Cell Biology: Cell Division, DNA Replication, Transcription, and Translation

Comprehensive practice flashcards covering cell division, mitosis, meiosis, ploidy, DNA replication, transcription, and translation.

What are the three core principles of Cell Theory?

1. All living organisms are made of one or more cells.
2. The cell is the basic unit of life.
3. All cells arise from pre-existing cells.

How do prokaryotes and eukaryotes differ in their mechanisms of cell division?

Prokaryotic cells divide by binary fission, whereas eukaryotic cells divide by either mitosis (asexual division for growth, replacement, and healing) or meiosis (sexual division for gamete production).

What are the primary phases of the eukaryotic cell cycle, and what occurs during each?

Interphase consists of $G_1$ (metabolic activity and growth), S phase (DNA replication/synthesis), and $G_2$ (growth and preparation for mitosis). M phase consists of nuclear division (mitosis) followed by cytoplasmic division (cytokinesis).

What is the function of the $G_0$ phase in the cell cycle?

It is a resting state where cells perform their normal metabolic functions but do not actively prepare for or undergo cell division.

How do cell cycle checkpoints function, and how do cancer cells behave regarding them?

Checkpoints act as regulatory gates monitored by molecular signals that determine whether a cell should proceed to the next phase; cancer cells bypass these checkpoints and divide uncontrollably despite errors.

What is the general rule for counting the number of chromosomes in a eukaryotic cell?

Count the number of centromeres.

What is ploidy ($n$), and how do haploid and diploid cells differ?

Ploidy ($n$) represents the number of unique chromosomes in a set. Haploid cells ($1n$) contain one set of unique chromosomes, whereas diploid cells ($2n$) contain two sets (one maternal and one paternal).

What is the distinction between homologous chromosomes and sister chromatids?

Homologous chromosomes are pairs of chromosomes (one from each parent) that share the same size, shape, and gene loci but may carry different alleles. Sister chromatids are identical replicated copies of a single chromosome held together at the centromere.

What is an allele?

An alternative version of a gene.

What are the three labeled components of the mitotic apparatus indicated in this diagram?

The centromere (the DNA region linking sister chromatids), the kinetochore (the protein complex assembled on the centromere), and the spindle (microtubules that attach to the kinetochore to pull chromatids apart).

What sequential events occur during prophase of mitosis?

Chromatin condenses into distinguishable chromosomes, centrosomes migrate toward opposite poles of the cell, and the mitotic spindle fibers begin to assemble.

What events characterize prometaphase and metaphase in mitosis?

In prometaphase, the nuclear envelope breaks down and spindles attach to kinetochores. In metaphase, chromosomes align along the center of the cell at the metaphase plate.

What occurs during anaphase and telophase of mitosis?

In anaphase, centromeres split and sister chromatids separate toward opposite poles as spindle fibers shorten. In telophase, nuclear envelopes re-form around the two daughter nuclei and chromosomes decondense.

How does cytokinesis differ between animal cells and plant cells?

Animal cells divide by pinching the cell membrane with an actin contractile ring, whereas plant cells form a rigid cell plate that develops into a new cell wall.

Why must meiosis reduce chromosome number by half during sexual reproduction?

To maintain a constant diploid chromosome number ($2n = 46$ in humans) across generations; without reduction, the chromosome number would double every generation.

How do the chromosome reduction outcomes of Meiosis I and Meiosis II differ?

Meiosis I separates homologous chromosomes, reducing ploidy from diploid to haploid ($2n \rightarrow 1n$). Meiosis II separates sister chromatids, maintaining haploid ploidy ($1n \rightarrow 1n$).

What is synapsis and crossing over, and during which phase of meiosis does it occur?

Synapsis is the pairing of homologous chromosomes, and crossing over is the reciprocal physical exchange of genetic segments between non-sister chromatids; both occur exclusively during prophase I of meiosis.

How does gamete formation differ between human males and females following meiosis?

Meiosis in human males yields $4$ functional sperm cells, whereas in human females it yields $1$ mature oocyte and polar bodies.

What chromosomal abnormality is shown in this karyotype?

Trisomy 21 (Down Syndrome), characterized by three copies of chromosome 21 and a total count of $47$ chromosomes ($2n + 1$).

What is nondisjunction, and what are its consequences for zygote ploidy?

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis, resulting in gametes with $n + 1$ or $n - 1$ chromosomes and aneuploid zygotes ($2n + 1$ or $2n - 1$).

Who are the researchers shown in this image, and what hypothesis did they experimentally confirm in 1958?

Matthew Meselson and Franklin Stahl; they demonstrated that DNA replication is semi-conservative using isotope density gradient centrifugation.

What does semi-conservative replication mean?

Each replicated double-stranded DNA molecule consists of one conserved parental template strand and one newly synthesized daughter strand.

What chemical rule governs the direction of nucleic acid synthesis by DNA and RNA polymerases?

Nucleic acids are synthesized exclusively in the $5' \rightarrow 3'$ direction because the $3'\text{ OH}$ group of the growing strand attacks the high-energy $\alpha$-phosphate of an incoming nucleotide triphosphate, releasing pyrophosphate.

