All – 第 2 页 – AP Biology

the blueprint of life [10]: eukaryotic structure of DNA 3

let’s go over the terms again. Make sure you all know what they mean.

•Nucleus: 细胞核; Nucleolus: 核仁; Nucleoid: 类核

• Mitosis: 有丝分裂; Meiosis: 减数分裂

Interphase: 分裂间期; Prophase: 分裂前期; Metaphase: 分裂中期; Anaphase: 分裂后期; Telophase: 分裂末期

• Histone: 组蛋白

• Nucleosome: 核小体

•Chromosome: 染色体; Chromatin: 染色质; eu- 真染色质; hetero- 异染色质

Sister chromotid 姐妹染色单体;
mitotic spindle 纺锤体
spindle microtubule纺锤丝

• Centromere: 中心粒; Telomere: 端粒

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The familiar picture of a chromosome is actually that of the most highly condensed state at mitosis(which we reviewed in the blueprint of life [8]: eukaryotic structure of DNA 1(chromatin structure)).

As the daughter chromosomes are pulled apart by the mitotic spindle at cell division, the fragile centimeters-long chromosomal DNA would certainly be sheared by the forces generated, were it not in this highly compact state.

picture above is from Instant Notes in Molecular Biology

As we can see from the picture, the chromosomal loops fan out from a central scaffold or nuclear matrix region consisting of protein(which we talked about last section). One possibility is that consecutive loops may trace a helical path along the length of the chromosome.
The centromere is the constricted region where the two sister chromatids are joined in the metaphase chromosome. This is the site of assembly of the kinetochore, a protein complex which attaches to the microtubules of the mitotic spindle.

(The microtubules act to separate the chromotids at anaphase)

The DNA of the centromere has been shown in yeast to consist merely of a short AT-rich sequence of 88 bp, flanked by two very short conserved regions, although in mammalian cells, centromeres seem to consist of rather longer sequences, and are flanked by a large quantity of repeated DNA, known as satellite DNA.

Telomeres are specialized DNA sequence that form the ends of the linear DNA molecules of the eukaryotic chromosomes. A telomoere consists of up to hundreds of copies of a short repeated sequence(5′-TTAGGG-3′ in humans), which is synthesized by the enzyme telomerase in a mechanism independent of normal DNA replication.

The telomeric DNA forms a special secondary structure, the function of which is to protect the ends of the chromosome proper from degradation.(Independent synthesis of the telomere acts to counteract the gradual shortening of the chromosome resulting from the inability of normal replication to copy the very end of a linear DNA molecule—we will talk about this when reaching DNA replication)

Interphase chromosome

In interphase(S phase, to be exact), the genes on the chromosomes are being transcribed and DNA replication takes place. During this time, the chromosmes adopt a much more diffuse structure and cannot be visualized individually. It is believed, however, that the chromosomal loops are still present, attached to the nuclear matrix.

The blueprint of life [9]: eukaryotic structure of DNA 2(nucleosome)

Eukaryotic chromosome is packaged in hierarchical levels, mediated by various proteins.

DNA duplex→Nucleosome → Chromatin →Chromosome

We talked about DNA duplex and chromatin; now it’s time to meet the basic unit of chromatin structure–nucleosome.

Definition: nucleosome is the chromatin subunit that consists of DNAand a set of eight histone core proteins(complex of (H2A)₂(H2B)₂(H3)₂(H4)₂, →octamer. More loosely with one molecule of H1 )

Comparison

The proteins protect the DNA from the action of micrococcal nuclease.

Micrococcal Nuclease is an endo–exonuclease that preferentially digests single-stranded nucleic acids The enzyme is also active against double-stranded DNA and RNA and all sequences will be ultimately cleaved.(wikipedia)

Digestion with nuclease leads to the loss of H1, yielding a very resistant structure consisting of 146 bp of DNA associated very tightly with the histone octamer. The structure, known as the nucleosome core, is structurally very similar whatever the source of the chromatin.

Adjacent nucleosome is connected by a varied length (10-100 bp, average 55bp)of DNA, called “linker DNA”

One molecule of linker histone H1 binds to the linker DNA between nucleosome.

H1 also acts to stabilize the point at which the DNA enters and leaves the nulceosome core.

