Source: Professor Partridge, University of Texas at Austin
Difficulty: Intermediate | Prerequisites: You need a solid grasp of mitosis and the cell cycle (especially the stages of division and chromosome structure) before this material will make sense. Review DNA replication and gene expression as well.
Meiosis is the specialised cell division that produces gametes (sex cells). Where mitosis makes two identical diploid daughters, meiosis makes four genetically unique haploid cells. This is how sexual reproduction generates the genetic variation that drives evolution. The second half of these notes covers mutations, nondisjunction (what happens when meiosis goes wrong), and the biotechnology tools that let scientists read, cut, and edit DNA. These tools, from restriction enzymes to CRISPR, build directly on the molecular biology covered earlier in the course. If you understand DNA replication and gene expression, the biotech section is a set of applications rather than new theory.
Meiosis divides a diploid cell into four haploid gametes through two rounds of division. Meiosis I separates homologous chromosomes (and introduces genetic variation via crossing over); meiosis II separates sister chromatids, much like mitosis. Nondisjunction, where chromosomes fail to separate properly, disrupts the normal chromosome count. On the biotech side, restriction enzymes cut DNA at specific sequences, plasmids carry genes into bacteria, PCR amplifies tiny DNA samples, and CRISPR-Cas9 allows precise genome editing.
Meiosis
A type of cell division that reduces chromosome number by half, producing four genetically unique haploid cells (gametes). Think of it as the division that makes sperm and eggs.
Homologous Chromosomes (Homologs)
A pair of chromosomes, one from each parent, that carry genes for the same traits at the same loci. They are the same size and shape but may carry different alleles.
Haploid (n)
Having a single set of chromosomes. Human gametes are haploid (n = 23).
Crossing Over (Recombination)
The exchange of DNA segments between homologous chromosomes during prophase I of meiosis. This is a major source of genetic variation. In simple terms, mum's and dad's chromosomes swap pieces so each gamete is a unique mix.
Independent Assortment
The random orientation of homologous pairs at metaphase I, which means each gamete gets a random combination of maternal and paternal chromosomes.
Nondisjunction
The failure of chromosomes (or chromatids) to separate properly during meiosis, resulting in gametes with too many or too few chromosomes. This can cause conditions such as Down syndrome (trisomy 21).
Gamete
A reproductive cell (sperm or egg) that is haploid. When two gametes fuse at fertilisation, they form a diploid zygote.
Zygote
The diploid cell formed by the fusion of two haploid gametes at fertilisation.
Plasmid
A small, circular piece of DNA found in bacteria that replicates independently of the chromosome. Scientists use plasmids as vectors to insert genes of interest into bacterial cells. Think of it as a USB drive for genes.
Restriction Enzyme (Restriction Endonuclease)
A bacterial protein that recognises and cuts DNA at a specific short sequence. Bacteria use these to destroy invading viral DNA. Scientists use them to cut DNA at precise locations for cloning.
Methylation (in the context of restriction enzymes)
Bacteria protect their own DNA from their restriction enzymes by adding methyl groups to the recognition sequence. The methylated DNA is not cut.
CRISPR-Cas9
A genome-editing system adapted from a bacterial defence mechanism. A guide RNA directs the Cas9 protein to a specific DNA sequence, where Cas9 cuts both strands. Scientists can then delete, replace, or insert DNA at the cut site.
PCR (Polymerase Chain Reaction)
A laboratory technique that amplifies a small amount of DNA into millions of copies. In simple terms, a molecular photocopier for DNA.
Transposon
A DNA sequence that can change its position within the genome. Moves by a "cut and paste" or "copy and paste" mechanism, with no RNA intermediate.
Retrotransposon
A type of transposable element that moves via an RNA intermediate: DNA is transcribed to RNA, the RNA is reverse-transcribed back to DNA, and the new copy is inserted elsewhere in the genome. More common than transposons in the human genome. Think of them as DNA parasites.
Sanger Sequencing
A method of determining the nucleotide sequence of DNA, based on selective incorporation of chain-terminating nucleotides during DNA polymerisation.
Antibiotic Resistance Gene (on a plasmid)
A gene included in an engineered plasmid that allows only the bacteria carrying the plasmid to survive when an antibiotic is applied. This is the selection mechanism that guarantees the surviving bacteria have your gene of interest.
Meiosis produces gametes (sperm and egg cells). Gametes must be haploid so that when two fuse at fertilisation, the resulting zygote is diploid. Starting point: a diploid cell that has already replicated its DNA (so it has 2 sets of chromosomes, each consisting of 2 sister chromatids).
Meiosis I separates homologous chromosomes. This is the step that halves the chromosome number.
Prophase I: chromosomes condense. Homologous chromosomes pair up (synapsis) and crossing over occurs: segments of DNA are exchanged between non-sister chromatids of homologs. This is the primary source of genetic recombination.
Metaphase I: homologous pairs line up at the metaphase plate. The orientation of each pair is random (independent assortment), adding another layer of genetic variation.
Anaphase I: homologous chromosomes are pulled to opposite poles. Sister chromatids remain joined.
