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“Are on click the cc button at bottom right to turn off follow us on on twitter amoebasisters and facebook have you ever wondered how two siblings can nhave the mom and dad still look so different well today. We re going to talk about a nprocess that makes that possible a process called meiosis not to be confused with nmitosis. Which sounds. Unfortunately similar mitosis makes identical body cells like your nskin cells and stomach cells recall from our mitosis clip that since it nmakes identical body cells mitosis is important for growth and for repair of damage or to nreplace worn out cells.
But not meiosis meiosis is a process that contributes to genetic nvariety meiosis also doesn t make body cells. It makes sperm and egg cells. Otherwise known nas gametes. The fancier word you might recall that humans have 46 chromosomes that s how many chromosomes.
Most body cells nin your body have but there are some human cells that don t nhave 46 chromosomes human sperm cells and egg cells have 23 chromosomes. Why the number difference well if a sperm cell. Has 23 chromosomes and nan egg cell. Has 23 chromosomes.
When they come together. That makes. 46 chromosomes that will allow a newly formed fertilized negg to develop into a human meiosis is what we call a reduction division nbecause. You have a starting cell.
That has 46 chromosomes and your ending cells. The nsperm and egg cells have only has 23 chromosomes before we start getting into the stages of nmeiosis to make gametes. We have to remember what happens before meiosis can start actually. This also happens before mitosis.
It s the stage known as interphase. If you remember interphase..
It s when the ncell is growing it s replicating its dna. It s carrying out cell processes. Just like mitosis interphase happens. Before nmeiosis is going to start so the starting cell has 46 chromosomes and nyou have to duplicate those chromosomes in interphase before meiosis starts that basically means you re duplicating nyour dna since chromosomes are made of dna and protein ready for the tricky part because we tend to count chromosomes by the nnumber of centromeres present.
When the 46 chromosomes duplicate. We still say there nare 46 chromosomes. Because the sister chromatids are still attached and we re counting by ncentromeres. So 46 chromosomes here they replicate in ninterphase and you still have 46 chromosomes in this picture.
But you went from 46 to 92 chromatids little tricky. There we have a detailed video that explains these nchromosome numbers before and after replicating in interphase that can be useful for understanding. Nmeiosis. Ok.
So interphase checklist. Done now we can nmove into meiosis. You might remember. The mitosis stages.
Pmat. Nthe p. was for prophase the m for metaphase. The a for anaphase and nthe t for telophase.
The good news is that in meiosis you still nuse those terms. But because meiosis is actually a reduction division you re going from n46 chromosomes to 23..
Which means you actually divide twice so instead of mitosis where you divide one ntime and do pmat one time in meiosis you re going to divide twice and therefore do pmat ntwice. Because of this in meiosis you put numbers nafter the phases to indicate whether you re in the first division or the second division. So let s dive right in so let s start with the very first step n prophase. I one thing i like to remember about prophase nis pro.
This actually means before it kind of helps you remember that it comes nbefore. All the other stages start. This is where the chromosomes are going to ncondense and thicken. They are actually going to line up with their nhomologous pairs.
The word homologous means that the chromosomes nare approximately the same size and that they contain the same types of genes in the same nlocations. They are going to match up it is during this prophase. 1. That this amazing nprocess occurs called crossing over i know crossing over probably sounds like nsomething very different.
But this is a really awesome process because when these chromosomes nare lined up in homologous pairs. They have a way that they can transfer their genetic ninformation and exchange. It between each other. It s kind of like these chromosomes flop.
Nover. Each. Other. And they do a little genetic information exchange.
Here. It makes for what we call recombinant chromosomes..
Which ncan eventually contribute to the variety that we were mentioning that siblings can have neven when they have the same parents more about that later now we move into metaphase. I in metaphase. I think if the m as standing nfor middle the chromosomes are going to be in the middle nof. The cell.
It s a little different though from mitosis nbecause. They re still going to be in pairs in the middle of the cell. So it s not a nsingle file line. They are in pairs in the middle during anaphase.
I think a for away because nthe chromosomes are going to be pulled away by the spindle. Fibersthen we end with telophase ni. Where you have two newly formed nuclei and it becomes obvious. You will end meiosis n1 with two new cells cytokinesis follows with splitting the cytoplasm.
But we re not done yet on to meiosis. 2. The very first step in meiosis ii is prophase nii. It s not going to be nearly as eventful nas.
It was in prophase. I because they are not going to have homologous pairs. They also are not going to have that amazing nprocess called crossing over that doesn t happen again in prophase ii. You have your chromosomes and the spindles nstarting to form like in prophase.
I but prophase ii is not nearly as eventful of having that nprocess of crossing over in metaphase ii. Remember think m..
For middle nthe. Chromosomes are going to line up in the middle. This time. Though they are in a single file nline.
They are not in pairs like they were in metaphase ni anaphase ii remember a for away. But this time it s the chromatids that are getting npulled away by the spindle fibers chromatids are getting pulled away to opposite nsides of the cell in telophase ii. Nuclei reform and the 2 cells nare each going to divide so you can see here that 4 cells are going to be formed cytokinesis will follow to completely split nthe cytoplasm now keep in mind that meiosis in males produces nsperm cells and in females it produces egg cells. Because of independent assortment and also ncrossing over you re going to have variety for example in a male.
The four sperms cells nthat are produced each time they are all different from each other they are also different from the starting ncell because the starting cell had 46 chromosomes and the ending cells. Only have 23. So they are not identical to the original nand. They are not identical to each.
Other this is going to lead to variety a reason. Why two siblings with the same parents ncan look different from each other they still developed from an unique egg and unique nsperm cell. That came together one last thing to think about scientists are often looking into the process nof meiosis. Because sometimes the chromosomes don t separate correctly.
It s called nondisjunction when a cell can nreceive too many or too few chromosomes in the separation this contributes to some genetic disorders nwhich is something scientists continue to study ” ..
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