↑Chromatins at different packaging levels

30 nm chromatin fiber: condensed form

10 nm chromatin fiber : less-condensed form, like a thread of beads

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Summary of Nucleosome Structure:

NUCLEOSOME STRUCTURE, image from bricker.tcnj.edu

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Higher Order Structure

The organization of chromatin at the highest level seems rather similar to that of prokaryotic DNA(see THE BLUEPRINT OF LIFE [7]: PROKARYOTIC CHROMOSOME STRUCTURE OF DNA). Even the size of the loops is approximately the same, up to aorund 100 kb of DNA, although there are many more loops in a eukaryotic chromosome.

The loops are constrained by interaction with a protein complex known as the nuclear matrix. The DNA in the loops is in the form of 30 nm fiber, and the loops form an array about 300 nm across.

”Solenoid model “of 30-nm chromatin fiber

6 nucleosomes per turn

“Zigzag model” of 30-nm chromatin fiber

6 nucleosomes per turn, longer linker DNA may be required

HIGHER ORDER STRUCTURE, shown by prof. Dong

Genetics [9] Reproduction 2: Meiosis

During the process of reducing the number of chromosomes by half, the combination of alleles are rearranged to give recombinant gametes. Two distinct processes are involved. These are independent assortment of chromosomes and crossing-over.

Meiosis involves twosuccessive divisions, resulting in producing four cells, each containing half the number of chromosomes of the mother cell. There is NO replication between the two divisions.

We call the two divisions meiosis I and meiosis II, as in Chinese 减数第一次分裂 and 减数第二次分裂. Here we will introduce more of meiosis than meets in high school textbooks.

Meiosis I

meiosis I is divided into prophase, metaphase, anaphase and telophase. Although they have the same names as the four phases in typical mitosis, behavior of chromosomes in these four phases in meiosis is very different from that in mitosis.

In prophase each chromosome pairs with its homolog (copy of the same chromosome inherited from the other parent). The paired chromosomes are called bivalents. Each pair is held together by chiasmata(plural of chiasma). The exchange of genetic material is known as crossing-over.

A chiasma (plural: chiasmata), in genetics, is thought to be the point where two homologous non-sisterchromatids exchange genetic material during chromosomal crossover during meiosis (sister chromatids also form chiasmata between each other, but because their genetic material is identical, it does not cause any change in the resulting daughter cells). (from wikipedia)

Prophase of meiosis is subdivided into five stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.
In leptotene(literal translation:thin threads), the chromatin is seen to condense into very long thin strands, that appear tangled in the nucleus. As prophase proceeds chromosomes become shorter and thicker.
At zygotene(literal translation: yoked/linked threads), homologous chromosomes are seen as partially paired structures. They are still very elongated at this stage and chromosome pairs may overlap or interwine.
By patchytene(thick threads), pairing is complete, though it still isn’t possible identify clearly the individual chromatids in each bivalent.
As the homologous chromosomes begin to separate, transition from patchytene to diplotene(double threads) occurs.

This process begins at the centromeres and the bivalents are seen to be held together by chiasma.

The nuclear membrane and the nucleolus breaks down.

The four chromatids in each bivalent become identifiable and individual chiasma clearly to be identified.

Chromosomes carrying on condensing, the cell moves into diakinesis(moving apart), the final subdivision of prophase I.

The scheme below shows the five stages in prophase in a simple way:

At metaphase I the nuclear envelope breaks down, the bivalents lie across the equator of the cell with their centromeres attached to the microtbules←similar to the spindle in mitosis.

The dynamic action of the spindle causes one member of each homologous cell to move to the opposite poles of the cell.

↑WHY wouldn’t the action of the spindle tear apart the sister chromatids?

BECAUSE at metaphase the sister chromatids are held together by proteins called cohesions, which holds chiasmata in place and so holds the chromosomes together.

At anaphase I the cohesions in the chromosome arms are cut, allowing the homologs to separate.

In telophase I, two neclei form around the segregating chromosomes and a degree of chromosome decondensation is observed.

Meiosis II

The process of meiosis II closely resembles that of the typical mitosis.

In prophase II the chromosomes are seen to recondense within the two nuclei. At metaphase II, the nuclear membrane breaks down and chromosoms rearranged at the equator , the centromere splits and…At anaphase II and telophase II,the initial dipliod cell has divided into four…we already knew these in high school.

What we didn’t know in high school is that each of the four haploid cells has a different genotype. And in many instances the group of four haploid cells may remain together and is known as a tetrad.

SUMMARY OF MEIOSIS

Genetics [8]：reproduction 1

Reproduction takes place by one of two methods, asexual or sexual.

