Why are siblings different? Meiosis.

2 Oct

For a short version of the video, start it at 6 minutes, 20 seconds.

My sister is sweet, neat and petite; I am sweet, squat and incurably messy :).

One egg, one sperm:  Roughly 5.9029581 × 10⁴³possible combinations of chromosomes.

Meiosis (pronounced my-OH-sis) is a chromosomal dance that happens when bodies make egg and sperm cells, and it ensures the shuffling of the genetic deck.

In brief, it is the process by which a cell’s 46 chromosomes replicate, partner up, swap chunks of information, and twice divide in order to create eggs (inside a female fetus) or sperm (inside a man’s testes), with each resulting sex cell containing a single set of 23 chromosomes.

Chromosomes are thread-like strands, located in every cell in our bodies, which carry our genetic information.  Egg or sperm cells (gametes) are unique among our trillions of cells in that they contain just one set of 23 chromosomes (this makes them “haploid cells”) rather than the 23 pairs we see in the rest of our (diploid) cells.  That’s because egg and sperm cells are fitted to join forces and pass a total of 46 chromosomes—23 from mom, 23 from dad– to the next generation.

The creation of a sperm cell

To illustrate this, let’s look at the creation of a sperm cell.

A man, let’s call him Karl Rove, has stem cells inside of his testes that are able to divide and mature into sperm.  Scientists have an array of terms for these cells as they hit various stages of development, but to keep things neat here we’re just going to call them “pre-sperm cells.”

Pre-sperm cells become sperm when they are  biologically “tapped” to develop.  It’s as if they were drafted.  Maturation, which takes less than two months, happens through the process of meiosis (pronounced my-OH-sis).

Replication, recombination, division

The first step in this process is for the chromosomes to replicate.

Hold out your left index finger.  This is a “chromatid,” or one half of a replicated chromosome.  Now hold out your right finger.  This is the sister chromatid.  Now touch your two fingers at the knuckles.  This X-shape formation resembles your chromosomes, and your knuckles represent the “centromere,” the node that links the sister chromatids.

Find yer partner

Inside of Karl Rove’s pre-sperm cells are 23 pairs of chromosomes.  The 23rd chromosome, which determines gender, is the “sex chromosome,” while the others are “autosomes.”  The autosomes are numbered by size: One is the biggest and 22 is the smallest.

During the meiosis process, the genes that Karl Rove has from his mom and the genes that he has from his dad get the opportunity to combine right in his testicles, inside of his pre-sperm cells.  Chromosomes 1 through 23 line up in pairs. (Karl Rove’s chromosome1 from mom gets together with chromosome 1 from Karl Rove’s dad, 2 hooks up with 2, 8 with 8, etc.)  We don’t know how they recognize each other, but they do.

Then, as the arms of nonsister chromatids press together, they swap genetic information. Sort of like swapping hats and boots on the dance floor.

This “crossing over,” or the exchanging of parts between “homologous chromosomes” to produce new combinations of genetic information, makes it possible for both of Karl Rove’s parents to genetically contribute to the chromosomal package he passes on to his children.

The millions of possible combinations represent an endless stream of diversity that the body makes in order to try to get the best combination for living on the planet.

But the dance is not quite over.

Back to your corners, belles and beaus

After chromosomes have paired up and swapped chunks on the middle of the dance floor, they then migrate to separate corners (or opposite poles of the cell) in order to go separate ways as the cell divides.

The cell divides first to produce two cells containing the X-shaped chromosomes, 1 through 23, without their “dance” partners.

Those two cells then divide again, splitting the X-shaped chromosomes at the centromere, producing four cells containing single-stranded chromosomes.

The result is four haploid (23-chromosomed) “daughter” cells that are genetically unique.  Put a tail on each one (literally), and Karl Rove’s got himself four sperm ready to hit the gene pool.

Because of the genetic shuffling that takes place through meiosis, the possible number of genetic combinations that a couple can produce in their offspring has been estimated to be 80²³, or about 60 tredecillion.  For this reason, it is close to impossible to have two children who are genetically identical, unless they come from the same fertilized egg like identical twins.
For more on this topic, Google “spermatogenesis,” and enjoy this video animation.

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3 Responses to “Why are siblings different? Meiosis.”

Trackbacks/Pingbacks

  1. Scrambled eggs, mutant sperm « Genetics & Parenthood - July 6, 2011

    […] the process of meiosis, each of these cells that contain 23 chromosomes from Bob Saget’s mom and 23 from Bob […]

  2. Genetic math: Is Ben Stiller his parents? Or his own damn self? « Genetics & Parenthood - July 6, 2011

    […] is true because of meiosis, the chromosomal square dance that puts unique combinations of chromosomes into sperm and egg […]

  3. The Science of Pregnancy Timeline: Week One — Having sex to become pregnant: Why it is necessary? « - August 15, 2012

    […] Meiosis, or the jumbling and divvying up of genes inside of cells that will become eggs and sperm, creates genetic differences among kids. (Ask anybody from a large family, there’s always one that you’re sure probably is not related to the rest.) […]

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