Cell Division, DNA Replication & Protein Synthesis – Review

Comprehensive question-and-answer cards covering mitosis, meiosis, DNA replication, transcription, and translation.

What fundamental ability distinguishes living from non-living things and enables organisms to produce their own kind?

Cell division.

List four key roles of cell division in multicellular organisms.

Development from a zygote, growth, tissue renewal/repair, and maintenance of chromosome number across generations (via meiosis).

How does a unicellular organism such as Amoeba increase its population?

By asexual cell division that produces two identical daughter cells.

What is binary fission?

The asexual cell-division process by which prokaryotes (bacteria & archaea) reproduce, involving inward pinching of the plasma membrane to form two cells.

In prokaryotes, the genome is typically a DNA molecule.

single circular

How is the eukaryotic genome organised?

It consists of multiple linear DNA molecules packaged into chromosomes.

Name the two types of eukaryotic nuclear division.

Mitosis and meiosis.

Define diploid (2n) and haploid (n).

Diploid cells have two sets of chromosomes; haploid cells have one set.

What term describes the DNA–protein complex that condenses during cell division?

Chromatin.

What are sister chromatids?

Two identical copies of a chromosome joined at a centromere after DNA replication.

What are the centromere and telomere?

Centromere: constricted region joining sister chromatids; Telomere: protective DNA ends of chromosomes.

List the five levels of DNA packaging from least to most condensed.

DNA double helix → nucleosome → chromatin fibre → looped domains → chromosome.

Name the two main parts of the cell cycle.

Interphase and the Mitotic (M) phase.

Which cell-cycle phase is longest and what happens there?

Interphase (~90 %); the cell grows (G1, G2) and replicates its DNA (S phase).

State one major event of S phase.

DNA replication producing two sister chromatids per chromosome.

What are the subdivisions of M phase?

Mitosis (prophase, prometaphase, metaphase, anaphase, telophase) and cytokinesis.

Why is mitosis important for organisms?

It produces genetically identical cells for growth, repair, asexual reproduction and maintenance of chromosome number.

Describe two key events of prophase.

Chromatin condenses into visible chromosomes; mitotic spindle begins to form as centrosomes move apart.

What marks prometaphase?

Breakdown of nuclear envelope and attachment of spindle microtubules to kinetochores.

What happens at metaphase?

Chromosomes align on the metaphase plate with kinetochores facing opposite poles.

Which event characterises anaphase?

Sister chromatids separate at centromeres and move to opposite poles as kinetochore microtubules shorten.

List three events of telophase.

Chromosomes decondense, nuclear envelopes re-form, spindle disassembles.

Explain cytokinesis difference in plants vs animals.

Animals form a cleavage furrow via actin–myosin ring; plants build a cell plate from Golgi vesicles to form a new wall.

What is a centrosome?

A microtubule-organising centre containing a pair of centrioles that nucleates spindle fibres.

Define the mitotic spindle.

A structure of microtubules that orchestrates chromosome movement during mitosis.

What is an aster?

Star-shaped array of short microtubules radiating from each centrosome.

What is a kinetochore?

Protein complex at the centromere that attaches chromosomes to spindle microtubules.

During anaphase, microtubules shorten by at their kinetochore ends.

depolymerisation

State two mitotic differences between plant and animal cells.

Animals have centrioles/asters and form cleavage furrow; plants lack centrioles/asters and form a cell plate.

Why is meiosis biologically significant?

Produces haploid gametes, promotes genetic variation, and underlies sexual reproduction.

How many chromosomes are in human somatic cells and gametes?

46 in somatic (diploid), 23 in gametes (haploid).

Define homologous chromosomes.

Chromosome pair of same length and gene position, one inherited from each parent.

Which meiotic division is reductional and which is equational?

Meiosis I is reductional (2n→n); Meiosis II is equational (separates sister chromatids).

What is synapsis?

Pairing of homologous chromosomes during prophase I to form bivalents.

Define crossing over and chiasmata.

Exchange of DNA between nonsister chromatids; chiasmata are visible crossover sites.

List three key events of prophase I.