Asexual reproduction involves the production of a new individual(s) from cells or tissues of a pre-existing organism. This process is common in plants and in many microorganisms. It can involve simple binary fission(splitting into two) in unicelluar microbes or the production of specialized asexual spores(孢子).

NATURAL VEGETATIVE PROPAGATION. New plants grow from parts of the parent. (image from: leavingbio.net)

These processes may be exploited for commercial purposes as in the vegetative propagation of plants. More recently it has been possible to regenerate whole organisms from a single cell. This was first shown in carrots and frogs, but it has now been reported in mammals, and by implication, is possible in all mammals including humans.

Asexually reproduced organisms are genetically identical to the individual from which they were derived. A group of such genetically identical organisms is known as a clone.

Here I found an article at Buzzle , thinking it may help us learn more about animal cloning: http://www.buzzle.com/articles/animal-cloning/. Below is an excerpt of that article.

1, The Process of Animal Cloning
Initial attempts at artificially induced Animal Cloning were done using developing embryonic cells.

The DNA nucleus was extracted from an embryonic cell and implanted into an unfertilized egg, from which the existing nucleus had already been removed. The process of fertilization was simulated by giving an electric shock or by some chemical treatment method. The cells that developed from this artificially induced union were then implanted into host mothers.

The cloned animal that resulted had a genetic make-up identical to the genetic make-up of the original cell←the embryonic cell that contributes the DNA nucleus.

Since Dolly, of course, it is now possible to create clones from non-embryonic cells.

Now animal cloning can be done both for reproductive and non-reproductive or therapeutic purposes. In the second case, cloning is done to produce stem cells or other such cells that can be used for therapeutic purposes, for example, for healing or recreating damaged organs; the intention is not to duplicate the whole organism.

2. Ethics of Animal Cloning
While most scientists consider the process of animal cloning as a major break through and see many beneficial possibilities in it, many people are uncomfortable with the idea, considering it to be 'against nature' and ethically damning, particularly in the instance of cloning human beings.

The truth is that most of the general public are not aware of the exact details involved in cloning and as a result there are a lot of misconceptions about the entire matter.

In recent times, there have been a spurt of new laws banning or regulating cloning around the world. In some countries, animal cloning is allowed, but not human cloning. Some advocacy groups are seeking to ban therapeutic cloning, even if this could potentially save people from many debilitating illnesses.

3. Points against Animal Cloning
In a large percentage of cases, the cloning process fails in the course of pregnancy or some sort of birth defects occur, for example, as in a recent case, a calf born with two faces. Sometimes the defects manifest themselves later and kill the clone.

4. Points for Animal Cloning
On the favorable side with successful animal cloning - particularly cloning from an adult animal - you know exactly how your clone is going to turn out. This becomes especially useful when the whole intention behind cloning is to save a certain endangered species from becoming totally extinct.←for more info, please see http://www.buzzle.com/articles/cloning-extinct-animals.html

That this is possible was shown by cloning an Indian Gaur in 2001. The cloned Gaur, Noah, died of complications not related to the cloning procedure.

Speaking of saving extinct animals, I couldn’t help but think of Jurassic Park. Although in the movies looks like cloning extinct animals brings threat to humans, or maybe it truly would in reality, the idea of cloning them remains magically attractive to me. Especially, would the biodiversity allow, cloning those who became extinct because of our hunting.

What’s your idea?

Before I lead you off the topic, let’s go back and talk about Sexual reproduction, which differs from asexual reproduction, in that it involves fusion of cells (gametes←we knew in high school as配子), one derived form each parent, to form a zygote. The genetic processes involved in the production of gametes allow for some genetic changes in offspring.

The production of gametes is referred to as gametogenesis. This may be a complex process, involving sexual differentiation and the production of highly differentiated male and female gametes, or in lower eukaryotes identical cells may fuse–isogamy.

isogamy , in biology, a condition in which the sexual cells, or gametes, are of the same form and size and are usually indistinguishable from each other. Many algae and some fungi have isogamous gametes. In most sexual reproduction, as in mammals for example, the ovum is quite larger and of different appearance than the sperm cell. This condition is called anisogamy. (infoplease.com)

Whatever the biology of the process, one fact is obvious: gametogenesis must involve a halving of the chromosome number, otherwise each succeeding generation would have double the chromosome number of its parents.←That’s why, sexual reproduction is limited to species that are diploid or have a period of their life cycle in the diploid state.

