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What is on common X-like picture of chromosome?

Here is the image from Wikipedia:

enter image description here

Are (for example) lower petals homologous to each other?

If yes, then why (1) is entitled as chromatid: one-half of two identical threadlike strands of a replicated chromosome? Homologous parts are similar but not identical.

If no, then why does human chromosomes are often denoted by Xs:

enter image description here

Do we have 23 Xs (females) or 46 Xs?

Dims
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  • You should the wikipedia legends on the post – Remi.b Jun 06 '16 at 19:04
  • I have cited important one. – Dims Jun 06 '16 at 19:07
  • Sure, but if you give more info and thoughts, it might be easier for us to understand what is unclear to you. This seems particularly important as I don't understand your comments below my answer. – Remi.b Jun 06 '16 at 19:18
  • @Remi.b note that I am absolutely agree with your answer and this is my "zero hypothesis". But I am confused, that sometimes X-like patterns are apparently used to depict homologous chromosomes. – Dims Jun 06 '16 at 19:47
  • Ok. If the legend under the figure (wherever you found it) says that it displays homologous chromosomes, then the legend is wrong! A single X-like shape always represents a single chromosome. – Remi.b Jun 07 '16 at 00:19
  • Please indicate the source of your second image. – MattDMo Jun 07 '16 at 01:08

1 Answers1

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The "X" you see is a chromosome. The two strands that forms the X-shaped chromosome are called chromatids. As they are bound together (by the centromer), they are called sister chromatids. Note that chromosomes look like that only during the metaphase. Note also, that not all chromosomes have a centromer that is in the middle of the chromosome as it is obvious from the picture in your question.

Many species (including humans) have two sets of each chromosome. If you add the other chromosome in the picture, you would see a pair of chromosome. One chromosome of the pair is said to be homologous to the other one. Below is a cladogram during the metaphase of a human (I let you figure out the sex) showing each pair of chromosome.

enter image description here

Sister chromatids are identical (except for mutation during the last chromosomal replication). Homologous chromosomes are similar but not identical. Crossover occurs in between chromatids of homologous chromosomes.

Further comment on your figure

There are 23 pairs of chromosomes, that is 46 chromosomes. Your picture shows only one chromosome per pair, that is your picture shows only 23 chromosomes. In other words, your picture shows the haploid genome and not the diploid genome.

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Remi.b
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  • See the picture of human genome. It is depicted as 23 Xs (some not fully formed). – Dims Jun 06 '16 at 19:12
  • I don't understand your comment. Can you develop? Do you think I answered your question? – Remi.b Jun 06 '16 at 19:14
  • So, pictures, depicting 23-24 X-like feutures are just incorrect? – Dims Jun 06 '16 at 19:14
  • See my update. I have added a picture. These pictures are common. Search human genome in google images. – Dims Jun 06 '16 at 19:15
  • Sure, I see the picture. I don't understand what you think would be incorrect on this picture based on my answer. Can you please develop further what is unclear? Note that your picture only show a single chromosome (two chromatids) of each of the 23 pairs. – Remi.b Jun 06 '16 at 19:25
  • Regard object #1 on my picture. There are ~ 23 of such objects in my picture. Hence, this object is representing 1/23 of genome. Hence, it represents 2 chromosomes. Probably, homologous ones. This object is depicted as X. Hence, strands are homologous, not identical. But you said strands are identical (not regarding mutations). – Dims Jun 06 '16 at 19:42
  • Please see edit "further comment on your figure" and let me know if it helps. – Remi.b Jun 07 '16 at 00:16
  • To whoever is concerned: Please leave a comment when downvoting. Such downvotes are really not helpful at improving the answer. – Remi.b Jun 08 '16 at 18:43
  • I didn't downvote – Dims Jun 09 '16 at 11:28
  • I never thought you did. No worries! – Remi.b Jun 09 '16 at 18:22