Biology: Meiosis, Photosynthesis, and the Cell Cycle

Comprehensive review flashcards covering Meiosis and Sexual Life Cycles (Chapter 11), Photosynthesis and Carbon Fixation (Chapter 8), and Eukaryotic and Bacterial Cell Division (Chapter 10).

How do asexual reproduction and sexual reproduction differ in the number of parents involved and the genetic makeup of their offspring?

Asexual reproduction involves one parent and produces genetically identical offspring (clones), whereas sexual reproduction involves two parents and produces genetically unique offspring through the fusion of gametes.

What is the difference between diploid ($2n$) and haploid ($n$) cells in humans?

Diploid ($2n$) cells contain two complete sets of chromosomes ($46$ chromosomes arranged in $23$ pairs), while haploid ($n$) cells contain only one set of chromosomes ($23$ chromosomes).

How do germ-line cells differ from somatic cells in function and mode of division?

Germ-line cells belong to the cell lineage that gives rise to gametes, participate in sexual reproduction, and undergo meiosis; somatic cells make up the body, function in growth and maintenance, and divide through mitosis.

Why is meiosis I specifically designated as the reduction division?

Because it separates homologous chromosome pairs and reduces the number of chromosome sets from diploid ($2n$) to haploid ($n$).

What event defines synapsis during prophase I of meiosis?

Replicated homologous chromosomes find each other, pair up closely, and become physically joined along a synaptonemal complex to form a bivalent or tetrad.

What is crossing over, and at what visible microscopic structures does it occur?

Crossing over is the reciprocal exchange of DNA segments between nonsister chromatids of homologous chromosomes during prophase I, visibly occurring at X-shaped regions called chiasmata.

How does chromosome arrangement at the metaphase plate differ between metaphase I and metaphase II of meiosis?

In metaphase I, homologous chromosome pairs (tetrads) align side-by-side with monopolar attachment, whereas in metaphase II, individual duplicated chromosomes line up along the plate with sister chromatids attached to opposite spindle poles.

What formula calculates the number of possible chromosome combinations from independent assortment, and what is the resulting value for human gametes?

The formula is $2^n$ (where $n$ is the haploid number); for humans ($n = 23$), $2^{23} = 8{,}388{,}608$ combinations are possible from independent assortment alone.

What is interkinesis, and why is the absence of DNA replication during this stage critical?

Interkinesis is the interval between the end of meiosis I and the beginning of meiosis II; suppressing DNA replication is required to preserve the haploid state achieved in meiosis I before sister chromatids separate.

How do the gametic chromosome outcomes differ if nondisjunction occurs in meiosis I versus meiosis II?

Nondisjunction in meiosis I causes all four gametes to be abnormal (two $n + 1$ and two $n - 1$); nondisjunction in meiosis II results in two normal gametes ($n$) and two abnormal gametes (one $n + 1$ and one $n - 1$).

What four distinct features specifically differentiate meiosis from mitosis?

1. Synapsis and crossing over during prophase I; 2. Paired homologous chromosomes (tetrads) at metaphase I; 3. Separation of homologous chromosomes at anaphase I; 4. Suppression of DNA replication between divisions.

What is the overall balanced chemical equation for photosynthesis?

$$6CO_2 + 6H_2O + \text{Light energy} \rightarrow C_6H_{12}O_6 + 6O_2$$

What is the precise chemical origin of the oxygen gas ($O_2$) released during photosynthesis?

Water ($H_2O$), which is split and oxidized by Photosystem II, not carbon dioxide ($CO_2$).

How do autotrophs and heterotrophs differ in the way they acquire organic molecules?

Autotrophs ('self-feeders') produce their own organic molecules from inorganic substances, whereas heterotrophs ('other-feeders') obtain organic molecules by consuming other organisms or organic material.

Where do the light-dependent reactions and the Calvin cycle take place within the chloroplast?

The light-dependent reactions occur in the thylakoid membranes, while the Calvin cycle takes place in the stroma.

What is the relationship between the wavelength of electromagnetic radiation and its photon energy?

Photon energy is inversely proportional to wavelength: shorter wavelengths have higher frequency and higher energy, while longer wavelengths have lower frequency and lower energy.

Which colors within the visible light spectrum are absorbed and reflected by chlorophyll a?

