BIOL1400A Third Exam

roles of cell division

transmits gene information for reproduction, genetically identical daughter cells, asexual reproduction, sexual reproduction, zygote → adult, repair of tissues

asexual reproduction

involves 1 parent → clone offspring, yeast, sea stars, house plants

sexual reproduction

fusion of gametes, half the chromosome number, gene variation

mitosis

growth, maintenance, asexual, makes all body cells except egg and sperm

5 stages, prophase, prometaphase, metaphase, anaphase and telophase

meiosis

gametes, sexual, cuts chromosome # in half, ends with 4 haploid cells, each with 1 chromosome from the original pair

binary fission in prokaryotes

chromosome duplicates, cell elongates, membrane pinches inwards, cell wall forms → daughter cells, single cellular, asexual

How many chromosomes in humans

46 chromosomes

Chromatin

DNA+Protein, long thin fibers, proteins help regulate gene activity

chromosome compaction

before division chromatin coils, efficient sorting and transport 

chromosome duplication

each chromosome duplicates into identical sister chromosomes, sister chromosomes are joined at the centromere

cell division outcome

sister chromatins separate, become full chromosome, each receive identical set of chromosomes

mitotic spindle

chromosomes are moved with these, made of microtubules

centrosomes

organize the spindle fibers

prophase

chromosomes are visible but not lined up yet

cytokinesis

splitting the cell

cytokinesis in animal cells

split using a cleavage furrow, uses actin and myosin proteins to pinch the cell in two

cytokinesis in plant cells

build cell plate in middle, vesicles bring cell wall material to form the plate, plate becomes part of new cell wall

cancer

cell divides too much and ignores normal signals, starts with one cell that mutates and escapes the immune system

benign tumor

stays in one place

malignant tumor

spreads elsewhere(tissues and organs)

metastisis

when cancer cells travel to other parts of the body

cancer cell behavior

ignore cell cycle checkpoints, make their own growth signals, divide forever in labs

radiation

damages DNA in cancer cells more than normal cells

Chemotherapy

uses drugs to stop cell division

taxol- freezes spindle after it forms

vinblastin- stops spindles from forming

personalized cancer treatment

cancer treatments don’t work equally for everyone, early detection of breast cancer poses a lower risk of death, but younger women and black women have higher risk of death, controlled studies unethical because you don’t want to give someone cancer, observational studies are most often used and more ethical

genetic basis of cancer

when genes that control cell division stop working correctly

proto-oncogene

normal gene that helps regulate cell growth

oncogene

mutated version that causes cells to grow too much

Proto-oncogene → oncogene

mutation makes the gene too active

gene copies increase, making too much protein

mutation in control region causes over production

tumor suppressor genes

slow or stop cell division, if mutated can stop working and allow uncontrolled cell growth. Some help fix damaged DNA, if they’re broken more mutations can build up

how many mutations does cancer need to fully form

cancer needs multiple mutations, not just one

how does cancer start

starts with a mutation that activates an oncogene, causing cells to divide too often. Later, more mutations (like losing a tumor suppressor gene) lead to a benign tumor. Even more mutations can turn into a malignant tumor(metastasizes) around 6 mutations needed for full cancer development.

mutations build up

mutations build up overtime and are passed to other cells, this is why older people getting cancer is more common

epithelial cells

divide more often, more likely to have mutation, and becoming cancerous

somatic cells

body cells have 46 chromosomes (23 pairs) each pair is made of homologous chromosomes, same size, shape, and gene locations

homologous chromosomes

carry same gene, but have different variations of that same gene

sister chromatids

identical copies joined together

sex chromosomes

females XX(fully homologous) males XY(partially homologous)

autosomes

the other 44 chromosomes not related to sex

diploid cells

2n=46 have two sets of chromosomes, one from each parent

gametes

egg and sperm, are haploid (n+23), one set of chromosomes, each has 22 autosomes and one sex chromosome (X or Y)

fertilization

sperm and egg forms a zygote (diploid again)

