summary 7 th grade

8/7/2019 Summary 7 Th Grade

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English Themes

SIMPLE PRESENT

USE

We use for:

1. Facts

2. Routines

E.g.: I am a teacher FACT

They go to school every day ROUTINE

STRUCTURE VERB BE (am/is/are)

In this structure all people (I/he/she/it/you/we/they) change the same way.

We use NOT to make negatives.

And change places between the subject and the verb to be to make questions.

(+) They are students.

(-) They are not students.

(?) Are they students

(+) She is a doctor.

(-) She is not a doctor.

(?) Is she a doctor?

STRUCTURE OTHER VERBS (I/you/they/we)

In this structure we use the auxiliaries: dont (-) and do (?)(+) We work every day.

(-) Wedont work every day.

(?) Dowe work every day?

(+) You study at home.

(-) Youdont study at home.

(?) Doyou study at home?

STRUCTURE OTHER VERBS 3rd PERSON (he/she/it)

We add an S to the verb to make it affirmative (+).

In this structure we use the auxiliaries: doesnt ( -) and does (?)

(+) She workS every day.

(-) Shedoesnt work every day.

(?) Does she work every day?

(+) He runS to school.

(-) Hedoesnt run to school.

(?) Doeshe run to school?

SIMPLE PAST

USE


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We use for completed actions in the past.

STRUCTURE

The affirmative(+) structure is [VERB+ed] or irregular verbs. In the structure we use the auxiliaries: didnt (-) and did (?)

(+)You called Debbie.

(-)You didnt call Debbie.

(?)Did you call Debbie?

(+)He watched TV.

(-)He didnt watch TV.

(?)Did he watch TV?

PRESENT CONTINUOUS

USE

We use to describe an action happening NOW.

STRUCTURE

am/is/are + VERB+ ING

We use NOT to make negatives.

And change places between the subject and the verb to be to make questions.

(+) You are watching TV.

(-) You are not watching TV.

(?) Are you watching TV?

COMPARATIVES AND SUPERLATIVES

Introduction

Comparativesare special forms of adjectives. They are used to compare two things. Generally, comparatives are formed using -er .

1. Forming comparatives

How these forms are created depends on how many syllables there are in the adjective. Syllablesare like sound beats. For

instance, sing contains one syllable, but singing contains two singand ing. Here are the rules:

Adjective form Comparative

Only one syllable, ending in E. Examples: wide, fine, cute Add -r: wider, finer, cuter

Only one syllable, with one vowel and one consonant at the end. Double the consonant, and add -er: hotter, bigger,
http://www.englishpage.com/irregularverbs/irregularverbs.htmlhttp://www.englishpage.com/irregularverbs/irregularverbs.html


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Examples: hot, big, fat fatter

Only one syllable, with more than one vowel or more than one consonantat the end. Examples: light, neat, fast

Add -er: lighter, neater, faster

Two syllables, ending in Y. Examples: happy, silly, lonely Change y to i, then add -er: happier, sillier, lonelier

Two syllables or more, not ending in Y. Examples: modern, interesting,beautiful

Use more before the adjective: more modern, moreinteresting, more beautiful

2. How to use comparatives

ComparativesComparatives are used to compare two things. You can use sentences with than.

Jiro is taller than Yukio.

Social Themes

lvaro Uribe Vlez (nacido el 4 de julio de 1952 en Medelln, Antioquia) es un poltico y abogado colombiano que ejerci

como presidente de la Repblica de Colombia para los perodos anterior administracin (2002-2006) y 2006-2010 despus de que

presentara un acto legislativo que modific la constitucin.3La coalicin de gobierno de Uribe intent modificar la Consitucin

colombiana para permitir una segunda reeleccin, pero la Corte Constitucional fall en su contra, imposibilitando al mandatario para

ser a candidato a un tercer mandato.4

Uribe se gradu en derecho en la Universidad de Antioquia y curs posteriormente estudios en administracin, gerencia y

negociacin de conflictos en la Escuela de Extensin de la Universidad Harvard;56fue tambin "miembro senior asociado" de un

college de la Universidad de Oxford.76

Como poltico ha desempeado diferentes cargos en las Empresas Pblicas de Medelln, en el Ministerio del Trabajo y la AeronuticaCivil, sirviendo como Alcalde de la ciudad de Medelln durante (1982), ejerciendo de senador de la Repblica (1986-1994) y, ms

tarde, de gobernador de Antioquia(1995-1997), siendo finalmente elegido presidente de la Repblica de Colombia en 2002.

