Bio 101 Exam 3

Why do cells need to divide?

1. growth

2. development

3. replace damaged cells

4. replace old cells

5. immune response

6. maximize SA:V ratio

Mitosis

division of DNA in the nucleus

• happens in somatic cells

• 1 parent cell divides into 2 daughter cells

• each cell contained an identical copy of DNA from the parent

Stages of Cell Cycle

G1 → S → G2 → M → C

Stages of Mitosis

1. interphase (before)

2. prophase

3. prometaphase

4. metaphase

5. anaphase

6. telophase/cytokenesis

interphase

preparing to divide

G1 → S → G2

• G1 - growth, make protein, copy organelles

• S - copy DNA/centrioles

• G2 - growth, checkpoints

What does it mean for a cell to switch to the G₀ phase

cell exits cell cycle and stops dividing

Apoptosis

cell programmed death if the process has any mistakes

• removes damaged, infected, unnecessary cells to maintain health

Cyclins

signaling proteins and control passage from 1 stage to the next

• act as checkpoints

Cancer

uncontrolled cell division

• cyclins don’t work properly and cells speed through cell cycle ignoring cyclins (stop signs)

Contact inhibition + Cancer

when cells stop dividing when they touch neighboring cells

• cancer lose it → cells start piling up an form a tumor (mass of cells)

Chromatin

complex of DNA and proteins that make up eukaryotic chromosomes (stringy)

Chromosome

one very long condensed DNA molecule

• humans have 46 total (diploid); 23 from mom and 23 from dad (haploid)

Diploid

2 sets of chromosomes

• somatic cells

Haploid

1 set of chromosomes

• gametes

Somatic Cells

Body cells

Gametes

sperm/eggs

Parts of a duplicated chromosome

Prophase

• nucleus begins to disappear

• chromosomes condense

• centrosomes form + begin moving apart

Prometaphase

• Nucleus disappears completely, including nucleolus

• Spindle fibers extend and attach to the kinetochore (centromere region)

• Chromosomes begin to move and line up → moved by spindle fibers

Metaphase

• chromosomes align single file in middle of cell

Anaphase

• sister chromatids separate and move to opposite poles

Telophase

• Each daughter cell begins to form a nuclear membrane

• Nucleolus returns

• Chromosomes unravel → become chromatin again

• Happens at the same time/sequentially as cytokinesis (division of the cytoplasm)

Cytokinesis (animals)

• after mitosis 2 nuclei are in the same cell

• cell membrane starts to pinch in at the center of the cell

• cleavage furrow forms - acts as a belt tightening in the middle

• furrow deepens and separates 2 daughter cells with their own nuclei

Cytokinesis (plants)

• after mitosis 2 nuclei are in the same cell

• vesicles from golgi gather in middle of cell

• vesicles fuse to form a cell plate

• cell plate grows outward toward existing cell walls - reaches edges and splits cell into 2 daughter cells with their own nuclei

Mitosis vs. Cytokinesis

Mitosis -

• division of nucleus and DNA

• results in 2 identical nuclei

• 4 phases

Cytokinesis -

• division of cytoplasm and organelles

• results in 2 separate daughter cells

• happens after mitosis

Asexual Reproduction

• single parent

• no fusion of gametes

• offspring are (in most cases) identical to parent (creates clones)

• single cell organisms reproduce asexually through cell division

Sexual Reproduction

• 2 parents

• gametes

• Offspring have unique combinations of genes inherited from both parents - genetically different from siblings and parents

• genetic variation advantageous for environmental changes

Binary Fission

a single cell splits into two identical cells, each with the same DNA as the original

• how bacteria reproduce

• 1 chromosome

Genetics

field of study involving inheritance and hereditary variation

Genes

discrete unit of heredity information consisting of a specific sequence in DNA

• located on chromosome

• hereditary traits passed from 1 generation to the next

Locus

the specific location on a chromosome

• Gene A is always at the same locus in every individual within a species (same place on same chromosome)

Karyotype

ordered display of all chromosomes in a cell arranged in pairs of homologous chromosomes

• provides information about number of chromosomes, detect genetic disorders, and shows if there are missing or extra chromosomes

Homologous Chromosomes

paired chromosomes of the same size, shape, and genes

Autosomes

all other chromosomes (not sex chromosomes X and Y)

Alleles

different versions of the same gene that exist at the same locus

• can determine eye color, hair color, etc.

Ploidy

 the number of sets of chromosomes in a cell

Polyploidy

cells that have more than 2 sets of chromosomes (tetraploid, hexaploid, etc.)

Euploidy

correct number of chromosomes for a species

• ex: humans have 46 (2 sets of 23)

Aneuploidy

change in chromosome number resulting from nondisjunction

• ex. down syndrome 3 copies of chromosome 21 instead of 2

Nondisjunction

chromosomes fail to separate properly during meiosis

• one gamete gets too many chromosomes and the other gets too few

• can lead to aneuploidy

• affects offspring that develops from abnormal gamete

Human Life Cycle

• starts with 2 haploid gametes (n=23)

• fertilization - fusion of 2 nuclei of gametes (2 sets of chromosomes 2n=46)

  • creates zygote (diploid fertilized egg)

• repeated mitosis and development single cells become mature multicellular adult

Why does the human life cycle need to alternate between haploid and diploid

So that chromosome numbers remain constant from generation to generation

• Fertilization with 2 diploid cells would double the chromosome number (2n + 2n = 4n or 96 chromosomes)

Meiosis

how chromosome number gets reduced - counterbalances fertilization

• in gonads of sexually reproducing organisms

• created haploid gametes (sperm/egg)

• 2 rounds of cell division; 1 round of DNA replication

• Results in cells with half the number of chromosomes sets as the original cell

Meiosis is a ___ divison

reductional

What major events occur in prophase 1 of meiosis?

