Many enzymes are involved in the DNA replication fork.

Many enzymes are involved in the DNA replication fork.

Living organisms perpetuate themselves through DNA replication and cell division. DNA replication occurs in all lifeforms, a process of semiconservative replication in which each strand of the original DNA molecule serves as a template for the production of the complementary strand. Cell division usually occurs within the context of a larger cell cycle. Prokaryotes undergo a process celled binary fission. While binary fission is conceptually a simple process, its underlying biochemical and genetic mechanisms are a major research area. In eukaryotes, there are two distinct types of cell division. In mitosis each daughter cell is genetically identical to the parent cell. Meiosis is a reproductive cell division. In meiosis, the number of chromosomes is reduced by half, producing haploid gametes.

The genetic and biochemical processes underlying DNA replication and the cell cycle are major focuses for the MCAT. There is a large conceptual vocabulary to be mastered. For example, in just the topic of DNA replication the list of enzymes you need to be familiar with includes DNA helicase, DNA polymerase, DNA clamp, single strand binding proteins, topoisomerase, DNA gyrase, DNA ligase, primase, and telomerase. Like certain other topics in molecular biology, biochemistry, and physiology, it's best to start early in review and have more than one pass through this material. Multiple study cycles spaced out over time are more effective for learning and retention than a single, more intense period of dedicated study.

WikiPremed Resources

Cellular Reproduction Images
Image gallery for study with links to larger teaching JPEGs for classroom presentation

Question Drill for DNA Replication, Repair and Cellular Reproduction
Conceptual Vocabulary Self-Test

Basic Terms Crossword Puzzle

Basic Puzzle Solution

Learning Goals


Be able to describe the functions of the enzymes primase, helicase, DNA polymerase and DNA ligase in DNA replication.

Understand the differences between prokaryotic and eukaryotic origins of replication

Be able to distinguish the leading strand and the lagging strand.

Be prepared to describe the role of telomeres in compensating for incomplete semi-conservative DNA replication at chromosomal ends.

Understand how positive and negative control mechanisms work in eukaryotic DNA replication.

Be familiar with the process of post-replicative modification of DNA by methylation.

Be able to narrate the processes of prokaryotic cell division.

Have a thorough understanding of what occurs in each of the stages of the eukaryotic cell cycle.

Be able to describe how cyclins and cyclin dependent kinases regulate progress through the cell cycle and understand how tumor suppressor genes prevent the progression of the cell cycle.

Understand the processes of eukaryotic cell division and specifically, be prepared to describe what occurs within each phase of mitosis.

Be able to describe the condensed eukaryotic chromosome using the terminology chromatid, chromatin, kinetocore, and centromere.

Understand the main features of the spindle apparatus.

Know how to distinguish meiosis and mitosis and be able to narrate meiosis.

Be able to describe how the process of crossing over occurs during meiosis and be prepared to describe how this affects genetic diversity in offspring.

Be able to present various examples of extra-nuclear inheritance.

Suggested Assignments

Review terminology from DNA replication and cellular reproduction using the question server. Complete the fundamental terms crossword puzzle. Here is the solution to the puzzle.

Read pp. 46-49 and pp. 55-59 in ExamKrackers Biology I. Perform practice items 33-40 on pg. 50 (material also here from gene expression) and practice items 41-48 on pg. 64 (material also here from genetic recombination and Mendelian genetics).

Review the web resources for gene expression.