Repblica de Colombia / Republic of ColombiaCronologa de PresidentesChronology of Presidents

ltima actualizacin / Last updated: August 9, 2010

Aos / Years Presidente /President Partido / Party Aclaraciones /Notes

2010 - present Juan Manuel Santos

Partido Social de Unidad Nacional

(Partido de la U)
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2006 - 2010 lvaro Uribe VlezPartido Social de Unidad Nacional(Partido de la U)

Gabinete/Cabinet

2002-2006 lvaro Uribe Vlez Liberal (independiente) Gabinete/Cabinet

1998 - 2002 Andrs Pastrana Arango New Democratic Force (NFD)

1994 - 1998 Ernesto Samper Pizano Liberal

1990 - 1994 Csar Gaviria Trujillo Liberal

1986 - 1990 Virgilio Barco Vargas Liberal

1982 - 1986 Belisario Betancur Cuartas Conservative

1978 - 1982 Julio Csar Turbay Ayala Liberal

1974 - 1978 Alfonso Lpez Michelsen Liberal

1970 - 1974 Misael Pastrana Borrero Liberal

1966 - 1970 Carlos Lleras Restrepo Liberal

1962 - 1966 Guillermo Len Valencia Conservative

1958 - 1962 Alberto Lleras Camargo Liberal (Frente Nacional)

1957 - 1958 Military junta: Gabriel Pars Military

1953 - 1957 Gustavo Rojas Pinilla Military president by military coup

1951 - 1953 Roberto Urdaneta Arbelez Conservative

1950 - 1951 Laureano Gmez Castro Conservative

1946 - 1950 Mariano Ospina Prez Conservative

1945 - 1946 Alberto Lleras Camargo Liberal

1944 Daro Echanda Olaya

1942 - 1945 Alfonso Lpez Pumarejo Liberal 2nd term

1938 - 1942 Eduardo Santos Liberal

1934 - 1938 Alfonso Lpez Pumarejo Liberal

1930 - 1934 Enrique Olaya Herrera Liberal

1926 - 1930 Miguel Abadia Mndez Conservative

1922 - 1926 Pedro Nel Ospina Vsquez Conservative

1921 - 1922 Jorge Holguin Conservative 2nd term took over after Surez
http://pdba.georgetown.edu/Executive/Colombia/cabineturibe.htmlhttp://pdba.georgetown.edu/Executive/Colombia/cabineturibe.htmlhttp://pdba.georgetown.edu/Executive/Colombia/cabineturibe.htmlhttp://pdba.georgetown.edu/Executive/Colombia/cabineturibe.htmlhttp://pdba.georgetown.edu/Executive/Colombia/cabineturibe.htmlhttp://pdba.georgetown.edu/Executive/Colombia/cabineturibe.html


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1918 - 1921 Marco Fidel Surez Conservative resigns office of Presidency

1914 - 1918 Jos Vincente Concha Conservative

1910 - 1914 Carlos E. Restrepo Restrepo Republican elected by National Assembly

1909 - 1910 Ramn Gonzlez Valencia Conservativeelected by Congress afterresignation of Reyes

1909 Jorge Holgun Jaramillo

1904 - 1909 Rafael Reyes Prieto Conservative

1900 - 1904 Jos Manuel Marroquin Conservative takes power by coup

1898 - 1900 Manuel Antonio Sanclemente National

1894 - 1898 Miguel Antonio Caro Tovar Nationalvice-president, completes terminterrupted by death of Nuez

1892 - 1894 Rafael Nuez Moledo National 3rd term; died in office

1888 - 1892 Carlos Holgun National acting chief executive

1884 - 1888 Rafael Nuez Liberal 2nd term

1882 - 1884 Jos Eusebio Otlora Liberalpresidential designate, takes officeat death of Zalda