Telophase I and Cytokinesis: the cell divides into two cells. Each cell is now haploid (one set of chromosomes), though each chromosome still consists of two sister chromatids.
Ploidy tracking through meiosis I:
Interphase: diploid
Prophase I: diploid (no division yet)
Metaphase I: diploid
Anaphase I: diploid
Telophase I: diploid (the cell has not fully split)
After cytokinesis: haploid (two unique cells)
Meiosis II is mechanically similar to mitosis. It separates sister chromatids. There is no additional interphase or DNA replication between meiosis I and meiosis II.
Prophase II: centromeres move apart, spindle forms.
Metaphase II: sister chromatids align at the metaphase plate.
Anaphase II: sister chromatids are pulled apart to opposite poles.
Telophase II: cells pinch in, nuclear envelopes reform.
Cytokinesis: four haploid cells (gametes) are produced.
All stages of meiosis II are haploid.
Five main types of chromosomal mutations:
Insertion: extra DNA is added into a chromosome.
Deletion: a segment of DNA is lost.
Duplication: a segment is copied, so the chromosome has a repeat.
Inversion: a segment is flipped 180 degrees within the chromosome.
Translocation: a segment moves from one chromosome to a non-homologous chromosome.
Nondisjunction is the failure of chromosomes to separate correctly during cell division. It completely disrupts the normal chromosomal balance.
In meiosis I: homologous chromosomes fail to separate at anaphase I. Both homologs go to the same daughter cell. Result: two gametes with an extra chromosome, two with a missing chromosome.
In meiosis II: sister chromatids fail to separate at anaphase II. Result: one gamete with an extra chromatid, one missing a chromatid, and two normal gametes.
Nondisjunction can occur in both meiosis I and meiosis II.
A typical engineered plasmid has four key components:
Promoter: drives transcription of the inserted gene.
Restriction enzyme site: the location where the plasmid is cut open so a gene of interest can be inserted.
Origin of replication: allows the plasmid to replicate independently inside the bacterial cell.
Antibiotic resistance gene: a selectable marker. When you add an antibiotic to the culture, only bacteria carrying the plasmid (and therefore the resistance gene) survive. This guarantees every surviving bacterium has your gene.
Restriction enzymes are proteins that recognise specific short DNA sequences and cut the DNA at those points. They evolved as a bacterial defence against viral infection: when a virus injects its DNA into a bacterium, the restriction enzyme cuts the viral DNA, rendering it non-functional.
Bacteria protect their own DNA from their own restriction enzymes by methylating the recognition sequences. Methylated DNA is not cut.
Bacterial cells that produce restriction endonucleases also produce modification enzymes that add methyl groups to the bases at the recognition site.
CRISPR-Cas9 is a genome-editing tool derived from a bacterial immune system. The Cas9 protein, guided by a short RNA sequence, finds and binds to a complementary DNA sequence in the genome. Once bound, Cas9 cuts both strands of the DNA. Scientists can then let the cell repair the break (often disabling the gene) or supply a template for the cell to insert new DNA at the cut site.
PCR is a technique for making large amounts of a specific DNA sequence from a very small starting sample. It works by repeated cycles of heating (to separate DNA strands), cooling (to allow primers to bind), and extension (DNA polymerase copies the target region). Each cycle roughly doubles the amount of target DNA.
Transposons are DNA sequences that can move from one location in the genome to another. They do this by either:
Cut and paste: the transposon is excised from its original site and inserted elsewhere.
Copy and paste: the transposon is duplicated, and the copy inserts at a new location.
Transposons can be harmful (disrupting genes) or beneficial (creating genetic diversity).
Retrotransposons move via an RNA intermediate. The process: DNA is transcribed into RNA, the RNA is reverse-transcribed back into DNA, and that new DNA copy is inserted at a different location in the genome. The pathway is DNA to RNA to DNA.
Retrotransposons are more common than transposons in the human genome. They are sometimes described as DNA parasites.
The key difference: transposons do not involve an RNA step. Retrotransposons do.
Sanger sequencing and Illumina sequencing are both based on DNA polymerisation. They use slightly different conditions and chain-terminating nucleotides to read the order of bases in a DNA strand. Both are applications of the same fundamental enzymology covered in the DNA replication unit.
Students often confuse meiosis I and meiosis II. Meiosis I separates homologous chromosomes (reductive). Meiosis II separates sister chromatids (equational, like mitosis). Meiosis I is the step that makes cells haploid.
Students sometimes think crossing over happens in mitosis. It does not. Crossing over occurs during prophase I of meiosis, between homologous chromosomes.
Students sometimes assume nondisjunction always happens in meiosis I. It can occur in either meiosis I (homologs fail to separate) or meiosis II (sister chromatids fail to separate).
Students often confuse restriction enzymes with CRISPR-Cas9. Restriction enzymes recognise fixed, short DNA sequences and are a natural bacterial defence. CRISPR-Cas9 is programmable: the guide RNA can be designed to target any sequence.
Students sometimes think transposons and retrotransposons are the same. The key difference is the RNA intermediate: retrotransposons use one, transposons do not.
⚠️ Be able to track ploidy through every stage of meiosis I and meiosis II. This is a very common exam question.
