Expert: John Locke - 7/7/2009

Question
i cant differentiate that much about chromosomes, chromatid, chromatin and homologous chromosome. my friend said that chromatid is the one that had been condense in prophase. and homologous chromosome is te pairs. what about the sister chromatids and the daughter cell and stuff. i never get right when the question ask about daughter cells. and my school teacher wont help me. please help me solve this problem sir. thank you.

Answer
Thanks for using AllExperts. The exact differences between chromosomes, chromatids, chromatin, and homologous chromosomes can be difficult to understand if not explained well. I will try my best to explain them here, and I will include links to sites that explain the topic as well.

To begin with, chromatin is a combination of DNA and protein that makes up chromosomes. Chromatin consists mostly of DNA and histone proteins (some other proteins are also involved in the structure of chromatin, but I will ignore those for our purposes); DNA is coiled around the histone proteins to reduce its volume and to control its genetic expression. Chromatin is thus the structural material that makes up chromosomes. The structure of chromatin varies depending upon the stage of the cell cycle.

Compare that to chromosomes themselves. Chromosomes are the actual organized DNA/protein structures found within eukaryotic cell nuclei. You might think of chromatin as the building materials for a house, while chromosomes are the actual house that's built from those materials. Each chromosome consists of a single linear chain of DNA that undergoes a variety of coiling around the proteins mentioned above.

Humans have 23 unique chromosomes, and two copies of each chromosome (one maternal and one paternal). That will become important in a moment. See the website below for the image of a human karyotype: 46 total chromosomes, arranged in 23 pairs.

http://upload.wikimedia.org/wikipedia/commons/5/53/NHGRI_human_male_karyotype.pn...

During S phase of the cell cycle, all the DNA within the nucleus is replicated to give two identical copies of the cell's DNA. Those extra copies pair with each other and are attached at a centrosome; see the image below for what I'm talking about. The image on the right shows a replicated chromosome. It is important to realize that the structure on the right and the one on the left are both referred to as a chromosome. That is often a point of confusion for students. The structure on the right is a replicated chromosome, yes, but it is still a single chromosome.

http://staff.jccc.net/PDECELL/celldivision/chromosome1.gif

One of the two identical copies of DNA found in a replicated chromosome is known as a chromatid, sometimes called a sister chromatid because it is identical in sequence to the other. Chromatids are thus one-half of a replicated chromosome. When the cell undergoes mitosis and these strands are pulled apart, the structures are referred to as daughter chromatids. In short: chromatids are one half of a replicated chromosome; when attached, the strands are known as sister chromatids; when the chromatids are pulled apart, they're known as daughter chromatids.

Finally, homologous chromosomes. Homologous chromosomes pair with each other in meiosis and represent non-identical chromosomes that carry the same genes. Homologous chromosomes have the same structure as each other, carry the same loci for genes, and control the same functions as each other, but they are not identical in DNA sequence. In humans, each member of a pair of a homologous chromosomes is inherited from a separate parent. All humans have 22 pairs of homologous, non-identical, non-sex chromosomes (called autosomes), while females have an additional pair of homologous sex chromosomes (the X chromosomes). Males have a non-homologous set of X and Y chromosomes.

Homologous chromosomes represent two sets of replicated, paired sister chromatids that have undergone crossing-over in meiosis I. (And, by the way, homologous chromosomes only can be identified in meiosis; there is no analogous structure in mitosis). These chromosomes, as noted above, do not contain an identical genetic sequence.

Here are some more sites that you may wish to consider. Good luck!

http://www.johnkyrk.com/chromosomestructure.html
http://www.accessexcellence.org/RC/VL/GG/human.php
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chromosomes.html (The first diagram on the right succinctly identifies most of the terms I've discussed here).