Conceptual Vocabulary for DNA Replication, Repair and Cellular Reproduction

Cell division
Cell division is a process by which a cell, called the parent cell, divides into two cells, called daughter cells.
Binary fission
Binary fission is the form of asexual reproduction in most prokaryotes by which one cell divides into two cells of the same size.
DNA replication
DNA replication is the process of copying a double-stranded DNA molecule.
Cell cycle
The cell cycle is the series of events that take place in a eukaryotic cell leading to its replication.
A chromosome is a single large macromolecule of DNA, and constitutes a physically organized form of DNA in a cell.
Mitosis is the process by which a cell duplicates its genetic information in order to generate two, identical, daughter cells.
Prophase is a stage of mitosis in which chromatin condenses into a highly ordered structure called a chromosome.
Metaphase is the stage of mitosis in the eukaryotic cell cycle in which condensed chromosomes, carrying genetic information, align in the middle of the cell before being separated into each of the two daughter cells.
Anaphase is the stage of mitosis when chromosomes separate in a eukaryotic cell.
Meiosis is the process by which one diploid eukaryotic cell divides to generate four haploid cells often called gametes.
Semiconservative replication
Semiconservative replication describes the method by which DNA is replicated which produces two copies each containing one of the original strands and one entirely new strand.
Lagging strand
The lagging strand is the DNA strand opposite the replication fork from the leading strand.
Replication fork
The replication fork is a structure that forms during DNA replication having two branching prongs, each one made up of a single strand of DNA.
Leading strand
The leading strand is the DNA strand at the opposite side of the replication fork from the lagging strand.
DNA polymerase
A DNA polymerase is an enzyme that assists in DNA replication, catalyzing the polymerization of deoxyribonucleotides alongside a DNA strand.
DNA polymerase III holoenzyme
DNA polymerase III holoenzyme is the primary enzyme complex involved in prokaryotic DNA replication.
Interphase is a phase of the cell cycle, defined only by the absence of cell division.
A chromatid is one of two identical strands of DNA making up a chromosome that are joined at their centromeres, for the process of nuclear division.
A centromere is the region in the middle of a chromosome where sister chromatids join in the double chromosomal structure during mitosis, prophase and metaphase.
Sister chromatids
Sister chromatids are identical copies of a chromosome.
Homologous chromosome
Homologous chromosomes are non-identical chromosomes that contain information for the same biological features and contain the same genes at the same loci but possibly different genetic information at those genes.
The kinetochore is the protein structure in eukaryotes which assembles on the centromere and links the chromosome to microtubule polymers from the mitotic spindle during mitosis and meiosis.
Prometaphase is the phase of mitosis following prophase and preceding metaphase, in eukaryotic somatic cells.
Spindle apparatus
During cell division, the spindle apparatus pulls apart the chromosomes into the two daughter cells.
Telophase is a stage in either meiosis or mitosis in which nuclei reform and chromatin loses its condensed state.
Cytokinesis is the process whereby the cytoplasm of a single cell is divided to spawn two daughter cells.
Okazaki fragment
An Okazaki fragment is a relatively short fragment of DNA, with an RNA primer at the five prime terminus, created on the lagging strand during DNA replication.
A primer is a nucleic acid strand, or a related molecule, that serves as a starting strand of nucleotides for DNA polymerase to add to.
DNA polymerase I
DNA polymerase I is an enzyme that mediates the process of DNA replication in prokaryotes, an example of a processive enzyme - an enzyme which catalyzes a series of polymerisations.
Histones are the chief protein components of chromatin. They act as spools around which DNA winds, and they play a role in gene regulation.
A telomere is a region of highly repetitive DNA at the end of a linear chromosome that functions as a disposable buffer.
G1 phase
The G1 phase is a period in the cell cycle during interphase, after cytokinesis and before the S phase.
Synthesis phase
The S phase, short for synthesis phase, is a period in the cell cycle during interphase, between G1 phase and the G2 phase.
G2 phase
The G2 phase is the third, final, and usually the shortest subphase during interphase within the cell cycle in which the cell undergoes a period of rapid growth to prepare for mitosis.
Synaptonemal complex
The synaptonemal complex is a protein structure that forms between two homologous chromosomes during meiosis which is thought to mediate chromosome pairing, synapsis, and recombination.
Synapsis, also called syndesis, is the pairing of two homologous chromosomes that occurs during meiosis.
Crossing over
Crossing over is the process by which two chromosomes, paired up during prophase 1 of meiosis, exchange some portion of their DNA.
Origin of replication
The origin of replication is a particular DNA sequence at which DNA replication is initiated.
DNA primase is activated by DNA helicase to synthesize a short RNA primer as a starting point for replication.
DNA ligase
DNA ligase is a particular type of ligase that can link together strands that have double-strand breaks
The replisome is made up of two DNA polymerase III core enzymes, which are each made up of three subunits: one with polymerization activity, one with proofreading ability and one that stimulates the proofreading.
Ori is the DNA sequence that signals for the origin of replication, sometimes referred to simply as origin.
Heterochromatin is a tightly packed form of DNA. Its major characteristic is that transcription is limited.
Euchromatin is a lightly packed form of chromatin that is rich in gene concentration and is often under active transcription.
Cell cycle checkpoint
Cell cycle checkpoints are control mechanisms that ensure the fidelity of cell division in eukaryotic cells.
Ploidy is the number of homologous sets of chromosomes in a biological cell.