1882 - 1882 Francisco Javier Zalda Liberal died in office

1880 - 1882 Rafael Nuez Moledo Liberal

1878 - 1880 Julin Trujillo Liberal

1876 - 1878 Aquileo Parra Liberal

1874 - 1876 Santiago Prez Liberal

1872 - 1874 Manuel Murillo Toro Liberal 2nd term

1870 - 1872 Eustorgio Salgar Liberal

1868 - 1870 Santos Gutirrez Liberal

1867 - 1868 Santos Acosta Liberalpresidential designate, takes officeat deposition of Mosquera

1866 - 1867 Toms Cipriano de Mosquera Liberal 3rd term; died in office

1864 - 1866 Manuel Murillo Toro Liberal

1862 - 1864 Toms Cipriano de Mosquera Liberal 2nd term: president by civil war

1861 - 1862 Julio Arboleda Conservative


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1857 - 1861 Mariano Ospina Conservative

1854 - 1857 Manuel Mara Mallarino Conservative vice-president, acting executive

1853 Jos Mara Obando Liberal

1849 - 1853 Jos Hilario Lpez Liberal named president by Congress

1845 - 1849 Toms Cipriano de Mosquera Conservative named president by Congress

1841 - 1845 Pedro Alcntara Herran Conservative named president by Congress

1837 - 1841 Jos Ignacio de Mrquez named president by Congress

1832 - 1837 Francisco de Paula Santander

1830 - 1831 Rafael Urdaneta President dictator by military coup

1830 Joaqun Mosquera Elected by Congress

1821 - 1830 Simon Bolivar

Math Themes

Eratosthenes

276-195 B.C.E.

It can be said that Eratosthenes is most widely known as a famous Greek mathematician. What most peopleprobably do not know is that Eratosthenes is not only a famous mathematician but also a well known

geographer, astronomer and historian.

Before I get into a few of his accomplishments, let me tell you a little about his personal history. Eratostheneswas born in Cyrene, Greece, which is now known as Libya, in South Africa, in 276 B.C.E.. It is believed that hestarved himself to death in 195 B.C.E. due to the fact that he became blind and could no longer work. As ayoung man, Eratosthenes studied in Athens. Eventually, he made such a name for himself in his many fieldsthat he caught the attention of the ruler of Egypt, Ptolemy III. Ptolemy III invited Eratosthenes to Alexandria,Egypt for two reasons; to tutor his son and to be the librarian for the great Alexandrian University. Eratosthenesjumped at the chance. At the University, he was able to at most interested him and associate with otherscholars. Now on to his accomplishments. . .

One of his major accomplishments in mathematics is his creation of a sieve that determines prime numbers upto any given limit. This sieve, which is called, the Sieve of Eratosthenes, is still important today in number

research theory. Prime numbers are natural numbers greater than 1 that can be divided without remainder onlyby itself and by 1. Eratosthenes figured out that if you were to write down all the natural numbers from 2 toinfinity and "sieve out" every second number after two (or multiples of two), then move to the next availablenumber (3) and continue to "sieve out" every multiple of 3 and so on, one would end up with a list of primenumbers.

Eratosthenes is also known for his achievement in astronomy. Several astronomers and mathematicians beforeand after Eratosthenes tried to accurately measure the circumference of the Earth, but is was Eratosthenes thatcame through. He found the circumference of the Earth to be nearly 250,000 stadia (25,000 miles).Eratosthenes observed that the sun shone directly down a well at high noon on the day of the summer solsticein Syene and that it cast a shadow in Alexandria, directly south of where the well was. To calculate thecircumference of the Earth, Eratosthenes measured the angle of the shadow to the Earth. Until he realized this,Eratosthenes believed that the sun was so far away that its rays were parallel. It is also believed thatEratosthenes made a star catalog with approximately 675 stars and created a calendar that included leap

years.

As a historian, Eratosthenes decided to work on giving a systematic chronography of the known world byfiguring out the dates of literary and political events from the siege of Troy up until his time. However, this was


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only a beginning. Others built on his foundation.