meiosis 1

separates homologous chromosomes

meiosis 2

separates sister chromatids

chromosome vs chromatid

Chromosome- DNA strand with genes

always present

46 in human cells

carries genetic information

chromatid- 1 copy of a duplicated chromosome

only during cell division

92 during mitosis(before splitting)

helps DNA divide into new cells

Meiosis Rundown diploid → haploid

meiosis makes haploid gametes from diploid cells

fertilization joins egg and sperm to make a diploid zygote

meiosis keeps chromosome number stable across generations

starts with one chromosome duplication, followed by two divisions

M1 separates homologous chromosomes

M2 separates sister chromatids

ends with 4 haploid cells, each with half the chromosomes of the original cell

crossing over(prophase 1)

homologous chromosomes pair up and touch

nonsister chromatids can swap segments, called crossing over, nonsister chromatids exchange segments at chiasmata, creates recombinant chromosomes

crossing over increases genetic diversity in offspring

mitosis vs meiosis

mitosis- growth repair and asexual reproduction

only 1 division and makes 2 identical cells with the same chromosome number as parents (diploid)

no genetic variation

happens in somatic cells and they are the type made

meiosis- production of gametes

2 divisions and 4 unique cells made that are haploid

there is genetic diversity because of crossing over and independent assortment

only happens in reproductive organs

nondisjunction

chromosomes fail to separate during meiosis. Can lead to down syndrome or miscarriage because it results in gametes with too many chromosomes or too few chromosomes

What happens when there is an abnormal gamete and normal gamete joined?

zygote will have the wrong number of chromosomes

error is passed to all cells during mitosis can happen in 10-30% of human conceptions

a diploid gamete could form if all chromosomes fail to separate during meiosis

Trisomy 21

person with 3 copies of chromosome 21 instead of 2, total chromosome #47 instead of 46, most common serious birth defect in the US

symptoms: round face, eyes fold, flat nose bridge, small teeth, short height, heart problems, infections, risk of leukemia and Alzheimer’s, developmental delays and shorter life span

sterility with people with down syndrome

most males are sterile, half of females are sterile, a woman with down syndrome has 50% chance of passing it to her child

trisomy 21 and down syndrome associated with maternal age and risk

risk is higher with mothers age

younger than 30 less than 0.05%

40 around 1%

47 around 5%

gene variations on homologous chromosomes

homologous chromosomes have same genes at same loci but can have different versions(alleles)

gametes can carry different combos, depending on which chromosomes they inherit

Independent assortment and random fertilization

chromosomes line up randomly during metaphase of meiosis

each gamete has a 50% chance of getting either the maternal or paternal chromosome from each pair

formula 2n combination, where n= haploid number

fertilization adds even more variety

Mendel’s original thoughts about inheritance

thought we acquired traits which is incorrect

later after Mendell and before todays knowledge

scientists proposed blending hypothesis; parental traits mix like paint, rejected because traits can reappear in later generations

Gregor Mendel (1860s)

discovered how traits are inherited using pea plants, traits passed by heritable factors(genes) that stay unchanged

Mendel’s experiment set up

P generation= true breeding parent plants

F1 generation= hybrid offspring from P cross

F2 generation= offspring from F1 self fertilization

Mendel’s law of segregation

genes come in versions called alleles

each organism gets 2 alleles per gene, one from each parent

heterozygous Pp

homozygous PP

dominant alleles and recessive alleles

law of segregation- alleles separate during gamete formation(meiosis), so each gamete gets one

fertilization restores the pair- one allele from each parent

dominant alleles

show in appearance

recessive alleles

hidden unless both alleles are recessive

Mendel’s experiment (flower color example)

crossed purple(PP) x white(pp) → all F1= purple(Pp)

F1 self cross → F2= 3 purple: 1 white

genotype- 1 PP, 2Pp, 2pp

phenotype`- 3 purple 1 white

Alleles

located on homologous chromosomes at the same gene locus

monohybrid cross

tracks one trait (PpxPp for flower color)