Halving of chromosome numbers is achieved in a specialized form of cell division, meiosis(←we learned in high school as减数分裂), which is only observed in gametogenesis.

We will talk about meiosis more detailedly next section.

the blueprint of life [8]: eukaryotic structure of DNA 1(chromatin)

• Mitosis: 有丝分裂; Meiosis: 减数分裂

Interphase: 分裂间期; Prophase: 分裂前期; Metaphase: 分裂中期; Anaphase: 分裂后期; Telophase: 分裂末期

• Histone: 组蛋白 hhistidine 组氨酸

• Nucleosome: 核小体

•Chromosome: 染色体; Chromatin: 染色质; eu- 真染色质; hetero- 异染色质

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The total length of DNA in a eukaryotic cell depends on the species, but it can be thousands of times as much as in a prokaryotic genome.

Eukaryotic chromosome is made up of a number of discrete bodies called chromosomes. The DNA in each chromosome is believed to be a single linear molecule, which can be up to several centimeters long.

All these each contain a long linear DNA molecules, which must be packaged into the nucleus, a space of approximately the same volume as a bacterial cell

SO, much longer DNA chains packaged into a space of the same volume as a bacterial cell? → for example, 2 cm of DNA length versus ~10 µm of cell size for fruit fly; most condensed form of human chromosome is about ~2 µm long = 10,000× packing ratio

the obvious result is in their most highly condensed forms, the chromosomes have an enormously high DNA concentration: perhaps 200 mg/ml.!

The feat of packing is accomplished by the formation of a highly organized complex of DNA and protein, known as the chromatin, a nucleoprotein complex. (←our hero today, has finally showed up.)

Chromosomes greatly alter their level of compectness as cells progress through the cell cycle, vary between highly condensed chromosomes at metaphase(just before the cell division), and very much more diffuse structures in interphase.(This implies the existence of different levels of organization of chromatin)

More than 50% of the mass of chromatin is protein. Most of the protein in eukaryotic chromatin consists of histones, of which there are five families: H2A, H2B, H3 and H4, known as the core histones, and H1.

The core histones are small proteins, with masses between 10 and 20 kDa, and H1 histones are a little larger at around 23 kDa.

The unified atomic mass unit (symbol: u) or dalton (symbol: Da) is the standard unit that is used for indicating mass on an atomic or molecular scale (atomic mass). One dalton is approximately the mass of a nucleon and is equivalent to 1 g/mol.^[1] It is defined as one twelfth of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state,^[2] and has a value of 1.660538921(73)×10⁻²⁷ kg.^[3]

All histones proteins a large positive charge; between 20 and 30% of their sequences consist of the basic amino acids, lysine and arginine. This means that histones will bind very strongly to the negatively charged DNA in forming chromation.

amino acids in English?

Members of the same histone class(family) are very highly conserved between relatively unrelated species, for example between plants and animals, which testifies to their crucial role in the chromation.

Within a given species, there are normally a number of closely similar variants of a particular class, which may be expressed in different tissues, and at different stages in development.

There is not much similarity in sequence between the different histone classes, but structural studies have shown that the classes so share a similar tertiary structure, suggesting that all hisotnes are ultimately evolutionarily related.

H1 histones are somewhat distinct from the other histone classes in a number of ways; in addition to their larger size, there is more variation in H1 sequences both between and within species than in other classes. Histone H1 is more easily extracted from bulk chromatin, and seems to be present in roughly half the quantity of the other classes, of which there are very similar amounts.

Next section we will cover the distinct role of histone Hi in chromatin structure.

Stray Animals Not Safe In Winter

“Winter gives us the opportunity to stay inside and look outside. Snuggle up in thesofa, put a blanket over you,have a cup of hot cocoa, and enjoy the observations on this precious season.

“Winter is the time for comfort, for good food and warmth, for the touch of a friendly hand and for a talk beside the fire: It is the time for home. (the warmth in winter)

BUT what about those who don’t have a home? For them, winter is definitely not a time for comfort, but a time for freezing and hunger.

Homeless people could die of coldness, so could stray animals.

We see the homeless sleep in the subway stations or the underground, where it’s relatively warmer and free of the chilly wind. I don’t see many stray animals stay in the underground for night; perhaps they are afraid of humans. Most often they hide in trash cans, car engine compartment or inside the wheels or the tail pipes where it’s warmer, and to them, safer.