There is still more. Eratosthenes also contributed to geographic source of the river. Many scholars thatpreceded Eratosthenes in the study of the Nile river, tried to figure out the reason why parts of the river floodedwhile other parts did not. It was not until Eratosthenes that a correct answer was proposed. He believed thatheavy rains near the source of the Nile was the cause of Many of Eratosthenes' peers nicknamed him "Beta"which is the second letter of the Greek alphabet, indicating that he just fell short of first place. Eratosthenescontributed greatly to many different areas of knowledge, more than I could cover in this short paper. Maybe inhis time period, his peers did not feel that he contributed enough in one area or maybe they were jealous that

he had contributed so much to so many areas. For a man who was nicknamed Beta, it is pretty impressive thatso much of his work in these areas is still discussed today, so many years late

Pythagoras' Theorem

Years ago, a man named Pythagoras found an amazing fact about triangles:

If the triangle had a right angle (90) ...

... and you made a square on each of the three sides, then ...

... the biggest square had theexact same areaas the other two squares put together!

It is called "Pythagoras' Theorem" and can be written in one short equation:

a2 + b2 = c2

Note:

c is the longest side of the triangle

a and b are the other two sides

Definition

The longest side of the triangle is called the "hypotenuse", so the formal definition is:


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In a right angled triangle:

the square of the hypotenuse is equal to

the sum of the squares of the other two sides.

Sure ... ?

Let's see if it really works using an example.

Example: A "3,4,5" triangle has a right angle in it.

et's check if the areas are the same:

32 + 42 = 52

alculating this becomes:

9 + 16 = 25

works ... like Magic!

Why Is This Useful?

If we know the lengths of two sides of a right angled triangle, we can find the length of the third side. (But remember it only works on

right angled triangles!)

How Do I Use it?

Write it down as an equation:

a2 + b2 = c2

Now you can use algebra to find any missing value, as in the following examples:

Example: Solve this triangle.

2 + b2 = c22 + 122 = c2

5 + 144 = c2

69 = c2
http://www.mathsisfun.com/geometry/triangle-3-4-5.htmlhttp://www.mathsisfun.com/algebra/index.htmlhttp://www.mathsisfun.com/algebra/index.htmlhttp://www.mathsisfun.com/geometry/triangle-3-4-5.html


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= 169

= 169

= 13

You can also read about Squares and Square Roots to find out why 169 = 13

Example: Solve this triangle.

2 + b2 = c2

2 + b2 = 152

1 + b2 = 225

ake 81 from both sides:

2 = 144

= 144

= 12

Example: Does this triangle have a Right Angle?

oes a2 + b2 = c2 ?

a2 + b2 = 102 + 242 = 100 + 576

676

c2 = 262 = 676

hey are equal, so ...

Yes, it does have a Right Angle!

Pascal's Triangle

One of the most interesting Number Patterns is Pascal's Triangle (named

after Blaise Pascal, a famous French Mathematician and Philosopher).

To build the triangle, start with "1" at the top, then continue placing numbersbelow it in a triangular pattern.

Each number is just the two numbers above it added together (except for theedges, which are all "1").

(Here I have highlighted that 1+3 = 4)
http://www.mathsisfun.com/square-root.htmlhttp://www.mathsisfun.com/square-root.html


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Science Themes

Introduction to Cells

Cells are the basic units of life.There are seven characteristics of life which all cells and life forms do.

1) Living things require food for energy.

2) Living things respire. Respiration is the breakdown and use of energy contained in food.

3) Living things respond to the environment.- Plants turn towards the sun.- animals move to avoid danger and to obtain food.

4) Living things produce waste.

5) Living Things are able to repair themselves.

6) Living things grow and reproduce.

7) Living things have a limited lifespan.

ANIMAL CELL

The cell is the basic unit of life. All organisms are made up of cells (or in some cases, a single cell). Most cells are very small; most areinvisible without using a microscope. Cells are covered by a cell membrane and come in many different shapes. The contents of a cellare called the protoplasm.


