دورة الخلية : Cell Cycle‏ هي الفترة التي تمر بها الخلية بين انقسامين خلويين متتاليان يتم تقسيم هذه الدورة إلى عدة أطوار G1, S, G2 ويعرفوا باسم الطَّورُ البَينِيّ بالاضافة الى الطور الأخيرM الطور M : يتضمن حدوث عمليتين انقسام نووي وسيتوبلازمي. بعض الخلايا التي لم تعد تنقسم أو دخلت السبات توصف بأنها في الطور Gο.

تستمر دورة الخلية لمدة 14 ساعة كحد ادنى، ولا تنتقل الخلية من الطور الأول G1 حتى تجهز المركبات الكيميائية التي تحتاجها للانقسام من أحماض أمينية وليبيدات وسكريات ولذلك يعتمد وقت وسرعة انقسام الخلية على كمية المواد الغذائية التي يتلقاها الجسم. يؤدى الخلل في تتابع دورة الخلية إلى انقسام الخلايا بسرعة ودون تحكم مما يؤدى إلى وجود الخلايا السرطانية أو الخبيثة.

Interphase Prophase Metaphase Anaphase Telophase Cytokinesis لكي تنقسم الخليه فلابد ان تمر بالمراحل التاليه Interphase Prophase Metaphase Anaphase Telophase Cytokinesis M Phase

The Cell Cycle Interphase Phases include: Interphase – Preparation phases for mitosis Mitosis – Cell division or splitting Interphase G1 (Growth) S G2 (Growth)

The Cell Cycle

16 hour cell cycle Cell division 15 hours M phase G0 state G2 phase S-phase (DNA synthesis 5 hours 16 hour cell cycle

The cell cycle الدورة الخلوية النمو الخلوي و إستعداد الخلية للإنقسام الإنقسام النمو الخلوي و تضاعف العضيات تضاعف الدنا النمو الخلوي و إستعداد الخلية للإنقسام

Cell cycle timing Yeast 120 minutes (rich medium) Insect embryos 15-30 minutes Plant and mammals 15-20 hours Some adults don’t divide Terminally differentiated e.g. Nerve cells, eye lens Some quiescent unless activated Fibroblasts in wound healing

Interphase occurs before mitosis begins Chromosomes are copied (# doubles) Chromosomes appear as threadlike coils (chromatin) at the start, but each chromosome and its copy(sister chromosome) change to sister chromatids at end of this phase CELL MEMBRANE Nucleus Cytoplasm

الفترة الفاصلة الأولى (G1) وهي فترة نمو الخلية (Cell growth) حيث تزاول فيها الخلية نشاطها في مجال تخصصها، كتكوين العضيات، وبناء أو تكسير الجزيئات الكبيرة، إصلاح الأنسجة التالفة نتيجة الجروح، وتوزيع البروتينات. وتطول أو تقصر هذه الفترة بحسب ظروف الخلية، ولا يظهر في هذه الفترة بناء للحامض النووي (DNA). إلا أنه يزداد في نهايتها نشاط الإنزيمات التي يتطلبها بناء الحامض النووي (DNA). وهذه الإنزيمات مع عوامل أخرى تعمل على تهيئة الخلية للدخول في فترة البناء.

G1 phase Cell checks everything OK for DNA replication Accumulates signals that activate replication Chloroplast and mitochondria division not linked to cell cycle

فترة البناء (S phase) ويتم فيها مضاعفة حامض الديوكسي رايبوز النووي (DNA)، حيث يتم عمل نسخة من كل كروموزوم. كما يتم في هذه الفترة تكوين البروتينات الداخلة في تكوين الكروموزومات في الخلايا حقيقية النواة. وعملية مضاعفة الكروموزومات هي عملية معقدة تتم بتوجيه من الحامض النووي (DNA) الموجود في الخلية قبل المضاعفة

S-phase The chromosomes replicate Two daughter chromosomes are called chromatids Joined at centromere Number of chromosomes in diploid is four

الفترة الفاصلة الثانية (G2) بعد اكتمال فترة البناء تدخل الخلية في الفترة الفاصلة الثانية، ويزداد في هذه الفترة بناء جميع البروتينات وأنواع الحامض النووي الرايبوزي (RNA) وذلك كتمهيد لعملية انقسام الخلية. ويطلق على هذه الفترة فترة تهيئة الخلية

G2-phase Cell checks everything is OK for cell division Accumulates proteins that activate cell division

(انقسام الخلية) Mالطور تمر الخلية أثناء انقسامها بعمليتين متتاليتين هما: انقسام نواة الخلية والانقسام السيتوبلازمي (Cytokinesis). ويتضمن انقسام نواة الخلية نوعين من الانقسامات هما: الانقسام غير المباشر (Mitosis). الانقسام المباشر أو الاختزالي (Meiosis).

