The Cell Cycle - Interphase
The CELL CYCLE can be divided into two parts: INTERPHASE and DIVISION.
INTERPHASE - is a time of intense and organised activity during which the cell synthesises new cell commponents such as organelles and membranes, and new DNA. The formation of new cellular proteins occurs throoughout interphase, whereas DNA synthesis occurs during S (for synthesis of DNA) phase. The S phase seperates the G1 phase from the G2 phase - (It goes G1, S, G2, MITOSIS, then CYTOPLASMIC DIVISION.)
In the developing human embryo there is no INTERPHASE for the first few divisions - the ZYGOTE already contains the material needed to form the first 16 or so cells. In the first few divisions, the EMBRYO divides without growing in size, producing smaller cells with each cell cycle. This makes the embryonic cell cycle faster than those of other body cells.
The Cell Cycle - Interphase cont.
The S and G2 phases of most cells remain relatively constant in duration. The length of the G1 phase varies, cells can take weeks, months or even years to complete this phase. Eg - Liver cells may divide only once every one or two years due to an extended G1 phase. In seeds the cells of the embryo usually remain in the G1 phase until GERMINATION occurs. Some cells such as nerve and muscle cells never divide again, remaining permanently in a non-dividing state. In plants, cell division is localised in small groups of cells called MERISTEM, with most other cells not dividing.
The Cell Cycle - Interphase cont.
The Interphase nucleus is a fairly featureless structure with 1 or 2 darker-staining regions called NUCLEOLI (singular NUCLEOLUS). Ribosomes are formed in the Nucleoli, and they give the Nucleoli their dark appearance in electron micrographs. RIBOSOMES are made of PROTEIN and rRNA (ribosomal RNA). The rest of the nucleus contains the chromosomes, which are made up of DNA associated with proteins.
During Interphase, the individual chromosomes are unravelled. This allows access to the genetic material enabling new proteins to be synthesised. In preparation for CELL DIVISION, the cell synthesises additional CYTOPLASMIC PROTEINS and ORGANELLES. The cell must also copy DNA for the 2 new cells. It is vital that this DNA is identical in both structure and quantity to the DNA in the original cell. This is achieved by DNA REPLICATION.
The Cell Cycle - Cell Division
By the end of INTERPHASE, the cell contains enough CYTOPLASM, ORGANELLES and DNA to form two new cells. The next step is to share out both the DNA and the contents of the cytoplasm so that each new cell can function INDEPENDENTLY. The DNA is seperated in NUCLEAR DIVISION (MITOSIS). CYTOPLASMIC DIVISION follows this.
Cell division is a continuos process, where a single cell with double the usual amount of cell contents becomes new cells. However, it is possible to describe four different stages during NUCLEAR DIVISION by the behaviour of the chroosomes and other structures within the cell. The stages of mitosis are known as PROPHASE, METAPHASE, ANAPHASE and TELOPHASE.
INTERPHASE - in depth
During INTERPHASE, the new cell organelles are synthesised and DNA replication occurs. By the end if interphase, the cell contains enough cell contents to produce two new cells.
PROPHASE - in depth
During PROPHASE, the chromosomes condense, becoming shorter and thicker, with each chromosome visible as two strands called CHROMATIDS. Apart from the occasional mutation, the two strands are identical copies of once another, produced by replication. They are effectively two chromosomes joined at one region called the CENTROMERE.
During PROPHASE, microtubules from the cytoplasm form a three-dimensional structure called SPINDLE. THE CENTRIOLES move around the nuclear envelope and position themselves at opposite sides of the cell. These form the two poles of the spindle, and are involved in the organisation of the spindle fibres. The spindle fibres form between the poles. The widest part of the spindle is called the EQUATOR.
METAPHASE - in depth
The breakdown of the nuclear envelope signals the end of prophase and the start of METAPHASE. The chromosomes' centromeres attach to spindle fibres at the equator. When this has been completed the cell has reached the end of METAPHASE.
ANAPHASE - in depth
In the next stage of mitosis, ANAPHASE, the centromeres split. The spindle fibres shorten, pulling the two halves of each centromere in opposite directions. One chromatid of each chromosome is pulled to each of the poles. Anaphase ends when the separated chromatids reach the poles and the spindle breaks down.
TELOPHASE - in depth
The last stage of mitotic division is called TELOPHASE. This is effectively the reverse of prophase. The chromosomes unravel and the nuclear envelope reforms, so that the two sets of genetics information become enclosed in separate nuclei.
CYTOPLASMIC DIVISION -in depth
After nuclear division, the final reorganisation into the two new cells occurs. This is called CYTOPLASMIC DIVISION. In animal cells, the cell surface membrane constricts around the centre of the cell. A ring protein filaments bound to the inside surface of the cell surface membrane is thought to contract until the cell is divided into two new cells. It has been proposed that the proteins ACTIN and MYOSIN, responsible for the muscle contraction, may also be responsible for CYTOPLASMIC DIVISION. Instead of undergoing this constriction, plant cells synthesises a new cell plate between the two new cells.