• Chromosome condense 

• Nuclear envelope breaks down

• Centrosomes separate 

• Spindle fibers form and attach to kinetochores

• Homologous chromosomes pair up (synapsis) to form tetrads (akas bivalents)

• Crossing over occurs → causes genetic variation

Tetrads

2 homologous chromosomes pair up and each chromosome is replicated = 4 chromatids

• formed during meiosis in prophase 1

• break apart in anaphase 1

Crossing Over

genetic swap between homologous, non-sister chromatids (one from each chromosome in a tetrad exchange dna)

• occurs in prophase 1 of meiosis

• increases genetic diversity, helps chromosomes separate correctly

Anaphase 1

homologous chromosomes separate and move to opposite poles

• Sister chromatids remain attached

Anaphase 2

sister chromatids separate and move toward opposite spindle poles

• same as mitosis

Metaphase 1

Pairs of homologous chromosomes (tetrads) line up at the metaphase plate

Metaphase 2

Chromosomes align at the metaphase plate → single file

• sister chromatids may not be genetically identical because crossing over occured

• most similar to mitosis

daughter cells at the end of mitosis vs. meiosis

mitosis - 2 diploid daughter cells genetically identical to parent

meiosis - 4 haploid daughter cells, half as many chromosomes as parent + genetically different from parent

When do cells first become haploid in meiosis

telophase 1

Spermatogenesis

• occurs in testes

• makes 4 sperm cells

• small and mobile

• begins at puberty and continues through life

• cytoplasm divides evenly among 4 sperm

Oogenesis

• occurs in ovaries

• makes 1 functional egg and 3 polar bodies

• large and non-mobile

• begins before birth, pauses, and then continues from puberty to menopause

• cytoplasm divides unevenly producing 1 large egg

Gametogenesis

generic term for sperm and egg formation

Chromosome mutation

change in structure of number of chromosomes which can affect many genes at once

Trait

genetically determined characteristic → expression of proteins

ex. hair color, eye color, etc. (general category)

Phenotype

physical appearance of a trait (what you see)

Genotype

genetic makeup of a trait (the letters)

Dominant

always expressed if present → use capital letter (B)

Recessive

only expressed if there are 2 of the same no dominant allele is present → use lowercase letter (b)

P1 generation

• True breeding, parental generation

• homozygous dominant or homozygous recessive for the trait

F1 generation

• 1st filial generation

• Offspring from P generation

F2 Generation

• Second filial generation

• Offspring from the F1 generation

Law 1: Law of Dominance

an organism with a dominant allele will express the dominant trait

• An organism with a recessive allele will only express the recessive trait if the dominant allele is not present 

Law 2: Law of Segregation

• Punnet Square

• Each individual has 2 “factors” (alleles) for each trait - one from mom one from dad

• Alleles separate during the formation of gametes

  • Alleles are found on homologous chromosomes

• Fertilization gives each new individual 2 allele for each trait

  • Homozygous - 2 allele are the same

  • Heterozygous - 2 alleles are different

• Expressed in monohybrid cross

Law 3: Law of Independent Assortment

• Each pair of alleles segregates (assorts) independently of other traits in meiosis

• The inheritance pattern of 1 trait will NOT affect the inheritance pattern of another

• For a single human gamete, the possible ways for chromosomes to assort is an astounding 8388608 (2²³) possible combinations

• All possible combinations of alleles can occur in the gamete

• Expressed in a dihybrid cross (2 traits)

What factors / events contribute to the great diversity among individuals within a sexually reproducing species?

• random fertilization

• independent assortment of chromosomes

• crossing over (recombination)

• mutations

Why is diversity within a species so important that many organisms reproduce sexually?

creates genetic diversity

• adaptation to environments

• disease resistance

• evolutionary advantage

test cross

used to discover the unknown genotype of a known phenotype (the dominant phenotype)

• Cross a true breeding recessive individual (yy) with a dominant phenotype (YY or Yy unknown)

• ratio reveals genotype - if any with the recessive trait appear, unknown must be heterozygous

Multiple alleles

Some traits have more than 2 allele 

Ex: Blood type in humans (A, B, O alleles)

Incomplete Dominance

Occurs when 1 allele is not completely dominant over another

• Results in blending of traits

Codominance

In cases of multiple alleles sometimes more than 1 allele can act as a dominant allele

Ex: Blood type in humans (A, B, O alleles) - codominance A and B

Incomplete penetrance

Alleles have a “true” dominant/recessive relationship, but dominant does not always determine the phenotype

• Polydactyly: autosomal dominant, but not all who inherit the gene have additional fingers

Pleiotropy

single gene affects multiple traits

• sickle cell disease - affects blood, oxygen transport, and disease resistance

Epistasis

the action of 1 gene overrides the actions of another gene

Polygenic Inheritance

a phenotype is determined by more than one gene

• most human traits (skin color, height, etc.)

X-linked inheritance

Some traits are not located on autosomes but instead are on sex chromosomes - can lead to phenotypes based on sex

• males affected more - only have one X chromosome

• Females with 1 copy are considered carriers and can pass it to their offspring - half her sons will be affected, half her daughters will be carriers, and daughters can be affected if dad contributed affected X chromosome

Pedigree analysis

autosomal dominant - trait appears in every generation, affected need to have at least one affected parent

autosomal recessive - traits can skip generations, affected individuals may have carrier parents

X-linked recessive - trait often skips generations, affected fathers cant pass to son but daughters of affected fathers are all carriers