العلوم
Chromosome: What DNA Looks Like When It Is Packed Away
A chromosome is one long DNA molecule carrying many genes. Humans have 46 in 23 pairs, and the last pair, XX or XY, is the one that looks mismatched.
Chromosome
A chromosome is one long, coiled molecule of DNA in the nucleus of a cell, carrying hundreds or thousands of genes along its length.
- أين يقابله الطلاب
- Grade 7 science, when cell structure is first studied in detail, and again in Grade 9 and Grade 10 for cell division and inheritance.
الإجابة باختصار
A chromosome is a single very long molecule of DNA, coiled up with proteins inside the nucleus, carrying many genes along its length. A human body cell holds 46 of them arranged in 23 pairs, with one chromosome of every pair inherited from the mother and the other from the father.
مثال
46 chromosomes = 22 matched pairs + 1 sex pair (XX or XY)
Twenty-two of the pairs are matched: the two chromosomes look the same length and carry the same genes in the same order. The twenty-third pair is the exception. In a female it is XX and matches; in a male it is XY, and the Y is much smaller and carries far fewer genes. An egg cell always carries an X. A sperm cell carries either an X or a Y, which is why the father's gamete determines the sex of the child and why the outcome is close to an even split.
Nucleus, chromosome, gene, DNA: which sits inside which
These four words are often used as though they were four names for the same thing, and questions punish that. Working from the outside in: the nucleus is the compartment; inside it are the chromosomes; a gene is a short section of one chromosome; and DNA is the substance all of it is made from.
A chromosome is not a bag of genes. It is one continuous molecule of DNA, wound tightly around proteins so that a length of material measured in centimetres fits inside a cell you need a microscope to see. A gene is a stretch of that molecule — closer to a paragraph in a very long book than to a bead on a string.
What a gene does is coded in the order of its bases. That sequence specifies a sequence of amino acids, and the amino acids joined in that order make one particular protein. Everything inherited is inherited this way.
Why they come in pairs, and what the pairing means
Chromosomes come in pairs because you received a complete set from each parent: 23 from the egg and 23 from the sperm. The two chromosomes in a pair carry the same genes in the same positions, which is not the same as carrying the same information. At each position the two may hold identical versions of the gene or different ones, and that is where alleles come in.
The sex chromosomes break the pattern. Because a male has only one X, he has only one copy of most of the genes it carries, with nothing on the Y to mask a faulty version. This is why sex-linked conditions such as red-green colour blindness and haemophilia appear far more often in males, and why a female can carry one of these alleles without being affected.
Chromosome number measures nothing
It is tempting to read 46 as a mark of complexity. It is not. A dog has 78 chromosomes, a chimpanzee 48, a fruit fly 8. The number reflects how a species' genetic material happens to be packaged, not how much of it there is or how sophisticated the organism is.
What does matter is that the number is constant within a species and constant between generations. Mitosis keeps it at 46 every time a body cell divides, and meiosis halves it to 23 for gametes so that fertilisation restores 46 rather than doubling it.
- Chromosomes in a human body cell, arranged as 23 pairs
- 46Chromosomes in a human body cell, arranged as 23 pairs
أسئلة شائعة
How many chromosomes do humans have?
Forty-six in a normal body cell, arranged as 23 pairs. Gametes are the exception: a sperm cell or an egg cell carries 23 single chromosomes, so that a fertilised egg receives 23 from each parent and returns to 46. Red blood cells are another exception, because they lose their nucleus and so carry none at all.
What is the difference between a gene and a chromosome?
Scale. A chromosome is one entire molecule of DNA; a gene is a short section of that molecule coding for one protein. A single chromosome carries a great many genes strung along its length. Saying a chromosome is made of genes is roughly like saying a book is made of paragraphs — true, but it hides what the object actually is.
What are the sex chromosomes?
The twenty-third pair. Females have two X chromosomes, males have one X and one Y. The Y is considerably smaller and carries fewer genes, which is why the pair does not match under a microscope and why genes on the X have such visible effects in males, who carry only one copy of them.
Why do chromosomes come in pairs?
Because sexual reproduction gives you a full set from each parent. The egg supplies 23 and the sperm supplies 23, and the matching chromosomes line up as pairs carrying the same genes in the same order. Having two copies of each gene is exactly what makes dominant and recessive alleles possible.
Do all cells have 46 chromosomes?
No. Body cells do, but gametes have 23 because meiosis halves the number, and mature red blood cells have none because they lose their nucleus to make room for haemoglobin. Cells with an abnormal number occur too: Down's syndrome results from an extra copy of chromosome 21, giving 47 in total.
آخر تحديث
معرفة الكلمة ليست كاستخدامها
يستطيع المعلّم أن يرى الطالب وهو يستعمله في سؤال، فيتبيّن أين يتوقف الفهم بالضبط. الحصة الأولى مجانية.