But the truth is it’s not safe at all sleeping inside any of the places above. Especially the latter three. In China every year countless cats are critically injured, even killed inside their “warm spots”. In the morning most people go downstairs, directly get in their cars, start the engine, and head for office. Chances are, a poor kitty was soundly sleeping under the hood of the car and didn’t get a chance to wake up and escape before the car moved and hurt her.

SO next time before starting your car, please take time to check inside the engine compartment, the wheels and the trail tube, see if any small animal is inside. Not just cats, rats, little yellow weasels, which are common here in Hefei are all possible.If you’ve checked and don’t see any of them but still aren’t sure, it’s also a very good idea to sound your horn for a few times and wait for seconds, as my mom always does. Hearing the noise, the small animals are alarmed and would come out and escape.

IF you forget to “release”the little thing and hurt it, please contact the veterinary hospital and get the animal medical treatment as soon as possible. Don’t try moving the animal if he/she is stuck in the wheel or the engine compartment. Don’t move your car, either.

Most of the homeless animals are homeless because some of us bought them and then abandoned them. Most of the wild animals would “break into” our city because we took their homes. Now they are just trying to survive in a strange place without sofa, fireplace or warm woods.Is there any reason why we shouldn’t at least give them a safer spot for overnight?

Mendel’s Genetics [7]: handling problems

In college, while learning genetics, you may be faced with data obtained from F1 and F2 generations of the crosses. And you are required to be able to recognize ratios in order to decide how many genes are involved, and whether or not epistasis (which we talked about last section) is taking place.

Example(1)

A cross between two pure-breeding white-fruited tomato plants produced and F1 generation which all plants had purple fruit. In the subsequent F2 generation 160 plants were obtained; of these 94 had purple fuit, and the rest had white fruit.

As we know nothing about the genes controlling fruit color in tomato, we must first ask ourselves a question: “DOES THE DATA FIT ANY OF THE KNOWN MENDELIAN RATIOS?”

Since only two phenotypes are involved, it can’t be 9:3:3:1, or 9:3:4 or any mendelian ratios with more than 2 numbers included.

On examination of ratios with 2 phenotypes, 9:7 looks like a possible candidate, but 3:1 may also fit.

In this case, to decide which is the best to fit the data, we introduce a new approach to this problem: the Chi-square test.

Chi-square( test)=
sum[(obseved expected)ˆ2/expected]

(Xˆ2 is always calculated from original data, never from percentages, frequencies or proportions.)

If Xˆ2 is large, the data doesn’t fit. A perfect fit gives Xˆ2 a zero. BUT HOW LARGE IS LARGE?

In addition to the result of Xˆ2, we need another piece of info to determine “how large is large”. We need to know the degrees of freedom.

Degrees of freedom are one less than the number of classes. They tell us something about the number of independent numbers we have, which relates to the usefulness of our data.

In this example, we have two phenotypic classes, purple, and white. It means when we have counted the purple ones, the number of the white ones is fixed so we have only one degree of freedom.

If we had three classes, we would have two degrees of freedom: when the two classes have been counted, the number of the third is fixed.

As the degrees of freedom gets bigger, Xˆ2 gets bigger. So the answer of HOW LARGE IS LARGE depends on different degrees of freedom.

In this example, we determine which ratio is the best fit by comparing the value of Xˆ2

Observed result: 94 purple 66 white

Result predicted by 9:7 ratio 160×9/16 =90 160×7/16 =70

Xˆ2=[(94-90)ˆ2/90]+[(66-70)ˆ2/70]=0.41,with one degree of freedom

Result predicted by 3:1 ratio 160×3/4=120 160×1/4=40

Xˆ2=[(94-120)ˆ2/120]+[(66-40)ˆ2/40]=22.5,with one degree of freedom

Using Xˆ2 probability tables can help quickly get to the value of Xˆ2.

the x2 ditribution table — X2 PROBABILITY TABLE, from “Instant Notes in Genetics”

How to use it?

Follow the line for the one degree of freedom(top line) to find the nearest values of xˆ2 above and below our value.

We can see that the value of 0.41 is between the probability of 0.975 and 0.050 with an affinity to 0.975, which means that if we repeated the experiment 1000 times, we have a probability close to 97.5% that the observed ratio would fit 9:7. The value of 22.5 is way beyond way exceeded with the probability of 0.001, which suggests that 3:1 ratio doesn’t fit the data.

