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Mei Vita Cahya Ningsih, Ns.

KROMOSOM

Kromosom manusia 46 buah t.d 22 autosom dan 1 ps. Sex

kromosom

Satu bagian diturunkan dari ayah, satu bagian lain dari ibu

Pasangan yg homolog membawa informasi genetik yang

sesuai

Penurunan dilaksanakan melalui proses pembelahan

Terdapat dua macam pembelahan sel yaitu MITOSIS DAN

MEIOSIS

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Sel tubuh/somatik mempunyai

kromosom diploid

Sel gamet mempunyai kromosom

haploid

Sebelum mengalami pembelahan DNA

mengalami replikasi terlebih dahulu

Replikasi terjadi selama interfase

( S phase)

MORFOLOGI KROMOSOM

� Tiap kromosom t.d 2 kromatid yang

berkumpul di sentromer

�Sentromer berfungsi utk pegangan

fiber dari spindel pd waktu pembelahan

�Sentromer membagi kromosom menjadi

2 bagian p (lengan pendek) dan q

(lengan panjang)

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Berdasar letak sentromer, kromosom

dibedakan menjadi :

�Metasentris : letak sentromer di tengah, sehingga lengan kromosom hampir sama panjang

�Submetasentris : sentromer tidak ditengah, lengan kromosom tidak sama panjang, bila membengkok seperti huruf J

�Akrosentris : sentromer terletak di ujung sehingga membentuk satelit (13,14,15,21 dan 22)

Gambar Kromosom

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KLASIFIKASI KROMOSOM

Didasarkan pada besar dan letak

Sentromernya

� A : terbesar, metasentrik (1,2,3)

� B : submetasentrik besar, p ¼ lengan

q (4,5)

� C : submetasentrik, medium (6-12,X)

� D : akrosentrik, medium, satelit (13-15)

� E : metasentrik, medium (16) dan

submetasentrik kecil (17,18)

� F : metasentrik kecil (19,20)

� G : akrosentrik kecil, satelit (21,22)

dan akrosentrik tak bersatelit (Y)

Klasifikasi juga berdasar pada gambaran

kromosom waktu dilakukan pengecatan

1q24.3 = kromosom 1, lengan panjang,

regio 2, band 4 dan sub band 3

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Gambar Kromosom

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DNA and Protein

(Structure and

Functions)

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DNA carries the genetic information of a cell

and consists of thousands of genes. Each

gene serves as a recipe on how to build a

protein molecule.

Proteins perform important tasks for the cell

functions or serve as building blocks.

The flow of information from the genes

determines the protein composition and

thereby the functions of the cell.

DNA is the physical carrier of inheritance. It is like a giant book of information containing all the instructions for building and maintaining a living organism.

Replication followed by cell division is the answer to one of life's most interesting questions: How can the union of a single sperm and an egg become a five-trillion-cell baby, all containing the same DNA?

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The helix

makes a

complete

turn every

3.4 nm and

there are

about 10.5

base pairs

per turn.

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The building blocks of DNA are the 5-

carbon sugar deoxyribose linked

together by phosphodiester bonds

forming two strands of sugar-phosphate

backbones on the outside of the double

helix

The human genome contains 3x109 base

pairs of DNA divided into 23 chromosomes

which if linked together would form a thread

of 1 meter with a diameter of 2 nm.

This DNA codes for about 105 different

proteins. In fact only about 2-4 % of the total

coding capacity in the human DNA is used for

coding of different genes, the rest of it

probably has other more structural and

organizational functions.

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Chromatin

The DNA double helix in the cell is

packaged by special proteins called

histones to form a protein/DNA complex

called chromatin

The structural unit of chromatin is the

nucleosome.

It consists of a central protein complex,

the histone octamer, and two turns of

DNA, about 146 base pairs, which are

wrapped around the histone octamer

complex.

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There are four different types of core

histones which form the octamer

containing two copies each of H2A, H2B,

H3 and H4.

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There is a linker histone, H1, which contacts the exit/entry of the DNA strand on the nucleosome.

The nucleosome together with histone H1 is called a chromatosome.

Chromatosomes are held together by the continuous DNA strand, thus forming linker DNA of 30-50 base pairs in length.

The Gene

A functional and inheritable element in

the genome is referred to as a gene and

usually codes for a protein.

