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Jojie A. Frias Rating:

1:00-2:30/TF July 21, 2015

Homeor! "o. # A$ %oliti&al '&ien&e

1. Do you believe that:a. Just in a past second millions of your cells have divided into two?b. You begin life as a single cell but there are now more cells in your body 

that stars in the Milky ay?c. !ach sperm and egg produced in your reproductive organs carries one

of over eight million possible combinations of parental chromosomes?d. "he dance of the chromosomes in a dividing cell is so precise that only 

one error occurs in one hundred thousand cell divisions?

 #nswers:

a.   Yes. I believe that in a past second, millions of my cells have divided intotwo. The growth of cell populations can be modelled by assuming each cell

divides into two, and the rate of growth depends on the length of the cell

cycle. Based on a research, In order for a cell to be able to count the number of 

times it has divided, there must be some way to sense when the cell is dividing.

Apparently about 50 billion cells die and are replaced every day in the average

adult. That means we go through about ! trillion cells each year. These

numbers will be less during childhood because we"re smaller and then made up

of fewer cells, but this is #ust a rough guess anyway. If we wor$ with the rough

life e%pectancy of !0, then we will go through ,&'0,000,000,000,000 cells in our

lifetimes ( that is .&' )uadrillion cells*

 The average adult is made up of about &0 trillion cells. This would then

suggest that we rema$e ourselves about +5 times through our lives. This isn"t

actually really true because some cell types die and are replaced a lot faster

than others ( particularly red blood cells where apparently .& million are made

every second.

 b.   Yes. I believe that we begin life as a single cell but there are now more cells

in my body than stars in a -il$y ay. According to a blog I"ve read, nobody really

$nows how many brain cells anybody has, but typical estimates are around 00

billion. /Youve heard the late 1arl 2agan tal$ about billions and billions of starsin the universe. Thin$ about this. 3ach brain cell has many connections with

many other brain cells, by way of multi4branching dendrites and a%ons,

communicating across a mind4bogglingly large number of synapses.

ow many6 If each brain cell connected once to each of the others, that

would be & with 7eros after it, a number so large that it would put 2agan and

his billions of stars to shame. 8ne well4$nown researcher has even asserted that

a single human brain has more potential connections than the number of atoms

in the universe. Is that really true6 In mathematical terms, yes. owever, since

each connection depends on many atoms, theres no way that such a huge

potential could be reali7ed, even in theory. But su9ce it to say that the human

brain has a vast potential for thin$ing and learning.:;http<==www.sciencei).com=>acts=BillionsBillions.cfm?

c. Yes. I believe that each sperm and egg produced in my reproductive organs

carries one of over eight million possible combinations of parental

chromosomes. 3ach of us began as a single cell. This cell couldnt move, thin$,

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see, or do things li$e laugh and tal$. But the one thing it could do, and do very

well, was divide@and divide it did. The lone cell became two, and then four,

then eight and so on.

d. o I don"t believe that the dance of the chromosomes in a dividing cell is so

precise that only one error occurs in one hundred thousand cell divisions.

Based on research, there are several cases in which abnormalities or errors in

chromosomes appear. There are many types of chromosome abnormalities.

owever, they can be organi7ed into two basic groups< numerical

abnormalities and structural abnormalities.

umerical Abnormalities< hen an individual is missing one of the

chromosomes from a pair, the condition is called monosomy. hen an

individual has more than two chromosomes instead of a pair, the condition is

called trisomy.

An e%ample of a condition caused by numerical abnormalities is own

syndrome, which is mar$ed by mental retardation and other problems. An

individual with own syndrome has three copies of chromosome rather

than twoC for that reason, the condition is also $nown as Trisomy . An

e%ample of monosomy, in which an individual lac$s a chromosome, is Turner

syndrome. In Turner syndrome, a female is born with only one se%

chromosome, an D, and is usually shorter than average and unable to have

children, among other di9culties.

• 2tructural Abnormalities< A chromosomes structure can be altered in

several ways.

• eletions< A portion of the chromosome is missing or deleted.

• uplications< A portion of the chromosome is duplicated, resulting in

e%tra genetic material.•  Translocations< A portion of one chromosome is transferred to another

chromosome. There are two main types of translocation. In a reciprocal

translocation, segments from two diEerent chromosomes have been

e%changed. In a Fobertsonian translocation, an entire chromosome has

attached to another at the centromere.

• Inversions< A portion of the chromosome has bro$en oE, turned upside

down, and reattached. As a result, the genetic material is inverted.

• Fings< A portion of a chromosome has bro$en oE and formed a circle or

ring. This can happen with or without loss of genetic material.

