinesis

Normal telophase things may not happen – might go right to next stage

Cytokinesis occurs

• Results in 2 haploid cells with duplicated chromosomes

Chromosomes/tetrads/chromatids at the end of telophase I

Chromosomes: 23

Chromatids: 46

Tetrads: 0

Meiosis Summary

Starts with 1 diploid cell with duplicated chromosomes

Ends with 2 haploid cells with duplication chromosomes

Crossing over in prophase I

Homologous pairs separate in anaphase I

Sister chromatids do not separate

Meiosis I is when ploidy is reduced

Interkinesis

Time between 1 st and 2 nd meiotic divisions

Usually short, interphase-like stage

No S-phase, no DNA replication

Meiosis II

2 nd meiotic division

Chromatids separate into daughter cells
Basically just mitosis

Stages of Meiosis II

Prophase II

Metaphase II

Anaphase II

Telophase II (and cytokinesis)

Chromosomes/tetrads/chromatids at the end of meiosis II in humans

Chromosomes: 23

Chromatids:0

Tetrads:0

Mitosis vs Meiosis

                                               Mitosis                   Meiosis

Occurs in 2n Cells:                    yes                          yes

Occurs in n cells:                       yes                         no

Number of divisions:                    1                            2

Delta chromosome number:       No          reduced by ½

_{What separates in anaphase:} Sister chromatids   homologs, then chromosomes

Identical daughter cells:              Yes                     No

Homologous pairing:                   No                       Yes

Crossing Over:                           No                        Yes

Gene

Unit of heredity information

Allele

variant of a gene

Character

Observable, heritable feature

Trait(character state)

Detectable variant of a character

Genotype

Genetic make up; what alleles are present

Phenotype

observable physical trait

Gregor Mendel: Background

Austrian monk

First to determine basic rules of inheritance

Published in 1860s, little attention until early 1900s

Foundation of field of genetics

Experimental Organism: Pea Plant

Advantages of the experimental organism: Pea Plant

Inexpensive, easy to obtain

Many identifiable, heritable traits

Easy to grow, short generation time

Easy to control pollination

Many varieties available

True-breeding

always express same phenotype after self-fertilization, no exceptions

2 years of true breeding for each line before starting experiments

Blending inheritance hypothesis

Gametes contain sampling of fluids from parents

Fuse, fluids blend, offspring will have intermediate phenotype

Prevailing idea in mid 1800s

P generation

parental generation

F1 Generation

first filial generation (kids)

F2 Generation

Second filial generation (grandkids)

Mendel’s Prediction

if blending is accurate, F1 should be intermediate between P phenotypes

Mendel’s Experiments

Start with true-breeding p generation

Mate P with opposite phenotypes, get F1

Mate F1 with each other, get F2

Mendel’s observations

F1 resembled just one parent, was NOT intermediate

Other parent’s phenotype absent from F1

F2 were mix of both P phenotypes

• Traits absent in F1 reappear in F2

Consistent 3:1 phenotype ratio in F2

Mendel’s conclusion

No intermediate phenotypes appeared

Lost phenotypes reappeared

Blending cannot explain – hypothesis rejected

Particulate Inheritance

Characters determined by “heritable factors” (now called genes)

Each character controlled by 2 factors – 1 from each parent

4 components of Mendel’s model

4 Components of Mendel’s Model

1. Alleles – alternative versions of a gene

2. 2 “factors” for each character – diploidy

• Diploid individuals inherit 2 copies of each gene – 1 from each parent

3. Dominance

• Dominant allele determines phenotype if present

• Recessive allele only affects phenotype unless no dominant allele is present

4. 2 principles of heredity

Mendel’s Laws

Law of segregation

Law of independent assortment

Law of Segregation

2 alleles for a character segregate (separate) during gamete formation

Example: seed color

Y and y, yellow and green

Possible genotypes: YY, yy, Yy

Homozygous: having two of the same allele at a locus

Heterozygous: have two different alleles at a locus

Law of Independent Assortment

Genes on different chromosomes assort independently during meiosis

Due to random orientation of tetrads during metaphase I

2 nd mechanism to increase variation via sexual reproduction

Independent assorts + crossing over = lots of variation

When you have a genetic cross you…

Determine what gametes are possible from each parent

Determine what genotypes are possible in the offspring

Calculate frequency of each possible genotype

Genetic crosses can illustrate Mendel’s Laws

Monohybrid

Heterozygous for one character

Crosses between heterozygotes

Cross between heterozygotes

P: true breeding

1 homozygous dominant, 1 homozygous recessive

P: YY x yy

Possible gametes: Y or y

Possible F1 offspring: Yy, frequency of 1 (100%)