Gametogenesis is a process by which the diploid germ cells undergo a number of chromosomal and morphological changes to form mature haploid gametes.
A gametocyte is a eukaryotic germ cell that divides by mitosis into similar cells or by meiosis into gametes during gametogenesis.
Sometimes referred to as a tetrad, bivalents are a pair of associated homologous chromosomes formed after replication during meiosis.
A dyad is a pair of sister chromatids.
Helicases are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separating two annealed nucleic acid strands.
DNA clamp
A DNA clamp is a protein fold that serves as a processivity-promoting factor in DNA replication.
Single-strand binding protein
Single-strand binding protein binds single stranded regions of DNA to prevent premature reannealing.
DNA gyrase
DNA gyrase is a type II topoisomerase that introduces negative supercoils into DNA or relaxes positive supercoils.
Topoisomerases are isomerase enzymes that acts on the topology of DNA.
Pre-replication complex
A pre-replication complex is a protein complex that forms at the origin of replication during the initiation step of DNA replication.
Origin Recognition Complex
Origin recognition complex is a multisubunit complex existing in the replication procedure of DNA, marking the replication origin by binding to and marking ori sequences in all eukaryotes.
Cleavage furrow
The cleavage furrow is the indentation that begins the process of cleavage, by which animal and some algal cells undergo cytokinesis.
Germ cell
A germ cell is part of the sequence cells that have genetic material that may be passed to an offspring.
dnaB helicase
DnaB helicase is an enzyme in prokaryotes which opens the replication fork during DNA replication.
Advanced terms that may appear in context in MCAT passages
Processivity is a measure of the average number of nucleotides added by a DNA polymerase enzyme per association/disassociation with the template.
Rolling circle replication
Rolling circle replication describes a process of nucleic acid replication that can rapidly synthesize multiple copies of circular molecules of DNA or RNA such as plasmids or bacterial nucleoids.
Cell division cycle 6
Cell division cycle 6 is a protein essential for the initiation of DNA replication. Its role is to recruit a complex of six proteins, called the minichromosome maintenance complex to the sites of replication initiation.
Telomerase is an enzyme that adds specific DNA sequence repeats to the three prime end of DNA strands in the telomere regions of eukaryotic chromosomes.
Restriction point
The restriction point is a G1 phase checkpoint in the cell cycle of animal cells. Cells that progress through this point are committed to enter S phase.
Condensins are large protein complexes that play a central role in chromosome assembly and segregation in eukaryotic cells. Even in prokaryotes, ancestral forms regulate the segregation of nucleoids.
Cohesin is the protein responsible for binding the sister chromatids during mitosis after S phase.
Cyclins are a family of proteins involved in the progression of cells through the cell cycle. Their concentration varies in a cyclical fashion during the cell cycle.
Spindle checkpoint
The spindle checkpoint blocks the entry of a cell undergoing mitosis into anaphase until all chromosomes are properly attached to the meiotic or mitotic spindle.
Separase is a cysteine protease responsible for triggering anaphase by hydrolysing cohesin which is the protein responsible for binding sister chromatids during metaphase.
Septins are evolutionary conserved proteins with essential functions in cytokinesis, and more subtle roles throughout the cell cycle. They act as a scaffold, recruiting a plethora of proteins.
The chiasmata are thought to be the points where two homologous chromatids exchange genetic material during chromosomal crossover during meiosis.
Primosome is a complex of seven proteins including DnaG primase, DnaB helicase, and DnaC helicase assistant.
Cell plate
Cell plate formation is the process by which cytokinesis occurs in plants.
Holliday junction
A Holliday junction is a mobile junction between four strands of DNA.
Flap endonuclease
Flap endonucleases are a class of nucleolytic enzymes that act as both exonucleases and structure specific endonucleases on specialised DNA structures.
Septin ring
A septin ring is a collar-shaped structure which forms during cell division along the axis where a plant cell splits.
dnaG is a primase which synthesizes an RNA primer for Okazaki fragments in E. coli.
Klenow fragment
The Klenow fragment is a large protein fragment which is produced when DNA polymerase I from E. coli is cleaved by the protease enzyme subtilisin.
Histone deacetylase
Histone deacetylases are a class of enzymes that remove acetyl groups from an epsilon-N-acetyl lysine amino acid on a histone, causing the histones to wrap more tightly around the DNA.
Maturation promoting factor
Maturation promoting factor (also called mitosis-promoting factor or M-Phase promoting factor) is a protein which stimulates the mitotic and meiotic cell cycles.
Securin is a protein involved in anaphase triggering which helps the transport of separase to the nucleus and inhibits its catalytic activity.
Preprophase is a phase preceding prophase during mitosis in plant cells. This phase does not occur in other eukaryotes such as animals or fungi.
Preprophase band
The preprophase band is a microtubule array found in plant cells that are about to undergo cell division and enter the preprophase stage of the plant cell cycle.
The phragmosome is a sheet of cytoplasm forming in highly vacuolated plant cells in preparation for mitosis.
Aurora A kinase
Aurora A kinase is a member of a family of mitotic serine/threonine kinases implicated with important processes during mitosis in association with the mitotic poles and adjacent spindle microtubules.
Cyclin-dependent kinase
Cyclin-dependent kinases belong to a group of protein kinases originally discovered as being involved in the regulation of the cell cycle. CDK9 is an exception, playing no role in cell cycle regulation.
Anaphase-promoting complex
Anaphase-promoting complex is a complex of several proteins which is activated during mitosis to initiate anaphase.

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