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Data analysis after class:

A cross between pure-breeding white-fruited and purple-fruited tomato plants produced and F1 generation which all plants had purple fruit. In the subsequent F2 generation 160 plants were obtained; of these 99 had purple fuit, 25 had red fruit, and 36 had white fruit.

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卡方检验是以χ2分布为基础的一种常用假设检验方法，它的无效假设H0是：观察频数与期望频数没有差别。

该检验的基本思想是：首先假设H0成立，基于此前提计算出χ2值，它表示观察值与理论值之间的偏离程度。根据χ2分布及自由度可以确定在H0假设成立的情况下获得当前统计量及更极端情况的概率P。如果P值很小，说明观察值与理论值偏离程度太大，应当拒绝无效假设，表示比较资料之间有显著差异；否则就不能拒绝无效假设，尚不能认为样本所代表的实际情况和理论假设有差别。

the blueprint of life [7]: prokaryotic chromosome structure of DNA

First, let’s make sure the anatomy of prokaryotes are familiar to us:

Prokaryotes are the simplest living cells, typically 1~10μm in diameter, and are found in all environmental niches from the guts of the animals to acidic hot springs.

They are bounded by a cell (plasma)membrane comprising a lipid bilayer, in which are embeded proteins that allow the exit and entry of small molecules.
Most prokaryotes also have a rigid cell wall outside the plasma membrane, which prevents the cell from swelling or shrinking in environments where osmolarity differs significantly from that inside the cell.
Sometimes the cell wall is surrounded by an (often) polysaccharide envelope called capsule.
The cell interior (cytoplasm or cytosol) usually contains a single, circular chromosome compacted into a nucleoid and attached to the membrane.
There are no distinct subcelluar organelles in prokaryotes as in eukaryotes(except for the ribosomes核糖体).
The surface of a prokaryote may carry pili, which allow the prokaryote to attach to other cells and surfaces. Some prokaryotes also carry flagella, whose rotating motion allows the cell to swim.

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To talk about prokaryotic chromosome structure, we use E. coli (大肠杆菌)as the example.

E. coli chromosome contains a single supercoiled circular DNA molecule of length 4.6 million bp.
E. coli chromosome is highly folded: 1500 µm of DNA length versus ~1 µm of cell size, forming a structure called the nucleoid.
The nucleoid has a very high DNA concentration, perhaps 30~50 mg/ml, as well as containing all the proteins associated with DNA, such as polyerases, repressors(a protein that is determined by a regulatory gene, binds to a genetic operator, and inhibits the initiation of transcription of messenger RNA), etc.

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DNA domains (loops)

Remember this famous electron micrograph of an E. coli cell we showed before? The cell was carefully lysed, all the proteins removed and then, it was spread on an EM grid to reveal all of its DNA.

Several of such experiments revealed one level of organization of the nucleoid.

The DNA consists of 50~100 domains or loops, about 50~100 kb in size (kb: kilobase, a unit of measure of the length of a nucleic-acid chain that equals one thousand base pairs).
The ends of the loops are constrained by binding to a structure which probably consists of proteins attached to part of the cell membrane.
Not known whether the loops are static or dynamic, but one model suggests that the DNA may spool(wind) through sites of polyerase or other enzymic action at the base of the loops.

E coli DNA instant notes

image above is from”Instant Notes in Molecular Biology”

Supercoiling of the genome

The E. coli chromosome as a whole is negatively supercoiled, although there is some evidence that indicates individual domains may be supercoiled independently. Electron micrographs indicate that some domains may not be supercoiled, perhaps because the DNA has become broken in one strand, where other domains clearly do contain supercoils.

The domains may be topologically independent. There is, however, no real biochemical evidence for major differences in the level of supercoiling in different regions of the chromosome in vivo

DNA-binding proteins

The looped DNA domains of the chromosome are constrained further by inter-action with a number of DNA-binding proteins.

The most abundant of these are protein HU, a small basic (+charged) dimeric protein, which binds DNA non-specifically by the wrapping of the DNA aorund the protein.

And H-NS (formerly known as the protein H1), a monomeric neutral protein, which also binds DNA non-specifically in terms of sequence, but seems to have a preference for regions of DNA which are intrinsically bent.

These proteins are sometimes known as histone-like proteins, and have the effect of compacting the DNA, which is essential for the packaging of the DNA into the nucleoid, and of stabilizing and constraining the supercoiling of the chromosome.