Synapsis of homologues, crossing-over with chiasmata formation, spindle formation & nuclear envelope breakdown.

Describe metaphase I alignment.

Homologous chromosome pairs line up at metaphase plate, each pair attached to opposite poles.

What separates during anaphase I?

Homologous chromosomes (sister chromatids stay together).

What is produced after telophase I and cytokinesis?

Two haploid cells with duplicated chromosomes.

What major event does NOT occur between meiosis I and II?

No further DNA replication.

State the outcome of meiosis II.

Separation of sister chromatids to yield four genetically distinct haploid cells.

Name the three mechanisms that create genetic variation in sexual life cycles.

Independent assortment, crossing over, and random fertilisation.

Give two key genetic differences between mitosis and meiosis products.

Mitosis yields genetically identical diploid cells; meiosis yields genetically varied haploid cells.

Describe the Watson-Crick model of DNA.

A right-handed double helix with antiparallel sugar-phosphate backbones and complementary base pairs (A=T, G≡C).

Which nitrogenous bases are purines and which are pyrimidines?

Purines: adenine, guanine; Pyrimidines: cytosine, thymine, uracil.

Which DNA-replication model proved correct?

Semiconservative replication – each daughter molecule has one parental and one new strand.

What is an origin of replication?

Specific DNA sequence where replication begins, forming replication bubbles.

Differentiate leading and lagging strands.

Leading strand is synthesised continuously toward the fork; lagging strand is synthesised discontinuously away as Okazaki fragments.

Define Okazaki fragment.

Short DNA segment synthesised on the lagging strand between RNA primers.

Role of DNA helicase.

Unwinds the double helix at the replication fork.

Function of primase.

Synthesises short RNA primers to provide 3'-OH for DNA polymerase.

Function of DNA polymerase III in prokaryotes.

Adds nucleotides to the 3' end of primer, synthesising new DNA strand.

Function of DNA polymerase I.

Removes RNA primers and fills gaps with DNA nucleotides.

Role of DNA ligase.

Joins Okazaki fragments and seals nicks in the sugar-phosphate backbone.

What does topoisomerase (gyrase) do?

Relieves torsional strain ahead of replication fork by cutting and rejoining DNA.

Role of single-strand binding proteins.

Stabilise unwound DNA strands to prevent re-annealing.

In which direction is a new DNA strand synthesised?

5′ → 3′ direction.

Name the enzymes involved in DNA mismatch repair.

Nuclease removes error, DNA polymerase fills gap, DNA ligase seals nick.

State the central dogma of molecular biology.

Information flows DNA → RNA → Protein.

Define transcription and where it occurs in eukaryotes.

Synthesis of RNA from DNA template; occurs in the nucleus.

What is the promoter TATA box?

A DNA sequence (TATAAA) about 25 bp upstream of start site where transcription factors and RNA polymerase bind.

Function of RNA polymerase.

Catalyses RNA synthesis by linking ribonucleotides complementary to DNA template without primer.

List the three main stages of transcription.

Initiation, elongation, termination.

What is a codon and which codon initiates translation?

Triplet of mRNA bases specifying an amino acid; AUG (methionine) is the start codon.

Name the three stop codons.

UAA, UAG, UGA.

Define anticodon.

Triplet on tRNA complementary to an mRNA codon.

Give the roles of mRNA, rRNA and tRNA.

mRNA carries coding sequence; rRNA forms ribosome & catalyses peptide bond; tRNA delivers amino acids via anticodon pairing.

Identify the A, P and E sites of a ribosome.

A: aminoacyl-tRNA entry; P: peptidyl-tRNA holds growing chain; E: exit site for empty tRNA.

List the three stages of translation.

Initiation, elongation, termination.

What delivers the first amino acid in translation initiation?

Initiator tRNA carrying methionine (anticodon UAC).

What enzymatic activity forms peptide bonds and where is it located?

Peptidyl transferase activity of rRNA in the large ribosomal subunit.

What triggers termination of translation?

A stop codon enters A site, recognised by a release factor that hydrolyses the completed polypeptide.

Define polysome.

A cluster of ribosomes simultaneously translating a single mRNA molecule.