Modelling Meiosis Using Lego Bricks (VCE SSCE Biology): Revision Notes

Modelling Meiosis Using Lego Bricks

Investigation type: Modelling

Introduction to meiosis

Meiosis is a specialised form of cell division where a single diploid parent cell undergoes two successive divisions to create four genetically unique haploid gametes. Each gamete produced contains exactly half the genetic information present in the original parent cell, and importantly, no two gametes are genetically identical to each other.

Key processes that increase genetic diversity

Two particular stages during meiosis are responsible for generating the remarkable genetic variation we observe in gametes:

Crossing over in prophase 1: During this stage, sections of genetic material are swapped between homologous chromosomes. This exchange creates recombinant chromatids, which are chromosomes containing a mixture of genetic information from both parental sources.

Independent assortment during metaphase 1: In this phase, homologous chromosome pairs position themselves randomly along the cell's equator. Because this arrangement is random, different combinations of maternal and paternal chromosomes end up in different daughter cells.

Aim

To demonstrate the importance of crossing over and independent assortment in creating genetic diversity, using coloured Lego bricks to represent different chromosomes and alleles.

Materials required

• 4 rubber bands
• 40 red Lego bricks
• 40 yellow Lego bricks
• 2 black Lego bricks
• 2 white Lego bricks
• 2 blue Lego bricks
• 2 pink Lego bricks

Method

This investigation models meiosis in a diploid organism ($2n$) with two pairs of homologous chromosomes. Follow each step carefully.

Step 1: Creating homologous chromosome pairs

Using half of your red and yellow Lego bricks, construct two pairs of chromosomes with different lengths. In this model, red bricks represent chromosomes inherited from the mother (maternal chromosomes), while yellow bricks represent those inherited from the father (paternal chromosomes).

Each pair consists of one red chromosome and one yellow chromosome. These are homologous chromosomes because they carry genes for the same traits, although they may have different versions (alleles) of those genes.

Step 2: Adding genetic information

Now you'll add alleles to your chromosomes at two specific gene locations. Insert one coloured brick into each chromosome to represent different alleles. For this investigation, we'll examine two human traits:

Gene	Dominant allele	Recessive allele
Hitchhiker's thumb	Regular thumb (black Lego brick)	Hitchhiker's thumb (white Lego brick)
Hairline shape	Widow's peak (pink Lego brick)	Straight hairline (blue Lego brick)

Step 3: DNA replication

Using the remaining red and yellow bricks, create an exact duplicate of each chromosome. Connect the original chromosome to its replica using a rubber band, which represents the centromere - the structure that holds sister chromatids together.

At this stage, each chromosome now consists of two identical sister chromatids joined at the centromere. This replication occurs during interphase, before meiosis begins.

Step 4: Simulating crossing over

Create a chiasma (the point where chromosomes cross over) between each pair of homologous chromosomes. This represents the exchange of genetic material that occurs during prophase 1.

Step 5: Demonstrating independent assortment

Arrange your homologous chromosome pairs in all the different possible configurations they could adopt at the metaphase plate during metaphase 1.

Because each pair of homologous chromosomes can orient itself in two different ways, and these orientations are random and independent of each other, multiple different arrangements are possible. This random arrangement is the essence of independent assortment.

Step 6: Examining possible gametes

Select one arrangement from step 5 and separate the chromosomes to show what gametes would result from that particular configuration.

Each gamete receives one chromosome from each homologous pair. Depending on which arrangement you chose, the resulting gametes will have different combinations of maternal and paternal chromosomes, and different combinations of alleles.

Understanding the results

After completing the simulation, consider these important questions:

Question 4: What genotypes and phenotypes did your gametes have?

This will vary depending on your specific allele combinations and crossing over events.

Question 5: How do your gametes compare with classmates' gametes?

They should show considerable genetic variation, demonstrating the power of meiosis to create diversity.

Question 6: What would happen if you combined your gamete with a classmate's gamete?

You'd create a new organism with a unique genotype and phenotype.

Important concepts to remember

Diploid ($2n$): Cells containing two complete sets of chromosomes - one set inherited from each parent. In humans, diploid cells have 46 chromosomes arranged in 23 pairs.

Haploid ($n$): Cells containing only one complete set of chromosomes. Human gametes are haploid, containing 23 chromosomes.

Chiasma: The X-shaped structure formed when homologous chromosomes cross over and exchange genetic material.

Alleles: Different versions of the same gene. For example, black and white Lego bricks represent different alleles for the thumb shape gene.

Reflection on the simulation

When writing your conclusion, focus on:

• How crossing over and independent assortment contributed to genetic variation in your results
• Ways the simulation could be improved (perhaps using more chromosome pairs, or modelling both meiotic divisions)
• The limitations of using Lego to represent real biological processes
• What the model taught you about the importance of genetic diversity