Illustrations of wild animals [insect 8 Diptera]

双翅目DIPTERA

Diptera

True Flies / Mosquitoes / Gnats / Midges

The name Diptera, derived from the Greek words “di”meaning two and “ptera” meaning wings, refers to the fact that true flies have only a single pair of wings.

Classification & Distribution

Holometabola

complete development (egg, larva, pupa, adult)

The Diptera have traditionally been divided into three suborders:

Nematocera (flies with multisegmented antennae)
Brachycera (flies with stylate antennae)
Cyclorrhapha (flies with aristate antennae)

In some newer classifications, Brachycera includes the Cyclorrhapha.

Distribution: Abundant worldwide. Larvae are found in all fresh water, semi-aquatic, and moist terrestrial environments.

	North America	Worldwide
Number of Families	108	130
Number of Species	16,914	~98,500

Life History & Ecology

The order Diptera includes all true flies. These insects are distinctive because their hind wings are reduced to small, club-shaped structures called halteres – only the membranous front wings serve as aerodynamic surfaces. The halteres vibrate during flight and work much like a gyroscope to help the insect maintain balance.

All Dipteran larvae are legless. They live in aquatic (fresh water), semi-aquatic, or moist terrestrial environments. They are commonly found in the soil, in plant or animal tissues, and in carrion or dung — almost always where there is little danger of desiccation. Some species are herbivores, but most feed on dead organic matter or parasitize other animals, especially vertebrates, molluscs, and other arthropods. In the more primitive families (suborder Nematocera), fly larvae have well-developed head capsules with mandibulate mouthparts. These structures are reduced or absent in the more advanced suborders (Brachycera and Cyclorrhapha) where the larvae, known as maggots, have worm-like bodies and only a pair of mouth hooks for feeding.

Adult flies live in a wide range of habitats and display enormous variation in appearance and life style. Although most species have haustellate mouthparts and collect food in liquid form, their mouthparts are so diverse that some entomologists suspect the feeding adaptations may have arisen from more than a single evolutionary origin. In many families, the proboscis (rostrum) is adapted for sponging and/or lapping. These flies survive on honeydew, nectar, or the exudates of various plants and animals (dead or alive). In other families, the proboscis is adapted for cutting or piercing the tissues of a host. Some of these flies are predators of other arthropods (e.g., robber flies), but most of them are external parasites (e.g., mosquitoes and deer flies) that feed on the blood of their vertebrate hosts, including humans and most wild and domestic animals.

Physical Features

Immatures:

Culiciform
- Head capsule present with chewing mouthparts
- Legs absent
Vermiform (maggots)
- Without legs or a distinct head capsule
- Mouthparts reduced; only present as mouth hooks

Adults:

Antennae filiform, stylate, or aristate
Mouthparts suctorial (haustellate)
Mesothorax larger than pro- or metathorax
One pair of wings (front); hind wings reduced (halteres)
Tarsi 5-segmented

Major Families

Biting flies: In most cases, only the adult females take blood meals.♦
Herbivores: larvae feed on plant tissues.
Scavengers: larvae feed in dung, carrion, garbage, or other organic matter.
Predators: adults and/or larvae attack other insects as prey.:
Parasites: larvae are parasites or parasitoids of other animals.
Bug Bytes ♣
- Although they have only two wings, flies are among the best aerialists in the insect world – they can hover, fly backwards, turn in place, and even fly upside down to land on a ceiling.
- Flies have the highest wing-beat frequency of any animal. In some tiny midges, it may be as high as 1000 beats per second. Male mosquitoes are attracted by the wing-beat frequency of a virgin female.
- Larvae of some shore flies (family Ephydridae) live in unusual habitats that would kill other insects. For example, Ephydra brucei lives in hot springs and geysers where the water temperature exceeds 112 degrees Fahrenheit; Helaeomyia petrolei develop in pools of crude oil; and the brine fly, Ephydra cinera, can survive very high concentrations of salt.
- The arista in the antenna of higher flies is an air speed indicator. It allows the insect to sense how fast it is moving.
- As they mature, black fly pupae become inflated with air. Upon emergence, the pupal skin pops open and the adult fly floats to the water surface inside a bubble of air. It never even gets its feet wet!
- The little scuttle fly, Megaselia scataris (Phoridae), is truly an omnivore. It has been reared from decaying vegetation, shoe polish, paint emulsions, human cadavers pickled in formalin, and even lung tissue from living people.