During meiosis II, the chromosomes condense and become attached to a new spindle apparatus (prophase II). They then move to positions in the equatorial plane of the cell (metaphase II), and their centromeres split to allow the constituent sister chromatids to move to opposite poles (anaphase II), a phenomenon called chromatid disjunction.
During telophase II, the separated chromatids—now called chromosomes—gather at the poles and daughter nuclei form around them. Each daughter nucleus contains a haploid set of chromosomes. Mechanistically, meiosis II is therefore much like mitosis. However, its products are haploid, and unlike the products of mitosis, the cells that emerge from meiosis II are not genetically identical. Pair of homologous chromosomes Homologue 1 Centromeres One chromatid Chiasma Two chiasmata Homologue 2 Synapsis and crossing over Tetrad Recombinant chromatids
One reason these cells differ is that homologous chromosomes pair and disjoin from each other during meiosis I. Within each pair of chromosomes, one homologue was inherited from the organism’s mother, and the other was inherited from its father. During meiosis I, the maternally and paternally inherited homologues come together and synapse. They are positioned on the meiotic spindle and become oriented randomly with respect to the spindle’s poles. Then they disjoin. For each pair of chromosomes, half the daughter cells produced by the first meiotic division receive the maternally inherited homologue, and the other half receive the paternally inherited homologue. Thus, from the end of the first meiotic division, the products of meiosis are destined to be different. These differences are compounded by the number of chromosome pairs that disjoin during meiosis I. Each of the pairs disjoins independently. Thus, if there are 23 pairs of chromosomes, as there are in humans, meiosis I can produce 223 chromosomally different daughter cells—that is, more than 8 million possibilities. To test your understanding of this concept go to Solve It: How Many Chromosome Combinations in Sperm?
Author Archives: Sir Gwakaka
Introduction to HTML
Remember what HTML stands for? It stands for HyperText Markup Language.
Let’s look in more detail what this means.
‘Markup’ in computing means adding extra data to text in order to tell computers more information about that text. For example, in HTML we tell the computer which parts of the HTML document are to be displayed, which parts make up the navigation, even which parts are titles and which are content.
What is a HTML document?
A HTML document is just what it sounds like – a document that contains HTML! It’s nothing more special than that. All we have to do is type our HTML into a document in a writing program – for example Notepad, or even Microsoft Word – and we have a HTML document.
Starting from the next lesson of this course we’re going to make our own simple HTML pages on our own computers, and learn all about HTML as we go.
First let’s understand what makes HTML special.
What are HTML tags?
HTML is just normal text, with additional information. This additional information is delivered through tags. Tags are the fundamental building blocks of HTML.
A tag looks like this:
<tag>
Anything inside the “<” and “>” symbols defines the tag.
Can anything be a HTML tag?
Yes and no. Technically you could put anything inside a “<” and “>” symbol. However, browsers only understand certain tags, so unless you use the tags that are officially defined as part of HTML then your tags will not work correctly.
We will learn all about the important HTML tags as we proceed through this course.
More about tags
Tags can be both opened and closed. For example, let’s look at a <span> tag (don’t worry about what a <span> tag is for now – we’ll get to it later):
<span>This is inside my span! I can put anything I like here.</span>
You can see that a tag is opened by having an initial tag such as <span>. It is then closedwith a second tag that has an additional “/” symbol. Anything can go between the opening and closing tags – this is known as the tag contents.
This whole example from the opening tag to the tag contents to the closing tag is known as a HTML element. In this case we have created a span element, because we are using a span tag to define the element.
Elements are made up of tags.
Self-closing Tags
Some tags don’t require opening and closing tags, they can effectively open and close themselves in just one tag. These are often known as self-closing tags.
Technically in HTML 5, which is currently the most advanced version of HTML, self-closing tags don’t actually close themselves, but it’s a helpful way to picture what is happening. (If you’re interested in why: it’s because HTML is similar to a markup language called XML. In XML tags are required to either be closed in a pair, or to self-close. HTML is not as strict as XML, so you don’t have to self-close your tags. However, it doesn’t hurt, and keeps your HTML neat so it is not a bad habit to have.)
A self-closing tag looks like this: <tag />
Tags that can exist on their own, i.e. be self-closed include: <img />, <link /> and <meta />, amongst others.
Again, we will understand more about each of these tags as we encounter them when we start building our own website!
Tag attributes
The last important concept to understand about tags is that tags can have attributes as well as contents. Let’s add an attribute to our <span> tag from before:
<span class=”exampleTag”>This is inside my span! I can put anything I like here.</span>
Now our tag has a class attribute. Attributes can have values. In this case our attribute has the value exampleTag.
