eyes open – AP Biology

Sample Answers of [Assignment: DNA & RNA]

Provided by Professor Dong, College of Life Sciences, Anhui University.

1) Compare the differences between DNA and RNA, structurally and functionally. Can you explain why DNA is more favorable as a genetic material?

Answer:

The differences between DNA and RNA in the structure

(1) DNA contains deoxyribose in contrast to the ribose in RNA.

(2) DNA has the thymine base, while RNA has the uridine instead of thymine. The thymine base-pairs with adenine only. The uridine can also pair with guanine in some cases, beside the normal U-A pairing.

(3) DNA is commonly present as a double-stranded helix, while RNA is single stranded with a few exceptions.

(4) In the secondary structure, DNA follows the double-helix model. Cruciforms and hairpins are rare; for RNA, usually single-stranded, hairpins and pseudo-knots are common, leading to more complicated structures like the “cloverleaf” of tRNA.

The differences between DNA and RNA physiologically

(1) RNA is more vulnerable for alkaline hydrolysis than DNA and more chemically active due to the extra –OH group at 2C

(2) The UV absorbance feature is different: A260/A280=1.8 (DNA) 2.0(RNA)

The differences between DNA and RNA functionally

(1) DNA acts as the genetic material, containing all inheritable information in most organisms

(2) RNA has more diversified functions.

(a) Normally, RNA is the intermediate for protein translation. mRNA works as the template; tRNA is the adaptor between mRNA and amino acids; rRNA is the essential parts of protein-synthesizing ribosomes

(b) RNA can also be genetic material in some viruses

(d) Some small RNAs are important functional regulators, e.g. siRNA in RNA interfering.

Why DNA is more favorable as a genetic material

(1) DNA is more resistant to hydrolysis than RNA. Furthermore, the double-helical structure confers much higher level of stability on DNA compared to RNA, which is critical for storage and vertical transfer of genetic information.

(2) The semi-conservative replication of DNA and the high fidelity of DNA polymerases make it possible that the DNA is precisely copied from generation to next generation. While RNA cannot.

(3) DNA has a sophisticate repair system which corrects damages and thereafter minimizes the loss or change of genetic information. While RNA has not.

2) Give your explanation about the semidiscontinuous mode of DNA replication? Can you describe the steps of replication for lagging strand?

(4) Answer:

(5) Replication of the lagging strand is discontinuous compared to the continuous replication of the leading strand. Therefore, multiple primers are synthesized for the lagging strand while a single primer is needed for the leading strand.

(6) Reason: The DNA polymerase can work only from 5’à3’ orientation. To ensure the replication fork moves in one direction, different patterns of replication are applied to the two complementary strands.

(7) Steps:

(8) Initiation is the same for the both strands. When the replisome is assembled, the lagging strand is looped to make the both polymerase core-enzymes move in the same direction

(9) A short Okazaki fragment is synthesized along the lagging strand. The sliding clamp releases and reloads onto the newly primed location to generate the next Okazaki fragment.(“trombone” model)

(10) RNA primers are removed and replaced with DNA by “nick translation” mechanism. The discontinuous patches are stitched together to form a continuous strand.

3) What is an insertion element? Describe the differences between the conservative and replicative transposons.

(11) Answer:

(12) IS: a simple transposon consisting only two inverted repeats surrounding the coding region of a transposase.

(13) Differences btw. the two transposition mechanisms:

(14) “cut-and-paste” vs. “copy-and-paste” patterns: conservative transposition leaves the original sequence broken while the replicative transposition make a copy either in the original and target sequence.

(15) For conservative transposition, transposase is sufficient along with the common polymerase and ligase. For replicative transposition, an extra resolvase is required to initiate a recombination process and catalyze the resolvation of the “cointegrate”.

4) Can you explain how to assure the fidelity of the DNA replication in bacteria?

(16) The basic fidelity of DNA replication depends on the correct matching of base pairs in the catalytic center of DNA replicase, assuring the error rate as low as 10-5. The extra “proofreading” exonuclease activity of DNA polymerase improves the overall accuracy by recognizing and removing the new-coming mismatched bases (down to 10-7 error rate). The following repair system (MMR) can restore the remaining mismatches efficiently. Hence the overall error rate is controlled as low as 10-9 to 10-10 error/bp.

(17) Mismatch Repair (MMR) System: Base pairs with incorrect hydrogen bonding occur spontaneously during replication process, causing distortions in the double helix of parental and daughter strands. The MutS protein in MMR system detects the distorted mismatched site and MutH identifies the erroneous daughter strand by recognizing the strand-specific methylation of adenine in GATC. MutL links MutS and H, forming a loop and promoting MutH to nick the non-methylated strand. The following Exonuclease cuts the strand from 3’ to 5’ orientation, removing the wrong base. Finally the Pol III re-synthesizes the region and the ligase fills the gap.

5) Thymine dimer is among the major DNA damages due to UV exposure. Name and describe several possible mechanisms that E. coli uses to repair those dimmers.