Why have a cell cycle? Comprises gaps and distinct phases of DNA replication and cell division If replicating DNA is forced to condense (as in mitosis) they fragment Similarly if replication before mitosis Unequal genetic seperation Important to keep DNA replication and mitosis separate

Important to have divisions in mitosis e.g. Important metaphase complete before anaphase. If not segregation of chromosomes before attachment of chromatids to microtubles in opposite poles is possible Down syndrome due to extra chromosome 21

Gaps provide cell with chance to assess its status prior to DNA replication or cell division During the cell cycle there are several checks to monitor status These are called checkpoints

Checkpoints Checkpoint if G1 monitors size of cell in budding yeast (Saccharomyces cerevisae) At certain size cell becomes committed to DNA replication Called start or replication site

Evidence of size checkpoint Yeast cells (budding yeast) grown in rich medium Switch to minimal medium Cells recently entering G1 (buds) delayed in G1 (longer to enter S-phase) Large cells above threshold size still go to S-phase at same time as in rich medium

Evidence of size checkpoint Yeast in rich medium 120 minute cell cycle Short G1 phase Yeast in minimal medium Eight hour cell cycle primarily because of long G1 phase

Checkpoints Checkpoint 2 in G1 monitors DNA damage Evidence? Expose cells to mutagen or irradiation Cell cycle arrest in either G1 phase or G2 phase The protein p53 involved in cell cycle arrest Tumour suppresser

Checkpoints Checkpoint in S-phase monitors completion of DNA replication Cell does not enter M-phase until DNA synthesis is complete Checkpoint in G2 DNA breaks cause arrest Otherwise when chromosomes segregate in mitosis DNA distal to breaak won’t segregate

Checkpoints Checkpoint in mitosis Senses when mitotic spindles have not formed Arrests in M-phase Otherwise unequal segregation of chromosomes into daughter cells Described cell cycle, now I will talk about genes and proteins that control this process

MPF Protein identified that causes mitosis Called maturation promoting factor MPF in all mitotic cells from yeast to humans Renamed mitosis-promoting factor

Mitosis Promoting Factor Cycles in M-Phase Promoting Factor (MPF) activity control mitosis. As a protein kinase, MPF likely acts via phosphorylation of the major histone protein H1 and the major nuclear envelope protein lamin. This leads to the degradation of the nuclear envelope and condensation of chromatin into chromosomes in anticipation of mitosis. Found in all organisms, MPF is composed of cyclin-dependent kinase (cdk1) and cyclin B.

Properties of MPF MPF activity changes through the cell cycle MPF activity appears at the G2/M interphase and then rapidly decrease

How does MPF cause mitosis? It’s a protein kinase Phosphorylates proteins Phosphorylates proteins involved in mitosis Phosphorylates histones causing chromatin condensation Phosphorylates nuclear membrane proteins (lamins) causing membrane disruption

Characterisation of MPF Consists of two subunits; A and B Subunit A: Protein kinase Subunit B: Regulatory polypeptide called cyclin B Protein kinase present throughout cell cycle Cyclin B gradually increases during interphase (G1, S, G2) Cyclin B falls abruptly in anaphase (mid-mitosis)

Protein kinase (subunit A) Cyclin B (subunit B) MPF} Protein kinase (subunit A) Metaphase Ubiquitin Anaphase Prophase Interphase (G1-S-G2) Proteosome Telephase

How do Cyclin B levels decrease abruptly Proteolytic degradation Degraded in a protease complex present in eukaryotic cells called “The Proteosome” Specific proteins degraded by complex when tagged by a small peptide called ubiquitin

