Structural Organization of Chromosomes and DNA

8/3/2019 Structural Organization of Chromosomes and DNA

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an organized structure of DNA and protein found in cells.

a single piece of coiled DNA containing many genes, regulatory

elements and other nucleotide sequences.

contain DNA-bound proteins, which serve to package the DNA and

control its functions. comes from the Greek (chroma, colour) and (soma,

body) due to their property of being very strongly stained by

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Human Chromosomes


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Sister Chromatids - these are two identical copies of the chromosome.During cell division all the DNA of each chromosome is replicated. Theyeventually separate to become a full chromosome.

Centromere - the central region of the chromosomal structure. It'swhere the two sister chromatids are joined to each other.

DNA - dubbed the molecule of life. Each chromosome contains a long

linear molecule of DNA and there's more than two metres of it in eachone. So for it to be able to fit it's tightly wound around histone proteins.

Telomeres - repetitive DNA sequences that are situated at the tips ofchromosomes (the repeated sequence reads - TTAGGG). They arevital to the survival and life of the chromosome. Telomeres protect theends of the chromosomes from damage and stop them from becomingattached to each other. They shrink after every cell division and therehas been a great deal of research interest into the relationship between

telomere length and cellular senescence and theories about thepossibilities of increasing life spans if telomere length can bemaintained.

Short Arm - all human chromosomes possess two arms - a short armand a long arm. It is termed the 'p' arm.

Long Arm - is termed the 'q' arm.


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Chromosomes are made of 50% proteins (structure,protection, turn genes on and off), and 50% DNA(deoxy-ribonucleic acid).

Chromosomes are made of many chromatin threads,each containing DNA and proteins.

Somatic cells contain 46 chromosomes, gametic cellscontain 23 chromosomes.

44 of our chromosomes are AUTOSOMES, while 2 are

called sex chromosomes.

Each chromosome contains many GENES that act asa blueprint or set of instructions for the cell.


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DNA The DNA in chromosomes forms the basis for the genetic code. Italone contains the language of life that determines how a cell operatesand all of your characteristics. DNA is made of long chains of nucleotides. DNA has a DOUBLE-

HELIX structure where 2 strands of nucleotides join and twist around toform a spiral staircase or twisted ladder.

Each nucleotide has 3 parts:

1. phosphate group - helps form the backbone of the DNAmolecule

2. sugar (deoxyribose) - acts as a glue, forms the backbone withphosphate

3. nitrogen bases - 4 types (guanine, cytosine, adenine, thymine) the base of one nucleotide forms a hydrogen bond with

the base of another nucleotide, thus the bases formthe steps of the DNA ladder.

the sequence of the bases makes up the genetic code

deoxyribonucleic

acid


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LAW OF COMPLEMENTARY BAS

EPAIRINGS:adenine and thymine always bind to one another (A-T) and

cytosine and guanine bind together (C-G).


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DNA is composed of units called nucleotides, the basic

structural unit of DNA and RNA. Each nucleotide contains a

phosphate group, a deoxyribose sugar, and a nitrogenous base.Tohelp protectyour privacy, PowerPointprevented thisexternalpicturefrom being automatically downloaded. Todownload and display thispicture, click Optionsin theMessageBar, and then click Enableexternalcontent.

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The nucleotides joined together to

form a chain. The phosphate end ofthe chain is referred to as the 5'

end. The opposite end is the 3' end.


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DNA is composed of two chains of nucleotides linked together in

a ladder-like arrangement with the sides composed of alternating

deoxyribose sugar and phosphate groups and the rungs being the

nitrogenous bases as indicated by the diagram below.Tohelp protectyour privacy, PowerPointprevented thisexternalpicturefrom being automatically downloaded. Todownload and display thispicture, click Optionsin theMessageBar, and then click Enableexternalcontent.

The "A" of one strand is always paired with a "T" on the other.

Similarly, the "G" of one strand is paired with a "C" on the other.

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DEOXYRIBOSE

sugar in DNA. The sugar inRNA is r ibose. The deoxy- prefix

indicates that this form of the sugar contains one less oxygen

atom.

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NITROGENOUS BASES

PYRIMIDINES

There are three pyrimidine bases, and each consists of a 6-

member ring containing both nitrogen and carbon atoms. Two

pyrimidines, Thymine and Cytosine , are found in DNA. RNA

also contains two pyrimidines, but they are Cytosine and Uracil .

Bases are known by their first letter, hence the three pyrimidines

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PURINES

Again, the purines are made from two heterocyclic rings of carbon

and nitrogen. This time, a 6-member and a 5-member ring have fused.

There are two purines, Adenine and Guanine , and both are found inDNAand RNA.



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PHOSPHATES

The addition of one or more phosphate groups to a nucle oside,

transforms it into a nucleotide . This is the base-sugar-phosphate

unit which, when polymerised, forms DNA.

The nucleotides which contain three phosphates

(deoxyribonucleotide 5'triphosphates) are the precursors for the

synthesis of DNA. The two phosphate groups at the end are lost

during DNAsynthesis, hence the sequence only contains

deoxyribonucleotide monophosphates.

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The two strands of nucleotides are held together by hydrogen

bonds (electrostatic attraction). Two hydrogen bonds hold adenine to thymine.Three bonds attach cytosine to guanine as indicated in the diagram below.

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During the process of cell division, the DNAbecomes tightly coiled, forming structurescalled chromosomes. The diagram below isa portion of a double-stranded chromosome

showing the centromere and a portion of thebase sequence. The diagram does not showthe extensive looping and coiling and theproteins associated with coiling. Notice thatthe base sequence in the two chromatids isidentical.


Structural Organization of Chromosomes and DNA

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