تعريفات مهمة في علم الgenetic engineering

الأحد مارس 22, 2009 2:31 pm

Gene:

a sequence of nucleotides that represents a functional

unit of inheritance; a region of DNA that codes for a

product, either RNA or protein.

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Chromosome:

a highly ordered structure composed of DNA and

proteins that carries the genetic information. In humans,

there are 46 chromosomes ordered in pairs.

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Autosome:

all chromosomes other than the X and Y chromosomes,

which are designated the sex chromosomes.

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Homologous chromosomes or homologs:

sister chromosomes, the members of a pair of

chromosomes in which one is inherited from the mother

and the other from the father.

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Locus:

the position of a gene on a chromosome.

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Allele:

an alternative form of a gene occupying the same locus.

An allele may be the result of a mutation. There is a

maximum of two alleles per diploid chromosome

complement (one allele per chromosome), but multiple

alleles may exist within a population.

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Mutation:

a permanent heritable change in the sequence of

genomic DNA. This may manifest at both the molecular

and cytogenetic levels. Not all mutations are negative

events. Many are benign (e.g., blue eye color) and some

have positive effects (e.g., sickle cell trait in countries

with a significant risk of malaria). Individuals with a

constitutional mutation (i.e., a mutation present in every

cell of the body) may pass that mutation on to their

progeny by germ-line transmission. In some cases,

notably cancer, an acquired mutation may arise in a

single somatic cell, which then divides mitotically, giving

rise to a new clone of cells. The mutation will be limited

to this clone and will not be transmitted to progeny of

the individual. In rare instances of gonadal mosaicism, a

de novo acquired mutation may arise in the gonads,

resulting in a mixed population of normal and mutant

gametes. Progeny receiving the new mutation may

display a phenotype not present in either parent.

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Karyotype:

the chromosome constitution of an individual.

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Karyogram:

a figure showing the paired chromosomes from a cell

arrayed in a standard sequence

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Diploid:

the presence of two copies of each unique chromosome

per cell. In humans, the chromosomes occur in pairs and

the diploid (2N) number is 46.

*****************

Haploid:

one copy of each unique chromosome. In humans, the

gametes are haploid (N = 23).

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Homozygous:

both alleles at a locus are the same. (In the ABO

system, an AA complement represents homozygosity.)

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Heterozygous:

the two alleles at a locus are different. (In the ABO

system, an AO complement represents heterozygosity.)

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Hemizygous:

the presence of only one chromosome or chromosome

segment rather than the usual two; applies to males with

a single X chromosome.

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Genotype:

the genetic constitution of an individual or organism (i.e.,

what alleles are present). (In the ABO system, AA, AO,

BB, BO, AB, and OO are genotypes.)

*****************

Phenotype:

the appearance of an individual that results from the

interaction of environment and genotype. (In the ABO

system, A, B, and O blood types represent the

phenotypic expression of the alleles for a given

individual.)

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Dominant allele:

an allele that is expressed when present in only a single

dose (i.e., it ‘dominates' over the other allele present).

(In the ABO system, A is dominant over O such that an

AO genotype results in an A blood type phenotype.

Similarly, the presence of pigment (T) is dominant to the

absence of pigment (t) (i.e., albinism), such that Tt

results in pigmentation.)

****************

Recessive allele:

in a diploid organism, an allele that is only expressed

when homozygous. (In the ABO system, the O blood

group is only seen with a OO genotype; O is recessive to

A and B. Similarly, albinism (t) is recessive to

pigmentation (T), and an albino phenotype only occurs

with a tt genotype.)

*****************

Codominant alleles:

in a diploid organism, alleles that show no dominance or

recessivity to each other but, when present together

are both fully expressed. (In the ABO system, A and B

are codominant such that an AB genotype expresses

both A and B antigens.)

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Independent assortment:

random assortment of chromosomes (paternal and

maternal) in the gametes; 50:50 chance of inheriting a

given chromosome from one parent.

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Linkage:

the presence of two or more genes on the same

chromosome that tend to be inherited together.

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Crossing over:

the physical exchange of genetic material between

homologous chromosomes.

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Recombination:

the generation of new allelic combinations on

chromosomes, usually by crossing over.

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Mitosis:

somatic cell division in which the DNA replicates and is

evenly distributed to two equal daughter cells.

*****************

Meiosis:

cell division in the gonads that produces the gametes. A

single DNA replication is followed by two cell divisions

which reduces the total DNA content of a cell from 2N to

N. Recombination occurs to increase genetic diversity

within a population.

***************

Nondisjunction:

failure of chromosomes or chromatids to separate to

opposite poles in cell division. Usually results in one too

many or one too few chromosomes in a cell.

المصدر كتاب :

McPherson & Pincus: Henry's Clinical Diagnosis and Management by Laboratory Methods

الأحد مارس 22, 2009 2:32 pm

معلومات رائعة في الهندسة الوراثية....

تكفي
المعلومات التي يحتوي عليها الجينوم البشري لملء كتب ورقية يبلغ ارتفاعها
61 متراً، أي ما يوازي المعلومات التي يحتوي عليها 200 دليل للهواتف يحتوي
كل منها على 500 صفحة!

************************

;فيما
بيننا نحن البشر، يختلف DNA من فرد لآخر بنسبة 5,2% فقط، أو 1 من كل 50
حرفاً، ويضع ذلك في الإعتبار أن الخلايا البشرية تحتوي كل منها على نسختين
من الجينوم.

************************

;إذا
أردنا أن نقرأ الجينوم البشري بسرعة حرف واحد في الثانية لمدة 24 ساعة
يومياً، فسيستغرق الأمر قرناً كاملاً للانتهاء من قراءة كتاب الحياة!

************************

;إذا
تم فرد جميع DNA الموجود في الجسم البشري طرفا لطرف، يمكن للخيط الناتج أن
يصل من الأرض إلى الشمس وبالعكس 600 مرة [100 تريليون ×1.8 متر مقسومة على
148.800.000 كيلومتر = 1200].

************************

;يقوم الباحثون في مشروع الجينوم البشري بفك شفرة 12.000 حرف من DNA البشري في الثانية الواحدة.

************************

;إذا
تم فرد جميع الحروف (3 بلايين) المكونة للجينوم البشري بحيث يكون كل منها
على بعد 1 ملم من الآخر، فستمتد لمسافة 3000 كيلومتر - أو نحو 700 ضعف
لارتفاع مبنى الإمباير ستيت، وهي ناطحة السحاب الشهيرة في مدينة نيويورك

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يتشابه DNA الخاص بالبشر مع مثيله في الشمبانزي بنسبة 98%.

*****************

Øيبلغ
العدد التقديري للجينات في كل من البشر والفئران 60.000 - 100.000 أما في
الديدان المستديرة فيبلغ العدد 19.000 وفي الخميرة yeast يبلغ عدد الجينات
6.000 تقريباً، بينما يبلغ عدد جينات الجرثومة المسببة للتدرن 4.000.

**********************

Øتظل وظيفة الغالبية العظمى (97%) من DNA الموجودة في الجينوم البشري، غير معروفة لدينا حتى الآن.

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Øكان
أول كروموسومchromosome بشري تم فك شفرته بالكامل هو الكروموسوم رقم 22،
وقد تم ذلك في المملكة المتحدة في ديسمبر 1999، وتحديداً في مركز (سانجر)
بمقاطعة كمبردج.

تعريفات مهمة في علم الgenetic engineering

تعريفات مهمة في علم الgenetic engineering

رد: تعريفات مهمة في علم الgenetic engineering