At anaphase a chromosome can look like a V, an L, a J, or just a rod, an I. Four shapes, one cause. Where the centromere sits along the chromosome decides which shape you get.
1. How many centromeres?
Start by counting. By the number of centromeres, chromosomes are of five kinds: monocentric, dicentric, polycentric, diffused and acentric.
One centromere makes a chromosome monocentric, and that is the usual case in most species. Two make it dicentric. More than two make it polycentric, found in some species of banana (Musa sp).
The last two are the odd ones out. With no clear centromere at a fixed place, a chromosome is diffused. With no centromere at all it is acentric: a piece of a freshly broken chromosome that takes no part in cell division.
2. Four shapes by position
Now forget the count and look at position. By where the centromere sits, chromosomes have four shapes: metacentric, submetacentric, acrocentric and telocentric. At anaphase they look like the letters V, L, J and I.
Metacentric: the centromere is exactly in the middle, so the two arms are equal, and at anaphase the shape is a “V”. All the chromosomes of Solanum nigrum are metacentric. It is the primitive feature.
Submetacentric: the centromere sits a little off-centre, the arms are slightly unequal, and at anaphase the shape is an “L”.
Acrocentric: the centromere is near one end. One arm is much longer and the other short, and at anaphase the shape is a “J”.
Telocentric: the centromere is right at the end, so the chromosome looks one-armed. At anaphase it is an “I”, a rod. It is usually absent in plants.
Before you learn the four names, slide the centromere yourself. Watch the shape change as it moves from the middle to the end.
slide the centromeredrag the handle
V, L, J, I at anaphase
Four are drawn below as in the book's Figure 1.20, points 1 to 4 from left to right. Tap the picture or the chips.
Figure 1.20tap a point
Chromosome shapes
Metacentric. V-shape. The centromere is in the middle, so the arms are equal.
Submetacentric. L-shape. The arms are slightly unequal.
Acrocentric. J-shape. One arm is very short.
Telocentric. I-shape. One arm.
The centromere. There is one in each picture. The shape depends on where it sits.
The arm. The part on either side of the centromere. How equal the two arms are is what separates the four shapes.
All seen. As the centromere slides from the middle toward the end, the shape goes V, L, J, then I.
Drawn by hand after the book's figure, not to scale.
These four pairs come straight into exams. Pick up a letter, then drop it in its shape's box.
matchtap a chip, then a box
Which letter?
Pick up a letter, then drop it in a shape's box.
Right. Next one.
Not yet. Ask where the centromere is: in the middle, a little off, near the end, or right at the end?
All right. Metacentric is V, submetacentric L, acrocentric J, telocentric I.
One species can carry several kinds at once. The dark purple variety of Typhonium trilobatum has 11 metacentric, 4 submetacentric and 2 acrocentric chromosomes.
The two classifications blur easily: one is by number, one by position. Say which box each description belongs in.
sort themtap a chip, then a bin
Number or position?
Pick up a chip, then drop it in a bin. One question: how many, or where?
Right. Next one.
Not yet. Does the description count centromeres, or say where the centromere sits?
All in place. Two is dicentric, none is acentric, more than two is polycentric: those are counts. V in the middle (metacentric), J near the end (acrocentric), I at the end (telocentric): those are positions.
3. Autosomes and sex chromosomes
Chromosomes can also be split by job. A chromosome carrying genes for body traits is an autosome, and the autosome set is written A. In humans, 22 of the 23 pairs are autosomes.
The rest are sex chromosomes, which decide sex. They are X and Y. A female has two alike, XX. A male has two different ones, XY.
4. What a chromosome is made of
A chromosome has three main chemical components: nucleic acids (DNA and RNA) and protein.
DNA (Deoxyribo Nucleic Acid) is the permanent component of a true chromosome, about 45%. Its structure is a double-stranded spiral polynucleotide. About 90% of an organism's DNA lies in its chromosomes.
By Swift and Bonner (1968), the ratio of DNA to histone protein in a chromosome is 1 : 1.
RNA (Ribo Nucleic Acid) is only 0.2-1.4% and is not a permanent component. It is usually single-stranded, made of ribose, phosphate and the bases A, G, U and C.
Protein is about 55% and builds the main framework. It comes in two kinds: low-molecular-weight (protamine or histone) and high-molecular-weight acidic protein.
Protamine and histone are both basic proteins. Most chromosomes have histone, while protamine occurs only in sperm chromosomes. Histone attaches directly to DNA because the positive side groups of arginine, lysine and histidine bind the negative phosphate groups of DNA.
Notable acidic proteins are DNA polymerase and RNA polymerase. Small amounts of Mg, Ca, lipid, enzyme and Fe also occur.
The percentages blur together. Sort eight facts into three boxes.
sort themtap a chip, then a bin
Chromosome contents
Pick up a chip, then drop it in a bin.
Right. Next one.
Not yet. Recall which one is permanent, which is smallest, and which builds the framework.
All in place. DNA: 45%, permanent, a double-stranded spiral. RNA: not permanent, 0.2-1.4%, usually single-stranded, uracil in place of thymine. Protein: 55%, the framework, histone is basic, DNA polymerase is acidic.
5. What a chromosome does
A chromosome holds and carries heredity. It acts directly in cell division. It holds genes, makes protein through mRNA, and decides sex. Taken together, it works as the blueprint of life.
The role in division is the main one. If the DNA of the chromosome is not replicated, cell division does not even begin.
6. Check yourself
Three questions in the exam's style. The first one.
check yourselfpick one answer
Question 1
The functions of a chromosome are:
i. Acting as the carrier and bearer of heredity ii. Synthesising carbohydrate iii. Transmitting characters of the organism
i and ii: ii is wrong, since the chromosome does not synthesise carbohydrate.
Right. ii and iii: ii is wrong.
The chromosome is the carrier of heredity and passes characters to the next generation.
Not all three, because ii is wrong.
The second.
check yourselfpick one answer
Question 2
In anaphase an acrocentric chromosome looks like which letter?
‘V’ is the shape of a metacentric chromosome.
‘L’ is the shape of a sub-metacentric chromosome.
Right. An acrocentric chromosome looks ‘J’-shaped in anaphase.
‘I’ is the shape of a telocentric chromosome.
The last, with a moment's thought.
check yourselfpick one answer
Question 3
Look at the figure above: [Figure: two X-shaped chromosomes side by side, each with two chromatids; the chromatids of the left chromosome are marked B and A, those of the right one C and D.] Which is the permanent component of a eukaryotic chromosome?
Right. DNA is the permanent component of a eukaryotic chromosome.
RNA is present in small amounts (0.2-1.4%) and is not a permanent component.
Protamine is a basic protein, but the permanent chemical component is DNA.
Prolamine is a plant storage protein, not a chromosome component.
The whole note on seven cards.
cut from the notetap to turn over
Seven cards
QuestionInto how many kinds are chromosomes divided by centromere number?