⚠️ Know exactly when and where crossing over occurs (prophase I, between non-sister chromatids of homologous chromosomes).
⚠️ Be able to compare and contrast meiosis I, meiosis II, and mitosis. What is separated, and what is the ploidy outcome?
⚠️ Understand nondisjunction in both meiosis I and meiosis II. Be able to draw the outcome (which gametes have extra or missing chromosomes).
⚠️ Know the four components of a typical plasmid (promoter, restriction site, origin of replication, antibiotic resistance gene) and the function of each.
⚠️ Be able to explain how bacteria protect their own DNA from their restriction enzymes (methylation).
⚠️ Understand the difference between transposons (cut/copy and paste, no RNA) and retrotransposons (DNA to RNA to DNA).
True or False: Meiosis produces two diploid daughter cells. (False: meiosis produces four haploid cells.)
Fill in the blank: Crossing over occurs during ______ of meiosis. (Prophase I)
True or False: After meiosis I, the resulting cells are haploid. (True)
Fill in the blank: A plasmid includes an antibiotic resistance gene so that scientists can ______ for bacteria that carry the plasmid. (Select / screen)
True or False: Retrotransposons move without an RNA intermediate. (False: retrotransposons use an RNA intermediate. Transposons are the ones that do not.)
Q: Compare the outcomes of meiosis I and meiosis II. What is separated in each, and what is the ploidy of the resulting cells?
A: Meiosis I separates homologous chromosomes, reducing the cell from diploid to haploid. Each resulting cell has one set of chromosomes, but each chromosome still has two sister chromatids. Meiosis II separates sister chromatids (like mitosis), producing four haploid cells, each with single-copy chromosomes.
Q: Explain how crossing over during prophase I increases genetic variation.
A: During prophase I, homologous chromosomes pair up and exchange segments of DNA. This means each chromatid in the resulting gamete carries a unique combination of maternal and paternal alleles, rather than being an exact copy of either parent's chromosome. Combined with independent assortment, this ensures each gamete is genetically unique.
Q: A student observes that a gamete has 24 chromosomes instead of 23. What went wrong, and at which stage could the error have occurred?
A: This is nondisjunction. If it occurred in meiosis I, a pair of homologous chromosomes failed to separate at anaphase I, so one daughter cell received both homologs. If it occurred in meiosis II, sister chromatids failed to separate at anaphase II, giving one cell an extra chromatid. Both produce gametes with abnormal chromosome numbers.
Q: You want to insert a human gene into bacteria using a plasmid. Describe the key components your plasmid must have and why.
A: The plasmid needs (1) a promoter to drive transcription of the inserted gene, (2) a restriction enzyme site where you can cut the plasmid open to insert the gene, (3) an origin of replication so the plasmid can replicate inside the bacterium, and (4) an antibiotic resistance gene so you can add an antibiotic to the culture and kill any bacteria that did not take up the plasmid, selecting only the ones carrying your gene.
Q: How does CRISPR-Cas9 differ from restriction enzymes as a tool for cutting DNA?
A: Restriction enzymes recognise fixed, naturally occurring short sequences and cut at those points. You cannot reprogram a restriction enzyme to target a different sequence. CRISPR-Cas9 uses a custom guide RNA that can be designed to match virtually any DNA sequence, making it a programmable genome-editing tool.
Q: What is the difference between a transposon and a retrotransposon?
A: A transposon moves directly as DNA (cut and paste or copy and paste). A retrotransposon is first transcribed into RNA, the RNA is reverse-transcribed back into DNA, and the new DNA copy inserts at a different genomic location. The key distinction is the RNA intermediate that retrotransposons require and transposons do not.
Meiosis builds directly on mitosis: the stages of meiosis II are nearly identical to mitosis, and the chromosome mechanics (spindle fibres, kinetochores, cohesins) are the same. If you have mitosis down, meiosis II should feel familiar.
Nondisjunction connects to genetics and inheritance: conditions like Down syndrome (trisomy 21), Turner syndrome (monosomy X), and Klinefelter syndrome (XXY) result from nondisjunction events. Understanding the mechanism here explains the chromosomal basis of these conditions.
The biotechnology section connects to DNA replication and gene expression: PCR is an in vitro application of DNA polymerase, restriction enzymes exploit the same sequence-specific binding that underlies transcription factor activity, and CRISPR-Cas9 targets DNA the same way complementary base pairing governs all nucleic acid interactions.
Meiosis, meiosis I, meiosis II, gamete, haploid, diploid, zygote, crossing over, recombination, synapsis, independent assortment, homologous chromosomes, homologs, nondisjunction, trisomy, monosomy, aneuploidy, genetic variation, prophase I, metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II, telophase II, cytokinesis, mutations, insertion, deletion, duplication, inversion, translocation, biotechnology, plasmid, vector, restriction enzyme, restriction endonuclease, methylation, CRISPR, Cas9, guide RNA, genome editing, PCR, polymerase chain reaction, DNA amplification, transposon, retrotransposon, reverse transcriptase, Sanger sequencing, Illumina sequencing, antibiotic resistance, selectable marker, origin of replication