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The following is a glossary of animal cell terms:

cell membrane - the thin layer of protein and fat that surrounds the cell. The cell membrane is semipermeable, allowing somesubstances to pass into the cell and blocking others.centrosome - (also called the "microtubule organizing center") a small body located near the nucleus - it has a dense center andradiating tubules. The centrosomes is where microtubules are made. During cell division (mitosis), the centrosome divides and the twoparts move to opposite sides of the dividing cell. The centriole is the dense center of the centrosome.cytoplasm - the jellylike material outside the cell nucleus in which the organelles are located.Golgi body - (also called the Golgi apparatus or golgi complex) a flattened, layered, sac-like organelle that looks like a stack ofpancakes and is located near the nucleus. It produces the membranes that surround the lysosomes. The Golgi body packagesproteins and carbohydrates into membrane-bound vesicles for "export" from the cell.lysosome - (also called cell vesicles) round organelles surrounded by a membrane and containing digestive enzymes. This is wherethe digestion of cell nutrients takes place.mitochondrion - spherical to rod-shaped organelles with a double membrane. The inner membrane is infolded many times, forming aseries of projections (called cristae). The mitochondrion converts the energy stored in glucose into ATP (adenosine triphosphate) for

the cell.nuclear membrane - the membrane that surrounds the nucleus.nucleolus - an organelle within the nucleus - it is where ribosomal RNA is produced. Some cells have more than one nucleolus.nucleus - spherical body containing many organelles, including the nucleolus. The nucleus controls many of the functions of the cell(by controlling protein synthesis) and contains DNA (in chromosomes). The nucleus is surrounded by the nuclear membrane.ribosome - small organelles composed of RNA-rich cytoplasmic granules that are sites of protein synthesis.rough endoplasmic reticulum - (rough ER) a vast system of interconnected, membranous, infolded and convoluted sacks that arelocated in the cell's cytoplasm (the ER is continuous with the outer nuclear membrane). Rough ER is covered with ribosomes that giveit a rough appearance. Rough ER transports materials through the cell and produces proteins in sacks called cisternae (which are sentto the Golgi body, or inserted into the cell membrane).smooth endoplasmic reticulum - (smooth ER) a vast system of interconnected, membranous, infolded and convoluted tubes thatare located in the cell's cytoplasm (the ER is continuous with the outer nuclear membrane). The space within the ER is called the ERlumen. Smooth ER transports materials through the cell. It contains enzymes and produces and digests lipids (fats) and membraneproteins; smooth ER buds off from rough ER, moving the newly-made proteins and lipids to the Golgi body, lysosomes, andmembranes.vacuole - fluid-filled, membrane-surrounded cavities inside a cell. The vacuole fills with food being digested and waste material that ison its way out of the cell.


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PLANT CELL

Unlike animal cells, plant cells have a cell wall. The cell wall is made of cellulose, a rigid fiber layer. It gives the cell structure and

allows for adhesion with other plant cells. Inside the cell wall is the cell membrane, a fluid structure made up of lipids and proteins. Cell

membranes have channels and pumps that allow certain molecules in and out of the cell. Most pumps and channels are specific to

certain molecules, like sodium and potassium.

All the components inside the plant cell, called organelles, sit in a jelly-like fluid called cytoplasm. The vacuole, a fluid-containing

organelle, takes up most of the cell and allows the cell to maintain its shape.

The nucleus contains the DNA, the genetic material that encodes for all the plants proteins and also contains other organelles. The

nuclear membrane encloses it. Inside the nucleus is an organelle called the nucleolus, where the DNA synthesizes RNA. The smooth

endoplasmic reticulum (ER) is where the RNA goes to make proteins and the rough ER transports molecules throughout the cell. Next

to the nucleus is the Golgi Body, whose function is to package proteins and carbohydrates that will be leaving the cell. An organelle

called a centrosome sits near the nucleus and is important for cell division.

Two organelles only found in plants are amyloplast, which store starch compounds, and chloroplasts. Chloroplasts contain chlorophyll

and convert sunlight into energy the plant cell can use in a process called photosynthesis.

The mitochondrion is known as the powerhouse in al l cells. Adenosine triphosphate (ATP) is produced from glucose specifically inthe christae, the little projections that are inside the mitochondrial membrane. ATP drives many important chemical reactions in cells.

