Why is it important for the egg and sperm to only have half a set of chromosomes?
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If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked. Take a look at human chromosomes. HI! Let's look at human chromosomes. This is a photomicrograph of stained human chromosomes from one body cell. How many chromosomes are there? 92. No, count again. 23. No, try again. These are the broken pieces of one long chromosome. No, Walther Flemming did think that chromosomes exist as one long piece that broke up during mitosis, but this is not true. 46. That is correct. Human cells normally have 46 chromosomes; this is the diploid number. Chromosomes are usually arranged in a karyotype, where homologs are shown side by side. This is useful for identifying some human disorders. In this karyotype, how many pairs of chromosomes are there? 46. No, try again. 23. That is correct. [HUMAN KARYOTYPE] Humans have 23 pairs of homologous chromosomes. How many of these chromosomes will be found in an egg cell? All 46 chromosomes. No, sex cells have a half set of chromosomes. 23, one from each pair. That is correct. 23, pairs 1-11 and one selected randomly from the remaining pairs. Human sex cells do have 23 chromosomes, but not these 23. 23, 11 random pairs and one extra. Human sex cells do have 23 chromosomes, but not these 23. None, egg cells don't have chromosomes. No, sex cells do have chromosomes. Meiosis reduces chromosome number so that sex cells (eggs and sperm) have a half set of chromosomes–one homolog of each pair. This is the haploid number. CONGRATULATIONS! YOU'RE SO SMART! walther flemming, human karyotype, human chromosomes, egg cells, sex cells, haploid number, diploid number, photomicrograph, homolog, mitosis, body cell, homologs, homologous chromosomes, meiosis
Bitesize genomics 1Half + half = youYour genome is unique. A copy is found in almost every cell in your body and is organised into 46 chromosomes in 23 pairs. But where does your genome come from? To answer this, we must go back to the point of your conception, when your father’s sperm fused with your mother’s egg. The sperm and egg are specialised cells called gametes and are unique in comparison to most of the other cells in the body, as they only contain half the usual number of chromosomes. At fertilisation, half of your father’s genome is mixed with half of your mother’s genome to form your complete genome. 2It's all in the mixSo if our genomes come from our parents, why don’t all siblings look the same? This is because - unless you are an identical twin - the egg and sperm that created you contained different combinations of your parents' DNA than the egg and sperm that created your brother or sister. The different combinations are because of how gametes are formed, through a type of cell division called meiosis, where a cell containing 46 chromosomes divides to produce four cells, each containing 23 chromosomes. During meiosis, the chromosomes are duplicated, shuffled and separated, which ensures that each gamete produced is unique. Meiosis: How gametes are formed 3When sperm meets eggAt fertilisation, a cell is formed called a zygote, which then undergoes another type of cell division called mitosis. This type of cell division differs from meiosis, as rather than producing four cells with half the number of chromosomes, it produces two completely identical cells. The two cells, known as daughter cells, then divide to produce two new cells, which also divide and so on. Cells in our body undergo mitosis not only for our growth and development, but also to repair tissue or replace dead cells. Watch the animation to see this process in action. 4The significance for healthAs the genome is passed from both parent to offspring and from cell to cell in our body, any change in the DNA - known as a variant - can also be passed on. In healthcare, this becomes significant if the variant is associated with a particular medical condition. For instance, if a parent knows that there is a chance their child might inherit a condition, it could influence their reproductive choices or help them prepare for the care of their child. Equally, knowing that a variant cannot be inherited can be equally important to an individual. Watch this film to hear Beskida Fejzullahu talk about her family's journey to discovering the genetic variant present in her son Arvin, and how they approached a subsequent pregnancy. 5Summary points
6Learn moreDo you want to learn more about genetic inheritance? Try one of our courses and resources, below! Why is it important that sperm and eggs only have half the number of chromosomes?Gametes are reproductive cells, such as sperm and egg. As gametes are produced, the number of chromosomes must be reduced by half. Why? The zygote must contain genetic information from the mother and from the father, so the gametes must contain half of the chromosomes found in normal body cells.
Why is it important for gametes to have half the number of chromosomes?It is important that gametes have half the amount of DNA (haploid) so that at fertilisation when the egg and sperm combine it results in a cell with a full complement of DNA. Errors can occur during meiosis producing gametes with an extra or missing chromosome.
Why is it important for an egg or sperm cell to only carry one chromosome from each homologous pair?The cells that undergo meiosis are the gametes producing haploid sperm cell and egg cell. Haploidy is essential so that at fertilization the chromosomal number remains the same throughout generations.
Why is it important for both sperm and egg to have the haploid number of chromosomes?Why is it important that gametes are haploid cells? It is important that chromosomes are haploids, because when the sperm and the egg fuse together the cell will have 46 chromosomes. What is gametogenesis and where does it take place? Gametogenesis is the process of formation and the male and female sex cells.
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