(18) (1) Direct removal by the photoreactivation catalyzed by the photolyase;

(19) (2) Nucleotide excision repair by the UvrABC system

(20) (3) Recombinational repair

(21) (4) SOS repair

the blueprint of life [11]: DNA replication 1

DNA replication

Definition: the process of copying a parental DNA molecule to form two daughter DNA molecules.

Introduction to DNA replication

DNA replication is essential for cell proliferation, i.e. mitosis, meiosis.
DNA replication is a complex endeavor involving a series of enzyme activities.→see “DNA polymerases”
DNA replication is performed in a semiconservative and semidiscontinuous mode.→see “DNA replication is semi-conservative”
DNA replication has 3 stages: initiation, elongation and termination.→next section
DNA replication is tightly regulated, involving various protein-protein, protein-DNA interactions.
DNA replication of prokaryote and eukaryote shares similar features, but is distinctive in details.

Chemical Reaction of DNA replication

Essentials

1. Substrate: deoxynucleoside triphosphates(dNTPs)
2. Template: a primer-template junction

DNA is synthesized by extending the 3’ end of the primer (free 3’-OH is required)

– RNA primer or priming from a nick in DNA

3. Enzymes: DNA polymerases etc

4. Energy supply: Hydrolysis of pyrophosphate (PPi) is the driving force for DNA synthesis

5. Ions involved: Mg++ or Zn++

DNA polymerases

DNA polymerase I
- Pol I was the first enzyme discovered with polymerase activity, and it is also the best characterized one.
- Although abundant in cells (400/cell), Pol I is NOT the primary enzyme involved with bacterial DNA replication.
- Main functions of Pol I: (1) Fill any gaps in the new DNA that result from the removal of the RNA primer by its 5’ -3’ polymerase activity; (2) Remove a new mispaired base by proofreading (校读)3’-5’ exonuclease (外切酶) activity. (3)Remove the RNA primer by its 5’-3’ exonuclease activity

The 3′–>5′ exonuclease activity intrinsic to several DNA polymerases plays a primary role in genetic stability; it acts as a first line of defense in correcting DNA polymerase errors. A mismatched basepair at the primer terminus is the preferred substrate for the exonuclease activity over a correct basepair. (source)

DNA polymerase III
- The primary polymerase in DNA replication, although lower in abundance (15/cell)than pol →referred to as “replicase”
- functions: (1) 5’→3’ polymerase activity; (2) 3’→5’ exonuclease activity – proofreading
- Catalytic efficiency: much higher than pol I→High processivity and polymerization rate
- A multi-unit complex: “holoenzyme” (全酶)

DNA replication is semi-conservative

Bet you all have learned it in high school, and the famous experiment by Meselson and Stahl. We still need to go over the points again as they are essentially important for what we will learn next.

The key to the mechanism of DNA replication is the fact that each strand of the DNA double helix carries the same information-their base sequences are complementary (we talked about this in THE BLUEPRINT OF LIFE [2]: PRIMARY AND SECONDARY STRUCTURE OF DNA).

During replication, the two parental strands separate and each acts as a template (that’s right, the template for DNA replication is DNA itself!)to direct the enzyme-catalyzed synthesis of a new complementary daughter strand with the normal base-pairing rules (A-T, C-G)

This semi-conservative mechanism was demonstrated experimentally in 1958 by Meselson and Stahl.

Hypotheses:

In the experiment:

E. coli cells were grown for several generations in presence of the stable heavy isotope ¹⁵N so that their DNA became fully density labeled (both strands are ¹⁵N labeled: ¹⁵N/¹⁵N)

The cells were then transferred to medium containing only normal ¹⁴N and, after each cell division, DNA was prepared from a sample of the cells and analyzed on a CsCI gradient using the technique of equilibrium (isopycnic) density gradient centrifugation, which separates molecules according to differences in buoyant density.

After the first cell division, when the DNA had replicated once, it was all of hybrid density, in a position on the gradient half way between fully labled (¹⁵N /¹⁵N )and fully light (¹⁴N/¹⁴N). After the second generation in ¹⁴N, half of the DNA was hybrid density and half fully light.

Thus, two of the hypotheses were denied, left us with the semi-conservative mechanism.

After each subsequent generation, the proportion of ¹⁴N/¹⁴N increased, while some DNA of hybrid density persisted. Thus the semi-conservative mode of DNA replication is confirmed: each daughter molecule contains one parental strand and one newly-synthesized strand.

DNA replication & PCR, General Biology, Open Courses at UC-Berkeley

General Biology, Great open courses given by Gary L. Firestone, Michael Meighan Jasper D. Rine and Jennifer A. Doudna, professors at UC-Berkeley.

There is more to DNA replication than we talked about yesterday so I tried to upload this video to help you learn more.

“tried? ”

“Well, turned out ‘ DNA replication and PCR open course at Berkeley.mp4 exceeds the maximum upload size (8 MB)for this site.'”