CELL NUTRITION

We discussed how small particles can diffuse across the cell membrane through either passive transport or active transport. However,large food particles, whether they be grains of sugar or other organisms, cannot simply diffuseacross the membrane; they are just too big. Instead, they are engulfed by the cellmembrane through a process known as endocytosis.

As a cell approaches a food particle, either the food particle pushes into the cell membrane forming

an indentation, or pseudopodia are extended from the cell around the particle. When the twoextensions of the cell membrane meet on the other side of the particle, they close and forma vacuole around the food inside the cell. This process of called endocytosis.


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Exocytosis is a very similar process. In fact, it's just endocytosis run in reverse order. A vacuole within the cell moves toward and fuseswith the cell membrane. In this manner, the contents of thevacuole are expelled into the external environment. This may occur, forexample, after a cell has taken in a large particle through endocytosis, digested it using the enzymes in the lysosome, and then needsto expel the waste products.

Endocytosis and exocytosis are general terms which refer to the process by which anything is taken into or expelled fromthe cell through the action of vacuoles. Phagocytosis refers to endocytosis of a solid particle.Pinocytosis indicates the taking in ofliquid and dissolved molecules through endocytosis.

CELL REPRODUCTION

Before we can fully discuss cell reproduction, we must look briefly at the organization of DNA in cells. Recall from Chapter Onethat DNA is an indirect way of controlling what proteins a cell will produce; based upon the DNA's genetic code (which will bediscussed in Chapter Six), different proteins will be produced.

When a cell reproduces, it basically just splits down the middle. While the process is of course a bit more complex than that, what isimportant to realize is that two cells form from the original parent. The parent cell cannot simply cut it's DNA in half and pass half toeither of the new cells; doing so would result in cells which do not have the "code" for all of the proteins. Instead, cells must actuallymake a complete copy of their DNA so that each new cell has a full copy of the entire "code."

In prokaryotes, the organization of DNA is fairly simple. It is just one long circular molecule. As you mightexpect, eukaryotes have developed a more complex way of organizing their DNA. When the cell is not dividing,the DNA of a eukaryote appears is a large mass called chromatin. However, immediately before the cell reproduces,the chromatin (which has already been replicated) condenses into distinct structures. Each of these structurescontains two identical copies of portions of the cell's DNA. The individual copies are known as sister chromatids, andthey are attached at a region called the centromere. During division, the chromatids split, and each half is referred toas a chromosome. In this manner, each new cell gets an exact copy of the original cell's DNA.

Cell division in prokaryotic organisms is significantly simpler than that in eukaryotes. This isbecause prokaryotes have much less complex DNA, and they do not have to worry about ensuring that each of the newcells receivesan approximately equal number of organelles.

All cells reproduce by actually dividing down the middle until the cell membrane pinches closed and two new "daughter" cells areformed. In prokaryotes, there is very little else to discuss. Once the DNA of thecell is replicated (using a process which will bediscussed in the next chapter), each copy moves toward an opposite side of the cell by attaching to the cell membrane. The cell thenelongates until it is approximately double its original size. Finally, the cell membrane on either side pinches inward and forms twonew cells.

During their lifetime, cells undergo a process of growth which ends either in death or reproduction. Since the general stages of thiscycle are the same for all eukaryotic cells, it has been called the cell cycle. It has two main stages. The first stage, mitosis, refers tothe division of the nucleus and the DNA. The other stage is called interphase (which literally means "the phase between eachdivision"), which is in turn divided into three smaller periods. The three periods, listed in the order in which they occur, are G1, S, andG2.

After a cell divides, the daughter cells enter a period of growth called the G1 period. Since each daughter cell is roughly half the size ofthe normal-sized parent, the daughter cells must grow and synthesize new organelles. Once the cell has grown to normal size, it mayremain stagnant (stationary) in the G1 phase. However, usually some change (which is currently being researched) occurs in

the cell which causes it to enter the S phase. The S phase simply refers to the time when the cell's DNA is replicated in preparationfor cell division. Once this is completed, the cell enters the G2 phase, in which it synthesizes the structures required for cell division.


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Mitosis is the process by which the cell gives identical copies of its DNA to each of the daughter cells. Although the process does notstart and stop into different stages, biologists have separated it into four phases in which major changes occur. These phasesare prophase, metaphase, anaphase, and telophase.