Chlorophyll a strongly absorbs blue-violet and red light, while it does not strongly absorb and instead reflects green light, making it appear blue-green.

How do the roles of the antenna complex and the reaction center differ in a photosystem?

The antenna complex contains many pigment molecules that gather photons and pass excitation energy inward, whereas the reaction center contains a special chlorophyll a pair that uses the energy to transfer an excited electron to a primary electron acceptor.

How do Photosystem II (PS II) and Photosystem I (PS I) differ in their reaction centers and order of function?

Photosystem II functions first and possesses a $P680$ reaction center that best absorbs at $680\,nm$; Photosystem I functions second and possesses a $P700$ reaction center that best absorbs at $700\,nm$.

How is ATP synthesized via chemiosmosis during the light-dependent reactions?

Protons ($H^+$) accumulate inside the thylakoid lumen to form a concentration gradient, and their diffusion down their electrochemical gradient into the stroma through ATP synthase powers ATP synthesis from ADP and inorganic phosphate.

What reaction does rubisco catalyze in Phase 1 (Carbon Fixation) of the Calvin cycle?

Rubisco combines $CO_2$ with ribulose bisphosphate ($RuBP$) to generate an unstable six-carbon intermediate that immediately splits into molecules of $3\text{-phosphoglycerate}$ ($3\text{-PGA}$).

For six turns of the Calvin cycle, what are the net consumption amounts of ATP and NADPH, and what is the net carbohydrate yield?

Six turns consume $18\text{ ATP}$ and $12\text{ NADPH}$, producing $12\text{ G3P}$ molecules, of which $10$ are recycled to regenerate $RuBP$ and $2\text{ G3P}$ exit as net output to form one glucose molecule.

What are the four sequential steps of bacterial binary fission?

1. DNA replication of the single circular chromosome; 2. Chromosome separation to opposite ends; 3. Cell growth; 4. Septum formation (septation) to divide the cell into two identical daughter cells.

What is the function of the FtsZ protein in bacterial septation?

FtsZ proteins polymerize into a ring at the division site and contract radially to pinch the bacterial cell in two.

What is the structural organization of a eukaryotic nucleosome?

A nucleosome consists of negatively charged DNA coiled around an octamer of eight positively charged histone proteins, spaced approximately every $200$ nucleotides.

How do cohesin proteins and condensin proteins differ in their primary roles?

Cohesin proteins hold sister chromatids together at the centromere following replication, whereas condensin proteins facilitate the tight coiling and condensation of chromosomes during cell division.

What is the distinction between a centromere and a kinetochore?

The centromere is the narrow region of a duplicated chromosome where sister chromatids are joined, whereas the kinetochore is a protein complex assembled on the centromere to which spindle microtubules attach.

What are the three subphases of interphase and their principal functions?

$G_1$ (major portion of cell growth and normal metabolic function), $S$ (DNA replication producing sister chromatids), and $G_2$ (preparation for division, chromosome coiling, and tubulin synthesis).

What molecular and cellular events characterize anaphase of mitosis?

Centromeres split as cohesin proteins are cleaved, kinetochore microtubules shorten by depolymerizing to pull sister chromatids to opposite poles, and nonkinetochore microtubules slide past each other to elongate the cell.

How does the physical process of cytokinesis differ between animal cells and plant cells?

Animal cells divide by forming a cleavage furrow powered by a contractile ring of microfilaments pinching the membrane inward; plant cells divide by assembling a cell plate through the fusion of Golgi-derived carbohydrate vesicles that matures into a new cell wall.

If a diploid cell has $32$ chromosomes in $G_1$, how many chromosomes and chromatids are present at metaphase of mitosis and in each daughter cell following cytokinesis?

In metaphase, the cell contains $32$ chromosomes and $64$ chromatids; after cytokinesis, each daughter cell contains $32$ chromosomes and $32$ chromatids.

What specific conditions are evaluated at the $G_1$, $G_2$, and M checkpoints of the cell cycle?

The $G_1$ checkpoint assesses favorable environmental conditions and DNA damage before replication; the $G_2$ checkpoint verifies complete and accurate DNA replication before mitosis; the M (spindle assembly) checkpoint ensures all chromosomes are properly attached to spindle microtubules before anaphase begins.