So I would just provide the link where you can watch it online.

After watching it, maybe you will find more than just DNA replication: you may as well find how it feels going to college.

I look forward to it every time I have finished an open course online. Hope you do, too.

>>click here to watch the video

Genetics [10] reproduction 3: Meiosis 2

Last section we gave a general introduction of meiosis in terms of chromosome behaviors.

In this section, we will deal with two other respects of meiosis. First, a specific description of meiosis in animals; second, we will mention a situation when meiosis goes wrong.

Meiosis in animals

is found only in ovaries (卵巢)and testes(曲细精管), and even in these tissues is restricted to cells that are destined to form gametes(the germline).

Despite the fact that the mechanisms of gametogenesis differ somewhat between organisms,the steps involved in gametogenesis in mammals are relatively similar.

In male gametogenesis (spermgenesis).

precursors of germ cells go through many rounds of mitotic divisions in order to maintain a pool of spermatogonia(plural of spermatogonium 精原细胞).↓

Spermatogonia subsequently differentiate into primary spermatocytes(based on what you learned in high school. guess what this word mean? : D ). It is in these cells that meiosis takes place.↓

After meiosis I, these cells are referred to as secondary spermatocytes. These are haploid. The products of the second meiotic division are spermatids. ↓

Spermatids differentiate into motile spermatozoa with rounded or elongate head and a long posterior flagellum.(The final activation of spermatozoa takes place after copulation/sexual intercourse)

Without cytoplasm and many subcelluar organelles, the sperm is light and fast (almost all its weight concentrated in its head, where stored the key of a potential life, nuclei. ).

In female mammals, the pattern of oogenesis is superficially similar.

Here oogonia(plural of oogonium, 卵原细胞) go through mitotic divisions before differentiating into primary oocytes. These then undergo meiosis.↓

Both daughter cells of the primary oocytes are haploid but differ greatly in size. The larger daughter cell is the secondary oozyte, the smaller the first polar body. The two cells remain attached. Both undergo a second meiotic division.↓

The secondary oocyte undergoes an unequal division producing a large ovum and a small secondary polar body. The first polar body divides into two secondary polar bodies.

Only the ovum, which contains almost all the cytoplasm, will transmit genes into the next generation.

The first meiotic division is only completed at ovulation(the discharge of a mature ovum from the ovary), and the second occurs after fertilization.

Little known: In human females, primary oocytes can be held in meiotic arrest for up to 45 years. This may be important in the increased frequency of aneuploid births observed in older mothers.

Production of aneuploid gametes

The major cause of anueploidy(the situation of having or being a chromosome number that is not an exact multiple of the usually haploid number) is aberrant chromosome behavior at meiosis. In other words, it’s the failure of chromosmes to segregate properly(known as nondisjunction)

At anaphase I, if two homologous chromosomes move to the same pole, first division nondisjunction occurs. In this case, of the 4 cells arsing from meiosis, two will be disomic(contain two copies of the chromosome) and two nullisomic (contain no copy of the chromosome.

At anaphase II, if the chromatids in a cell remain together, division nondisjunction occurs. The resulting tetrad will contain two normal cells, one nullisomic, and one disomic.

Aneuploidy also arises due to nondisjunction at an early mitosis in the embryo, resulting in two populations of cytogenetically different cells in the individual, which is known as a mosaic.

MOSAIC (GENETICS)"Heterochromia iridum and iridus" image from simple.wikipedia.org — MOSAIC (GENETICS)”Heterochromia iridum and iridus” image from simple.wikipedia.org

Mosaic is common in Turners syndrome.

↑Turner syndrome or Ullrich–Turner syndrome is a chromosomal abnormality in which all or part of one of the sex chromosomes is absent or has other abnormalities . In some cases, the chromosome is missing in some cells but not others, a condition referred to as mosaicism or “Turner mosaicism”.

Occurring in 1 in 2000– 1 in 5000 phenotypic females, the syndrome manifests itself in a number of ways. There are characteristic physical abnormalities which affect many but not all people with Turner syndrome, such as short stature,swelling, broad chest, low hairline, low-set ears, and webbed necks. Girls with Turner syndrome typically experience gonadal dysfunction (non-working ovaries), which results in amenorrhea (absence of menstrual cycle) and sterility. Concurrent health concerns may also be present, including congenital heart disease, hypothyroidism (reduced hormone secretion by the thyroid), diabetes, vision problems, hearing concerns, and many autoimmune diseases.Finally, a specific pattern of cognitive deficits is often observed, with particular difficulties in visuospatial, mathematical, and memory areas.

Turner syndrome is named after Henry Turner, the endocrinologist who first described it in 1938.（wikipedia）

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

====================================================

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

——————————————————————————-

Summary of Nucleosome Structure:

NUCLEOSOME STRUCTURE, image from bricker.tcnj.edu

——————————————————————————-

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.

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?