Cyclin B is tagged for Proteosome degradation at anaphase Tagged at N-terminus at sequence called Destruction box DBRP binds to Destruction box Guides Ubiquitin ligase to add ubiquitin molecules to Cyclin B Why is Cyclin B only degraded in anaphase DBRP = Destruction box recognition protein

Ubiquitin ligase adds ubiquitin when DBRP binds to the destruction box Protein de-phosphorylase MPF? P DBRP (active) P DBRP (inactive) Ubiquitin ligase adds ubiquitin when DBRP binds to the destruction box

Possible MPF phosphorylates DBRP causing Cyclin B destruction DBRP is normally inactive and is only activated in anaphase via phosphorylation Possible MPF phosphorylates DBRP causing Cyclin B destruction Binds to the destruction box Activates ubiquitin ligase to add ubiquitin to Cyclin B Cyclin B then targeted to the Proteosome for degradation

Cell Cycle Regulation The cell cycle is regulated by cyclins, cyclin-dependent kinases (CDKs), and cyclin-dependent kinase inhibitors (CDKIs) and is divided into 4 distinct phases (G1, S, G2, and M). G0 represents exit from the cell cycle. The cell cycle is driven by CDKs, which are positively and negatively regulated by cyclins and CDKIs, respectively. The restriction point governs the transition point beyond which progression through the cell cycle is independent of external stimuli. Adapted from Shah and Schwartz. Clin Cancer Res. 2001;7:2168-2181, with permission.

Regulation of the Cell Cycle Cyclin: major control switch for the cell cycle Cdk (Cyclin-dependent kinase (phospohorylation)): major control switch; activated by cyclin; causes G1  S or G2  M. Cyclins form the regulatory subunit and Cdks the catalytic subunit Checkpoints DNA damage checkpoints, including tumor suppressor genes p53: protein that blocks the cell cycle if DNA is damaged and can cause apoptosis. A p53 mutation is the most frequent mutation leading to cancer. Spindle checkpoints

What Are Cyclin-Dependent Kinases? Of the many proteins involved in cell cycle control, cyclin-dependent kinases (CDKs) are among the most important. CDKs are a family of multifunctional enzymes that can modify various protein substrates involved in cell cycle progression. Specifically, CDKs phosphorylate their substrates by transferring phosphate groups from ATP to specific stretches of amino acids in the substrates. Different types of eukaryotic cells contain different types and numbers of CDKs. For example, yeast have only a single CDK, whereas vertebrates have four different ones.

As their name suggests, CDKs require the presence of cyclins to become active. Cyclins are a family of proteins that have no enzymatic activity of their own but activate CDKs by binding to them. CDKs must also be in a particular phosphorylation state — with some sites phosphorylated and others dephosphorylated — in order for activation to occur. Correct phosphorylation depends on the action of other kinases and a second class of enzymes called phosphatases that are responsible for removing phosphate groups from proteins.

All eukaryotes have multiple cyclins, each of which acts during a specific stage of the cell cycle. (In organisms with multiple CDKs, each CDK is paired with a specific cyclin.) All cyclins are named according to the stage at which they assemble with CDKs. Common classes of cyclins include G1-phase cyclins, G1/S-phase cyclins, S-phase cyclins, and M-phase cyclins. M-phase cyclins form M-CDK complexes and drive the cell's entry into mitosis; G1 cyclins form G1-CDK complexes and guide the cell's progress through the G1 phase; and so on. All CDKs exist in similar amounts throughout the entire cell cycle. In contrast, cyclin manufacture and breakdown varies by stage — with cell cycle progression dependent on the synthesis of new cyclin molecules.

Known CDKs, their cyclin partners, and their functions in the human Cyclin B M phase Cdk2 Cyclin E G1/S transition Cyclin A S phase, G2 phase Cdk3 Cyclin C G1 phase ? Cdk4 Cyclin D G1 phase Cdk5 p35 Transcription Cdk6 Cdk7 Cyclin H CDK-activating kinase, transcription Cdk8 Cdk9 Cyclin T

Cyclins and CDKs by Cell-Cycle Phase G0 C Cdk3 G1 D, E Cdk4, Cdk2, Cdk6 S A, E Cdk2 G2 A Cdk2, Cdk1 M B Cdk1