In what direction does DNA polymerase read the template strand, and what chemical group is required for it to initiate elongation?

DNA polymerase reads the template strand in the $3' \rightarrow 5'$ direction and requires a free pre-existing $3'\text{ OH}$ group provided by an RNA primer.

What is the function of RNA primase in DNA replication?

RNA primase synthesizes a short complementary RNA primer that provides the free $3'\text{ OH}$ group necessary for DNA polymerase to begin extension.

Why is the lagging strand synthesized discontinuously as Okazaki fragments?

Because the two strands of parental DNA are antiparallel and polymerase can only synthesize $5' \rightarrow 3'$, synthesis on the lagging strand must proceed discontinuously away from the unwinding replication fork.

What are the roles of DNA polymerase and DNA ligase in finishing the lagging strand?

DNA polymerase removes the RNA primers and fills the gaps with DNA nucleotides; DNA ligase seals the remaining single-stranded nicks by forming covalent phosphodiester bonds.

What is the correct sequential order of key enzymatic steps during DNA replication?

1. Initiator proteins bind the origin.
2. Helicase unwinds the double helix.
3. Primase synthesizes an RNA primer.
4. DNA polymerase extends the new strand $5' \rightarrow 3'$.
5. Exonuclease proofreading corrects mispaired bases.
6. Ligase seals nicks in the phosphate backbone.

How does replication initiation differ between bacterial and eukaryotic chromosomes?

Bacterial circular chromosomes replicate from a single origin of replication in less than $20\text{ minutes}$, whereas eukaryotic linear chromosomes initiate at thousands of origins simultaneously to replicate within a few hours at a rate of $\sim 50\text{ nucleotides/second}$.

What constitutes the Central Dogma of molecular biology?

The directional transfer of genetic information: DNA is transcribed into mRNA, and mRNA is translated into protein.

What are the roles of the template strand and coding strand in transcription?

The template strand is read $3' \rightarrow 5'$ by RNA polymerase to synthesize RNA. The coding (non-template) strand runs $5' \rightarrow 3'$ and shares the exact same sequence and polarity as the resulting RNA transcript, except thymine ($T$) is replaced by uracil ($U$).

What is a promoter, and how do transcription factors regulate transcription initiation?

A promoter is a specific DNA region upstream of a gene where transcription factors bind in a sequence-specific manner to recruit RNA polymerase and orient the direction of transcription.

Why do different cell types in the same multicellular organism exhibit distinct structures and functions?

Although all somatic cells contain the same genome and genes, different cell types express different combinations of genes and synthesize different proteins due to cell-type-specific transcription factors.

What specific $5'$ end mRNA modification is illustrated in this diagram?

The $5'$ cap, consisting of a $7\text{-Methylguanosine}$ residue attached to the terminal nucleotide through an atypical $5'\text{-to-}5'$ triphosphate linkage.

What are the structural features and functions of the poly(A) tail added to eukaryotic mRNA?

A string of $100\text{--}300$ adenine nucleotides added to the $3'$ end of the transcript that stabilizes the RNA against degradation and promotes translation.

What is RNA splicing, and what are the roles of introns versus exons?

Splicing is the excision of non-coding intervening sequences (introns) from pre-mRNA and the ligation of expressed coding sequences (exons) into a continuous mature mRNA transcript.

What is alternative splicing, and what is its biological significance?

A process where different combinations of exons and introns are spliced from the same primary pre-mRNA transcript, allowing a single gene to encode multiple distinct protein variants (averaging $5$ proteins per gene).

How is the genetic code structured in terms of codons, amino acids, and punctuation?

The code contains $64$ triplet codons reading $5' \rightarrow 3'$: $61$ code for $20$ amino acids (including the $5'\text{-AUG-}3'$ start codon for Methionine) and $3$ serve as stop codons ($5'\text{-UAA-}3'$, $5'\text{-UAG-}3'$, and $5'\text{-UGA-}3'$).

What function is performed by aminoacyl-tRNA synthetase enzymes?

They 'charge' tRNAs by covalently coupling a specific amino acid to its corresponding tRNA at the $3'$ CCA attachment site based on anticodon recognition.

In this model of the translating ribosome, what are the distinct functions of the E, P, and A sites?

The A (aminoacyl) site receives incoming charged aminoacyl-tRNA; the P (peptidyl) site holds the tRNA attached to the growing polypeptide chain; and the E (exit) site temporarily holds uncharged tRNA before it is released.

What occurs during translation elongation when the ribosome moves along an mRNA?

The ribosome shifts one codon toward the $3'$ end, transferring the uncharged tRNA to the E site and the peptidyl-tRNA to the P site, which reopens the A site to accept the next incoming aminoacyl-tRNA.

How does translation termination take place?

When a stop codon ($5'\text{-UAA-}3'$, $5'\text{-UAG-}3'$, or $5'\text{-UGA-}3'$) enters the A site, a protein release factor binds to the site, triggering hydrolysis of the bond linking the polypeptide to the tRNA in the P site and releasing the completed protein.