As with tag names, attributes can technically be anything. For example, we could have said:
<span lemon=”fruityAttribute”>This is inside my span! I can put anything I like here.</span>
This gives our <span> tag an attribute named“lemon” and with a value of “fruityAttribute”.
However, just like with tags there are certain attributes that are important and have meaning. We will learn about these as we learn about each tag.
Putting it all together
Here we can see all of the parts that make up an element.
Make sure you understand which part of the element is a tag, which part is an attribute, which part is an attribute value and which part is the element contents.
PROPERTIES OF RNA.
Let’s consider the general features of RNA. Although both RNA and DNA are nucleic acids, RNA differs from DNA in several important ways:
1. RNA is usually a single-stranded nucleotide chain, not a double helix like DNA. A consequence is that RNA is more flexible and can form a much greater variety of complex three-dimensional molecular shapes than can double-stranded DNA. An RNA strand can bend in such a way that some of its own bases pair with each other. Such intranwkcular base pairing is an important determinate of RNA shape.
2. RNA has ribose sugar in its nucleotides, rather than the deoxyribose found in DNA. As the names suggest, the two sugars differ in the presence or absence of just one oxygen atom. The RNA sugar contains a hydroxyl group (OH) bound to the 2′-carbon atom, whereas the DNA sugar has only a hydrogen atom bound to the 2′-carbon atom. As you will see later in this chapter, the presence of the hydroxyl group at the 2′-carbon atom facilitates the action of RNA in many important cellular processes. Like an individual DNA strand, a strand of RNA is formed of a sugar-phosphate backbone, with a base covalently linked at the 1′ position on each ribose. The sugar-phosphate linkages are made at the 5′ and 3′ positions of the sugar, just as in DNA; so an RNA chain will have a 5′ end and a 3′ end.
3. RNA nucleotides (called ribonucleotide,) contain the bases adenine, guanine, and cytosine, but the pyrimidine base uracil (abbreviated U) is present instead of thymine. Uracil forms hydrogen bonds with adenine just as thymine does. In addition, uracil is capable of base pairing with G. The bases U and G form base pairs only during RNA folding and not during transcription. The two hydrogen bonds that can form between U and G are weaker than the two that form between U and A. The ability of U to pair with both A and G is a major reason why RNA can form extensive and complicated structures, many of which are important in biological processes.
4. RNA—like protein, but unlike DNA—can catalyze biological reactions. The name ribozyme was coined for the RNA molecules that function like protein enzymes.
CHANGE OF CHROMOSOMAL STRUCTURE.
Changes in chromosome structure, called rearrangements, encompass several major classes of events. A chromosome segment can be lost, constituting a dele-tion, or doubled, to form a duplication. The orientation of a segment within the chromosome can be reversed, constituting an inversion. Or a segment can be moved to a different chromosome, constituting a translocation. DNA breakage is a major cause of each of these events. Both DNA strands must break at two differ-ent locations, followed by a rejoining of the broken ends to produce a new chromo-somal arrangement ( Chromosomal rearrangements by breakage can be induced artificially by using ionizing radiation. This kind of radia-don, particularly X rays and gamma rays, is highly energetic and causes numerous double-stranded breaks in DNA.
To understand how chromosomal rearrangements are produced by breakage, several points should be kept in mind:
1. Each chromosome is a single double-stranded DNA molecule.
2. The first event in the production of a chromosomal rearrangement is the generation of two or more double-stranded breaks in the chromosomes of a cell (see Figure 16-19, top row at left).
3. Double-stranded breaks are potentially lethal, unless they are repaired.
4. Repair systems in the cell correct the double-stranded breaks by joining broken ends back together (see Chapter 15 for a detailed discussion of DNA repair).
5. If the two ends of the same break are rejoined, the original DNA order is restored. If the ends of two different breaks are joined together, however, one result is one or another type of chromosomal rearrangement.
6. The only chromosomal rearrangements that survive meiosis are those that produce DNA molecules that have one centromere and two telomeres. If a rearrangement produces a chromosome that lacks a centromere, such an acentric chromosome will not be dragged to either pole at anaphase of mitosis or meiosis and will not be incorporated into either progeny nucleus.
Therefore acentric chromosomes are not inherited. If a rearrangement produces a chromosome with two centromeres (a dicentric), it will often be pulled simultaneously to opposite poles at anaphase, forming an anaphase bridge. Anaphase-bridge chromosomes typically will not be incorporated into either progeny cell. If a chromosome break produces a chromosome lacking a telomere, that chromosome cannot replicate
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