-ost chromosome abnormalities occur as an accident in the egg or sperm. Inthese cases, the abnormality is present in every cell of the body. 2ome

abnormalities, however, happen after conceptionC then some cells have the

abnormality and some do not.

 

$omatic %ell 

4 A somatic cell is generally ta$en to mean any cell forming the body of an

organism.4 A somatic cell is any cell of the body e%cept sperm and egg cells. 2omatic

cells are diploid, meaning that they contain two sets of chromosomes, one

inherited from each parent. -utations in somatic cells can aEect the

individual, but they are not passed on to oEspring.

4 In mammals, germline cells are the sperm and ova ;also $nown asGgametesG? which fuse during fertili7ation to produce a cell called a 7ygote,

from which the entire mammalian embryo develops.4 3very other cell type in the mammalian body, apart from the sperm and ova,

the cells from which they are made ;gametocytes? and undiEerentiated stem

cells, is a somatic cellC internal organs s$in, bones, blood and connective

tissue are all made up of somatic cells.

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  &aploid %ell and Diploid %ell 

 There are two types of cells in the body 4 haploid cells and diploid cells.

 The diference between haploid and diploid cells is related to the

number of chromosomes that the cell contains.

Haploid cells

o aploid cells have half the number of chromosomes ;n? as diploid 4 i.e. a

haploid cell contains only one complete set of chromosomes.o aploid cells are a result of the process of meiosis, a type of cell division

in which diploid cells divide to give rise to haploid germ cells. A haploid

cell will merge with another haploid cell at fertili7ation.

Diploid Cells

o iploid cells contain two complete sets ;n? of chromosomes.

o iploid cells reproduce by mitosis ma$ing daughter cells that are e%act

replicas.

 

$e' %ell 

4 

8rganisms that reproduce se%ually 

do so via the production of se% cells

called gametes

. These cells are very diEerent for the male and female of a

species. In humans, male se% cells or spermato7oa ;sperm cells?, are relatively

motile. >emale se% cells, called ova or eggs, are non4motile and much larger in

comparison to the male gamete. hen these cells fuse in a process

called fertili7ation, the resulting cell ;7ygote? contains a mi% of  

inherited genes 

from the father and mother.

uman se% cells are produced in reproductive system 

organs called gonads

.

Honads produce se% hormones needed for the growth and development of 

primary and secondary reproductive organs and structures.

  %hromosomes

4 omologous chromosomes are chromosome pairs, one from each parent, that

are similar in length, gene position, and centromere location. The position of 

the genes on each homologous chromosome is the same, however the genes

may contain diEerent alleles.

Example: A human $aryotype shows the complete set of humanchromosomes. uman cells contain + pairs of chromosomes for a total of 

&'. 3ach chromosome pair represents a set of homologous chromosomes.

In males, the se% chromosomes D and Y are homologues. In females,

both D chromosomes are homologues.

  %hromatids

4 3ither of the two strands #oined together by a single centromere, formed from

the duplication of the chromosome during the early stages of cell division and

then separate to become individual chromosome during the late stages of celldivision.

4 A copy of a newly replicated chromosome, which typically is #oined to the other

copy by a single centromere.

Before replication, one chromosome is composed of one  DNA ;double4heli%?

molecule. >ollowing replication, each chromosome is composed of two A

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moleculesC in other words, DNA replication itself increases the amount of A but

does not  increase the number of chromosomes. The two identical copies@each

forming one half of the replicated chromosome@are called chromatids.

(re synthesis )*1 (!+,-D

 The G1 phase, or Growth 1/Gap 1 phase, is the rst of four phases of 

the cell cycle that ta$es place in eu$aryotic cell  division. In this part

of interphase, the cell grows in si7e and synthesi7es mFA and proteins in

preparation for subse)uent steps leading to mitosis. H phase ends when the

cell moves into the 2 phase of interphase.

 

"he $ period 

S-phase ;synthesis phase? is the part of the cell cycle in

which A is replicated, occurring between H phase and Hphase. Jrecise andaccurate A replication is necessary to prevent genetic abnormalities which

often lead to cell death or disease. ue to the importance, the regulatory

pathways that govern this event in eu$aryotes are highly conserved. This

conservation ma$es the study of 24phase in model organisms such as Xenopus

laevis embryos and budding yeast relevant to higher organisms.

 

"he */ (eriod 

  G2 phase, or pre-mitotic phase, is the third and nal sub phase

of Interphase in the cell cycle directly preceding -itosis. It follows the successful

completion of 2 phase, during which the cell"s A is replicated. H phase endswith the onset of prophase, the rst phase of mitosis in which the

cell"s chromatin condenses into chromosomes.