Even the closest siblings are genetically unique, thanks to a microscopic DNA dance known as meiosis. Episode 63 begins with a recap of chromosome structure and homologous pairs.
Even the closest siblings are genetically unique, thanks to a microscopic DNA dance known as meiosis. Episode 63 begins with a recap of chromosome structure and homologous pairs (1:48). In order to reduce the chromosomes number in half, the cells will need to go through PMAT twice. (2:53) When does meiosis occur and why are the haploid cells different? (7:19)
The Question of the Day asks (9:35) True or false? Mitochondria are inherited through egg and sperm.
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap meiosis
Zoom out:
Unit 5 - Heredity
Topic - 5.1 and 5.2
Big idea - Information Storage and Transmission
You can’t pick your family but you can pick your friends. You can pick your nose but you can’t pick your friend's nose.. or something like that. Even though your closest besties seem to walk and talk just like you the longer you hang out, they still aren’t sharing the same gene pool. That lake is reserved for parents and grandparents, cousins, uncles, aunts - etcetera etcetera. Even the closest siblings are genetically unique, thanks to a microscopic DNA dance known as meiosis. Yes, I know identical twins have the same DNA - but that’s another episode.
Let’s Zoom in:
Meiosis is the process by which organisms distribute a random half of chromosomes into gametes. In humans, these gametes are sex cells known as egg and sperm. The haploid cells (represented algebraically as n) will combine during fertilization to produce a new and unique diploid cell (represented as 2n). The algebraic placeholder of n stands for the quantity of chromosomes in a haploid cell. For humans, n = 23 and so 2n = 46, the number of chromosomes in somatic body cells.
You remember chromosomes. Highly condensed linear DNA strands wrapped around histone proteins and organized by spindle fibers during cell divisions. In meiosis, you’ll first see DNA in the chromosome form during Prophase I. Well maybe see is the wrong word, you’ll need a microscope for sure and some tissue stain. What I should have said is that you can distinguish individual chromosomes for division. Prior to this phase of the cell cycle, DNA is in its looser chromatin form.
So humans have 46 chromosomes - that must be the greatest amount right? Wrong! Don’t be fooled - chromosome number can vary widely and doesn’t correlate to our perception of an “advanced organism.” Potatoes have 48, hermit crabs 254, roundworms only 2, and a species of fern is credited with 1,260! That’s a lot to keep track of during division for sure. For the sake of simplicity, we will stick with humans as our example in this episode.
Regardless of quantity, chromosomes are organized into homologous pairs. These homologous chromosomes are a similar size and have similar genetic information. The only homologous pair which might not match up with similar size is number 23 - If you are a listener who is genetically XY, then this is you! (the y chromosomes is really small!) The chromosomes you have in each of your cells right now came from your parent’s gametes, with homologous chromosomes pairing back-up during fertilization. Following the same logic in humans, the process of meiosis will be to separate the 23 homologous pairs into gametes. Meiosis occurs in sex organs, such as testes and ovaries, and will include the familiar steps of prophase, metaphase, anaphase and telophase. In order to reduce the chromosomes number in half, the cells will need to go through PMAT twice.
Prophase I begins with condensing DNA into chromosomes and the disappearing of the nuclear membrane. Additionally, homologous chromosomes pair up, and an exchange of DNA may occur between them by crossing over at a tetrad. This event is called recombination and is one of the main contributors of genetic variation in gametes. In metaphase I, the homologous chromosomes line up as pairs in a random order in the middle of the cell, with a spindle fiber attached to each centromere. This random arrangement, called independent assortment, is another contributor to genetic variation - where over 8 million different combinations are possible for humans. The alleles of genes located on non-homologous chromosomes will separate during meiosis I without influencing each other. For example, so long as two genes are on different chromosomes - there won’t be a greater chance of inheriting them together, they are not linked. When spindle fibers shorten in Anaphase I, homologous pairs are separated. This represents the law of segregation which states that the two alleles for each gene will separate during meiosis, as diploid cells become haploid. Telophase I and cytokinesis usually overlap - but the cell won’t reform the nuclear membranes or uncondense the DNA.
Meiosis I is now complete! With two of Gregor Mendel’s Genetics Laws included, the original parent cell has now formed two genetically unique haploid daughter cells, each with 23 chromosomes. Homologous chromosomes have separated but sister chromatids remain intact. Because centromeres are still connected, the chromosomes are still in the familiar X shape. This is a HUGE giveaway in diagrams when trying to identify the phase of meiosis - look for the X shape and count the chromosomes.
Meiosis I and Meiosis II are consecutive with no interphase. Meiosis II will also follow PMAT, with much similarity to Mitosis. We already have haploid cells, now we just need to increase the quantity. Prophase II has spindle fibers attaching to centromeres, pulling chromosomes to the middle of the cell in metaphase II, separating sister chromatids in anaphase II, and lastly uncoiling into chromatin and reforming nuclear membranes in telophase II. Division of the cytoplasm is the final step with cytokinesis and tada! A total of 4 haploid daughter cells, each genetically unique from each other and unique from the original diploid parent cell.
When does meiosis occur? Well - that depends on many factors! Which sex organ is undergoing gametogensis, the age, hormone levels, even envronmetental factors! Human males can produce sperm from puberty until death whereas female ovaries begin meiosis of their egg cells prior to their own birth, continue at puberty, and only finish meiosis II if fertilization occurs! I know I’m only mentioning humans - but google clown fish reproduction - they are protandrous. Trust me, biology is very different from the Finding Nemo portrayal. And are all these haploid cells alike? NOPE! Sperm will continue to undergo cell differentiation to have flagella, a midpiece, and chisel like acrosome. The egg will be HUGE by comparison, having had unequal cytoplasmic division and forming 3 degenerative polar bodies and one egg cell instead of 4 functional eggs.
Time for unit connections. Big compare and contrast with Unit 4 and Mitosis. Also, strong connection to Unit 6 - Gene expression and regulation - watching out for errors and their effects on phenotype. Don’t forget unit 7 - natural selection as meiosis ensures genetic diversity, a crucial component to the survival of a species. Alright - what about the exam? Be prepared to explain how meiosis results in the transmission of chromosomes from one generation to the next. You may be expected to predict the effect on a cell if there is an error in division (like nondisjunction or crossing over). Make sure to study and make models chromosomal exchange to prepare. You should also be prepared to answer a comparative question about mitosis and meiosis as well as meiosis I vs. meiosis II. You do not need to know any specific animal or plant reproductive cycles for the exam.
To recap……
Cell division by meiosis allows for heritable information to be passed on while creating genetically unique haploid gametes for sexual reproduction. This is accomplished through two divisions of PMAT, crossing over and independent assortment.
Coming up next on the Apsolute RecAP Biology Edition: Mendel’s Rules
Today’s question of the day is about inheritance.
Question of the day: True or false? Mitochondria are inherited through egg and sperm.