Dihybrid cross

tracks 2 traits (seed shapes and color)

testcross

used to find unknown genotype of a dominant trait. Cross unknown with homozygous recessive (BB or Bbxbb) if al offspring are dominant → parent is homozygous, if offspring are a mix→ parent is heterozygous

does inheritance follow probability laws

Yes, like coin tosses or drawing cards

probability scale

0= no chance

1= will happen

independent events

previous outcome doesn’t affect future ones

rule of multiplication

for combined independent events

rule of addition

used when an event can happen multiple ways

chromosome theory of inheritance

genes are located at specific loci on chromosomes, and chromosomes follow Mendel’s laws during meiosis

Law of segregation

homologous chromosomes separate in anaphase 1, so each gamete gets one allele

Law of independent assortment

chromosome pairs align randomly at metaphase 1, so genes on different chromosomes assort independently

Bateson and Punnett

studied sweet peas

Traits: flower color (P/p) and pollen shape (L/l)

Expected: 9:3:3:1 ratio

Observed: Mostly purple+long, reg+ round → not Mendelian

Linked genes conclusion

genes for flower color and pollen shape are linked(on the same chromosome) so they’re often inherited together

Morgan’s fruit fly experiment confirmed what

crossing over creates variety

Cross:GgLl x ggll

most offspring had parental traits but 17% were recombinants

this recombinant frequency shows how often crossing over occurs between linked genes

Recombinant phenotypes=

recombination frequency

recombinant gametes

gametes (sperm and egg cells) that carry a new combo of alleles- different from either parent due to crossing over during meiosis.

Which of the following characteristics must be common to all cells that undergo cellular division?

Cell gets bigger and produces more cellular molecules 

All DNA in a cell is copied before dividing

The copies of DNA produced by DNA replication must be distributed to each of the new cells

Prokaryotic cells are always haploid or diploid

are always haploid

A diploid cell has four pairs of homologous chromosomes. 46 Therefore it has a total of _______ individual chromosomes, and after DNA replication there will be _______ pairs of sister chromatids.

eight; eight

Compare the purposes of cell division in prokaryotes and eukaryotes.

Prokaryotes

Purpose	Description
Reproduction	Cell division is used to reproduce asexually via binary fission.
Population growth	Enables rapid multiplication of single-celled organisms.

• Prokaryotes are unicellular, so division creates a new organism.

• No mitosis or meiosis, just simple replication and splitting.

Eukaryotes

Purpose	Description
Growth	Adds cells to increase body size (multicellular organisms).
Repair	Replaces damaged or dead cells (e.g., skin, blood).
Reproduction

• Asexual: via mitosis (e.g., in fungi or some protists)

• Sexual: via meiosis to produce haploid gametes

• Eukaryotes can be unicellular or multicellular.

• Cell division supports complex life functions beyond reproduction.

Describe the differences between prokaryotic and eukaryotic cells/genomes that contribute to differences in the purposes and processes of cell division.

Prokaryotes circular , single chromosomes Simple, no nucleus or organelles Asexual reproduction only Always single celled	Eukaryotes Linear, multiple chromosomes Complex, with nucleus and organelles Asexual or sexual Single celled or multicellular

Describe the process of prokaryotic cell division

1. Binary Fission, two genetically identical offspring

   1. DNA replicates (single circular chromosome)

   2. Cell elongates

   3. DNA copies move to opposite ends

   4. Cell splits into two genetically identical daughter cells

Are homologous chromosomes present in a diploid cell prior to DNA replication? What about sister chromatids?

Homologous chromosomes: Present in diploid cells before DNA replication

Sister chromatids: Formed after DNA replication (identical copies of one chromosome)

A diploid cell has 14 different chromosomes. How many individual chromosomes (pieces of DNA) are in the nucleus? What about a haploid cell with 14 different chromosomes?

