6.3 Eukaryotic Cell Cycle

4 phases

G1, S, G2, M(includes mitosis and cytokinesis)

G1

chromosomes are single unreplicated DNA molecules with associated proteins and involves cell growth and preparation for DNA replication.

S phase

DNA replication occurs and each chromosome is duplicated = consist of 2 sister chromatids that remained joined till mitosis

G2

prepares for mitosis by synthesising and assembling structures that move chromatids to opposite ends

What causes progress through phases

the action of cyclins and cyclin-dependent kinases (CDKs), which regulate the cell cycle checkpoints. Specific CDKs control G1-to-S transition at restriction point and other CDKs control other parts. CDKs not active until bound to cyclins.

How CDKs work

Cyclin binds to CDK changing it by exposing an active site. A protein substrate and ATP binds to the CDK and protein substrate is phosphorylated (phosphate group transferred from ATP to substrate). Phosphorylated protein regulates the cell cycle by triggering specific cellular processes.

G1/S Cyclin-CDK role

catalyses the phosphorylation of protein retinoblastoma (RB) which acts as an inhibitor of cell cycle at restriction point. RB’s 3D shape changes inactivating it = cell cycle can proceed

What could cause the cell cycle to pause at each checkpoint

G1 = DNA damage

S = Incomplete replication or DNA damage

G2 = DNA damage

M = chromosome unattached to spindle

If there is a problem, a signaling pathway results in the temporary halt of the cell cycle to allow for repair or corrections before progression. If damage is too serve cell undergoes apoptosis to prevent defective cells from causing potential harm

how many checkpoints are there

4 - 3 during interphase and 1 during mitosis

apoptosis

is the process of programmed cell death that occurs in multicellular organisms. It plays a crucial role in development and maintaining tissue homeostasis by eliminating damaged or unnecessary cells.

growth factors

A chemical signal that stimulates cells to divide by binding to specific receptors on target cells and activate signal transduction pathways that end cyclin synthesis

chromatin

The nucleic acid–protein complex that makes up eukaryotic chromosomes

state of chromosomes during interphase

consist of 1 or 2 linear double-stranded DNA molecules bound with many proteins forming chromatin. Before S phase the chromosomes are in a relaxed, uncoiled state, allowing for transcription and DNA replication. After S phase the chromosomes are duplicated forming sister chromatids joined by centromere

centromere

The region where sister chromatids join and microtubules attach to chromosomes

histones

Proteins that DNA wraps around to form nucleosomes, helping to organize and compact the DNA. Held together by ionic bonds

why can’t DNA be replicated during M phase

During M phase, chromosomes are highly condensed and organized, making the DNA strand inaccessible for replication mechanisms to function effectively.

nucleosomes

A portion of a eukaryotic chromosome, consisting of part of the DNA molecule wrapped around a group of histone molecules, and held together by another type of histone molecule. A chromosome is made up of many nucleosomes.

mitosis phases

prophase, prometaphase, metaphase, anaphase, telophase, cytokinesis

mitosis

Nuclear division in eukaryotes leading to the formation of two daughter nuclei, each with a chromosome complement identical to that of the original nucleus. With cytokinesis, makes up M phase of cell cycle.

Role of centrosome

organelle consisting of a pair of centrioles which help organise the microtubules in animal and protist cells during nuclear division

centrosomes during cell cycle

During S phase centrosome duplicates and at the start of prophase the 2 centrosomes separate and move to opposite ends

prophase

chromatin condense into visible chromosomes. spindle apparatus begins to form

2 types of spindle microtubules

polar microtubules - extend from each centrosome and overlap in the middle of the cell; kinetochore microtubules - attach to the kinetochores on the chromosomes.

prometaphase

nuclear envelope breaks down. Kinetochores form on each side of centromeres and microtubule spindles attach to them = each sister chromatid attached to opposite poles

metaphase

spindles rearrange chromosomes to line up at middle of cell

anaphase

spindles retract separating sister chromatids and pulling them to opposite sides. Microtubules also elongate the cell

telophase

spindle apparatus disbands, nuclear envelopes and nucleoli reform, chromatin decondense and the cell prepares for division.

cytokinesis

The cytoplasm must divide to form the two new cells, each surrounded by a cell membrane, and a cell wall in organisms that have one.

cytokinesis in animal vs plant cells

animals - cleavage furrow forms, pinching the cell membrane; plants - a cell plate forms from row of vesicles that fuse together, developing into a new cell wall.