During prophase, the chromatin mass in the nucleus condenses into sister chromatid structures. Boththe nucleolus and the nuclear membrane disappear during prophase. Also, two structures calledthe centrioles (which were previously adjacent to one another near the nucleus) begin to move towardopposite ends of the cell. As they do this, a network of fibers begins to form between them. These fibersare called the spindle fibers, and they become very important later on.

When prophase ends, the centrioles are at opposite ends of the cell andthe chromatin has fully condensed. The cell is now said to enter metaphase, duringwhich the spindle fibersattach to the centromeres (or rather a part ofthe centromere called the kinetochore) and pull the sister chromatids to the center ofthe cell. Metaphase ends when all of the chromatid pairs are aligned in the center.

During the next phase, called anaphase, the centromeres of all of the sister chromatid pairs breaksimultaneously. Once separated, each chromatid is referred to as a chromosome. Thechromosomes fromeach pair are tugged toward opposite ends of the cell by the spindle fibers still attached to it. At the endof anaphase, the chromosomes are completely at opposite ends of the cell.

The final stage of mitosis is telophase. The spindle fibers, no longer needed to direct the motion ofthe chromosomes, break apart. New membranes begin to form around each of the two sets

of chromosomes, which begin to disperse back into the mass of chromatin. Nucleoli reappeararound each set as well, and a second centriole is formed next to each of the existing in preparation for thenext division. Finally, cytokinesis (the division of the cytoplasm, which is discussed in the next section) beginsto occur.

Cytokinesis is the final stage of cell reproduction; it is the division of the cytoplasm into approximately equalhalves, resulting in a roughly equal distribution of organelles in each of the daughter cells. Cytokinesis usually begins intelophase. Wewill discuss cytokinesis separately for organisms which do not have a cell wall (mostly heterotrophs) and those which do have a cellwall (generally autotrophs).

For a cell without a cell wall, cytokinesis begins with the pinching inward of the cell membrane caused by the action of a ring of specialcontractile proteins called actin and myosin. The pinching inward forms a deep grove, often called the cleavage furrow. Eventually, theopposite sides of the furrow meet and the cell splits into two new daughter cells.

Whereas cytokinesis in cells lacking a cell wall occurs through a pinching inward, organisms with a cell wall undergo a processthrough which a so-called cell plate is built from the middle of the cell outward to the membrane.Vesicles from the Golgi bodies flattenand fuse in the center of the cell, forming the cell plate, which elongates toward the cell membrane as more vesicle are added. Whenthe cell plate reaches the cell membrane, the parentcell is effectively separated into two new daughter cells, which then proceed tobuild a cell wall along the cell plate.

Viruses

Viruses are extremely small infectious agents that invade cells of all types. Once inside another cell, viruses become hijackers, using

the cells' machinery to produce more viruses. Whether viruses constitute living organisms or merely conglomerations of molecules has

been a source of debate for many years.

Are they alive?

One of the fundamental hallmarks of life is the ability to reproduce. Whether viruses have this ability is key to a debate over their status

as living things. Some argue that since viruses cannot reproduce independently, they are not alive. However, similar to viruses, there

are a few prokaryotes that are obligate parasites and cannot reproduce without a host. But these prokaryotes show another hallmark

of life that viruses lack: growth. Once assembled, a virus does not change in size or chemical composition. They lack the machinery

for producing energy to drive such biological processes. This makes them radically different from any known organism.

Viruses do, however, show some characteristics of living things. They are made of proteins and glycoproteins like cells are. They

contain genetic information needed to produce more viruses in the form of DNA or RNA. They evolve to adapt to their hosts. So while

it is doubtful viruses are truly alive, they are clearly very similar to living organisms.


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Virus Diversity

There is more diversity among viruses than among all groups of living organisms combined. New varieties are constantly being

described. It would be nearly impossible to describe all of the groups of viruses and their characteristics briefly. Instead, we will look at

the characteristics shared by all viruses, such as their basic structure and general replicative cycle. Then we will discuss the features

used to classify new viruses, such as shape and form of genetic material.

summary 7 th grade

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