F1 genotype: Yy

F1 phenotype: dominant

Possible gametes: Y or y

Possible F2 offspring: YY, Yy, or yy

Frequencies? Will calculate with Punnett square

Punnett Square

tool to determine possible offspring and frequencies in a genetic cross

F1 monohybrid cross: Yy x Yy

• Genotype ratio: 1:2:1

• Phenotype ratio: 3:1

• Ta-da! Same as Mendel observed

• Due to segregation of alleles during meiosis

Introduction to probability in genetics

Laws of segregation and independent assortment reflect basic rules of probability

• Same as flipping a coin, rolling a die, etc.

Genetic ratios expressed in terms of probability – fractions, decimals, or percentages

Multiplication Rule

Predicts of combined probabilities of independent events

Stated as P(event 1 and event 2)

• “And” - > multiply the separate probabilities

Multiplication rule example

• Aa x Aa

• Probability that first child will be homozygous recessive?

  • P(recessive allele in egg)? 0.5

  • P(recessive allele in sperm)? 0.5

  • P(homozygous recessive offspring)? 0.5 x 0.5 = 0.25

Can check with Punnett square

Independent Events

Events where the occurrence of one does not affect probability of the other

Addition Rule

Predicts combined probabilities of mutually exclusive events

Stated as P(event 1 or event 2)

• “Or” - > add the separate probabilities

Addition rule example

• Ee x Ee

• Probability that first child will be heterozygous?

  • 2 ways to get heterozygous child:

    • E + e OR e +E

    • (0.5 x 0.5) + (0.5 x 0.5)

    • 0.25 + 0.25 = 0.5

Mutually Exclusive Events

Events that cannot occur simultaneously

Chromosome Theory of Inheritance

Genes have specific loci on chromosomes

Chromosomes undergo segregation and independent assortment

Thomas Morgan and D. melanogaster

Early 20th century experimental embryologist

1860s-1900s – cytologists noted chromosomes behave like Mendel’s “heritable factors”

Morgan originally skeptical of Mendel’s work and chromosome theory

Thomas Morgan and D. melanogaster: Significance

1 st experimental support that genes are located on chromosomes

Chromosomal theory fit what was known about Mendel’s “heritable factors”

Drosophila melanogaster

Fruit fly – Morgan’s model organism

Drosophila melanogaster: Advantages

Lots of offspring, short generations (~2 weeks)

Known chromosome arrangement

• 4 pairs – 3 pairs of autosomes, 1 pair of sex chromosomes

Wild Type

most common phenotype for a character in natural populations e.g. red eyes

wild type =/= dominant

Mutant Phenotype

alternative to wild type e.g. white eyes

Drosophila melanogaster: Notation

Conventions differ – for drosophila, symbol based on first observed mutant

• Allele for white eyes = w

• Allele for red eyes = w+ (plus indicates wild type

Correlating Alleles and Chromosomes with D. melanogaster

P: red-eyed females x white-eyed males

F1: all offspring have red eyes

→ red eyes dominant over white eyes

• Cross red-eyed F1 female x red-eyed F1 male

F2: 3:1 eye color ratio

• ALL white eyed offspring were male

Sex Determination Explains

Y chromosomes responsible for sperm production, NOT sex determination

Ratio of X chromosomes to sets of autosomes determines sex

XX:AA – 1:1 – female

XY:AA – 1:2 – male

X0:AA – 1:2 – sterile male

Why no white-eyed F2 females?

Eye color gene located on X chromosome

No corresponding locus on Y

Eye-color monohybrid cross

         P: Xw+Xw+ x XwY

         F1: Xw+Xw x Xw+Y

Punnett square of that cross showing the only way to get white eyes is XwY

Morgan’s Findings

Specific gene carried on specific chromosome

Unique inheritance pattern for genes on sex chromosomes

Strong support for chromosome theory of inheritance