Diploid cell with 14 chromosomes: 14 individual chromosomes (7 pairs of homologs)

Haploid cell with 14 chromosomes: 14 individual chromosomes (no pairs)

Do two homologous chromosomes always have the same genes? The same alleles for each gene

Homologous chromosomes: Same genes, but may have different alleles

Sister chromatids: Same genes and same alleles (identical copies)

Elephants are diploid organisms. Can more than two alleles for a particular gene exist in an elephant population? Can an individual elephant have more than two alleles for a particular gene? Can an individual elephant have only one allele for a particular gene? Explain your answers

Can a population have >2 alleles for a gene? → Yes

Can an individual have >2 alleles? → No (diploid = max 2 alleles per gene)

Can an individual have only 1 allele? → Yes, if they are haploid or have a deletion

Sketch and label a picture to describe the relationships between the terms homologous chromosomes, sister chromatids, non-sister chromatids, and replicated chromosomes. Use your sketch to explain the relationships to someone that’s not in the class.

Two homologous chromosomes: same size, same genes, possibly different alleles

Each homolog replicates → forms two sister chromatids (identical)

Non-sister chromatids: chromatids from different homologs (same genes, possibly different alleles)

Replicated chromosome: two sister chromatids joined at the centromere

Explain why sister chromatids always have the same alleles for each gene. Explain why non-sister chromatids can have the same OR different alleles for each gene

Because they have different alleles for the gene(non sister) because they are replicated so they are the same (sister) 

Sister chromatids: Always same alleles (they’re exact copies)

Non-sister chromatids: May have same or different alleles (from different parents)

Thinking about the differences between mitosis and meiosis, why does it make sense that diploid cells can divide by meiosis, but haploid cells can’t?

Diploid cells can divide by meiosis because they have homologous pairs to separate

Haploid cells lack homologs → can’t undergo meiosis

Explain why the gametes of diploid organisms that reproduce sexually must be haploid, and not diploid.

Sexual reproduction combines two gametes → if both were diploid, offspring would have double the chromosome number

Haploid gametes ensure offspring remain diploid

Explain why it’s necessary for eukaryotic chromosomes to condense prior to mitosis. Explain why condensing of chromosomes prior to division is not necessary in prokaryotic cells.

Eukaryotic chromosomes are long, linear strands of DNA. -> Condensing them into compact structures prevents tangling and ensures accurate separation. Prokaryotic cells have a single, circular chromosome and no nucleus. -> Their DNA is already small and simple, so condensation isn’t necessary.

Describe the steps of mitosis. Draw and label pictures to illustrate your description, keep it simple and stick to a diploid cell with two different chromosomes (so four pieces of DNA total). You could also try drawing it out for a haploid cell with three different chromosomes

Start: Diploid cell with 2 chromosomes (1 from each parent)

DNA Replication: Each chromosome duplicates → forms 2 sister chromatids per chromosome

Chromosome Alignment: Chromosomes line up in the center

Chromatid Separation: Sister chromatids are pulled to opposite sides

Cell Splits: Two identical daughter cells form, each with 2 chromosomes

Humans have twenty three pairs of chromosomes. During division of a human skin cell, the two sister chromatids of one replicated chromosome fail to separate. How many individual chromosomes will each of the daughter cells have?

Humans have 23 pairs = 46 chromosomes If 1 pair of sister chromatids fails to separate, one daughter cell gets 47 chromosomes, the other gets 45 -> This is called nondisjunction

A mutation in the gene PTEN has been linked to cancer. The mutation increases the activity of the protein produced by expression of PTEN. Based only on this information, do you predict that this protein promotes or inhibits cell division? Explain your reasoning.

PTEN normally inhibits cell division (tumor suppressor)

If a mutation increases PTEN activity, it likely inhibits cell division more -> But most cancer-linked PTEN mutations reduce its activity, leading to uncontrolled division

Why does cancer development require mutations in multiple genes?

Cancer is complex: it involves uncontrolled growth, evasion of death, invasion, and more

One mutation might cause faster growth, but others are needed to:

Avoid immune detection

Resist cell death

Promote blood vessel growth

Spread to other tissues

Explain why cancer that occurs due to mutations in the genes of body cells cannot be passed on to an individual’s offspring.

Mutations in somatic (body) cells affect only that individual

Only mutations in gametes (egg/sperm) can be passed to offspring → That’s why most cancers aren’t inherited