In some cases genes also code for RNA

molecules that are not translated to

protein, e.g. ribosomal RNA.

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The gene regulatory segment, of which the proximal part is referred to as the promoter, usually consists of many different DNA segments defined by their particular base pair sequences

Each individual segment, usually involving about 6-12 base pairs of DNA, serves as a binding target for a DNA binding protein which functions as a transcription factor

Furthermore, the DNA binding capacity of

various transcription factors is usually

regulated via cellular signals through

extracellular hormones and receptor

pathways or via cell interactions with the

environment.

In this way a particular stimulus in the

surrounding of a cell will trigger the

binding of a set of transcription factors to

a certain set of genes,

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RNALike DNA, the RNA molecule, is built up

by nucleotides linked together in a chain.

There are some differences though :

� The RNA molecule is single stranded

� The four bases in the DNA nucleotides are

adenine, guanine, thymidine and cytosine. In

RNA thymidine is replaced by uracil.

� The sugar in DNA is deoxyribose. In RNA it is

ribose.

There are three major types

of RNA: 1) mRNA,

messenger-RNA, which

transfer the information about

the aminoacid sequence from

the DNA to the protein

synthesis. 2) rRNA,

ribosomal-RNA, which builds

up the ribosome together with

proteins. 3) tRNA, transfer-

RNA, which transfer

aminoacids to the ribosome

for protein synthesis.

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Transcription

After transcription has been initiated RNA

polymerase II, together with the necessary

transcription elongation factors, travels along

the DNA template and polymerizes

ribonucleotides into an RNA copy of the gene

The polymerase moves at a regular speed

(approximately 30 nucleotides per second)

and holds on to the DNA template efficiently,

even if the gene is very long

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RNA ProcessingThe primary transcription product of a gene is therefore called a precursor of mRNA, pre-mRNA.

Both ends of the pre-mRNA are modified. An additional nucleotide, a 7-methylguanosine is added to the 5'-end to form a cap-structure. This process is called capping (1). The 3'-end of the pre-mRNA is cleaved and polyadenylated (4). The pre-mRNA is cut at a specific site and 150-200 adenylate residues are added to the 3'-end to form a poly(A)-tail. The third major modification is splicing (3).

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Translation

Translation is the actual synthesis of a protein

under the direction of mRNA

the ribosome, provides the basic machinery for

the translation process. The major role of the

ribosome is to catalyse coupling of amino acids

into protein according to the sequence specified

by the mRNA.

The amino acids are brought to the ribosome by

tRNA (transfer RNA) molecules.

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Initiation

formation of the initiation complex

between mRNA, charged tRNA and

the ribosome

translation begins at a specific

codon, the initiation codon (AUG)

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the growing polypeptide chain is attached to an amino acid in the P site

the next codon to be read is present beneath the A site

the tRNA bearing the next amino acid to be inserted enters the A site

ELONGATION

a peptide bond is formed between

the new amino acid and the growing

chain, transfering the chain to the

tRNA in the A site

the ribosome moves down one

codon moving the peptide-tRNA to

the P site and the cycle repeats

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Termination

translation of a particular protein

ends when the ribosome encounters

one of three termination codons

(UAG, UAA or UGA)

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REPLIKASI

A. Cara kerja Polimerase DNA

� Sebelum pembelahan salinan

genom (Replikasi)

� Dikatalis oleh Polimerase DNA

� Enzym ini memerlukan DNA untai

tunggal Untai Cetakan

(matrix strand)

� Substrat Polimerase DNA :

dATP,dGTP, dCTP, dTTp

B. Replikasi pada E. coli

• Dimulai pada titik awal mula replikasi dan

dilanjutkan ke kedua arah garpu replikasi

• Setiap garpu memiliki 2 molekul

Polimerase

DNA III + enzym pembantu (topoisomerase

DNA dan helikase) mengendurkan dan

memisahkan ikatan DNA

• Arah pembacaan 3’ – 5’

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DNA polymerase III

Newly

synthesized

leading strand

3'

5'

5'Replication fork

3'

5'

Formation of the leading strand

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� Pertama terbentuk fragmen okazaki yang

diperlukan u/ polimerase DNA dapat berfungsi

� Ada beberapa tempat fragmen okazaki dan

tidak saling

berhubungan

� Enzym polimerase DNA I menyambung dan

menutupinya

dengan enzym ligase