fertilisation + the Cell cycle

to produce a new induvidual the nuclei from the gametes have to combine in the process of fertilisation

in mammals the nucleus from one sperm enters the ovum, and the genetic material from the ovum and sperm fuse.

this forms a fertilised ovum called a zygote

the cell now contains genetic material from both parents.

in flowering plants the nuclei from the gametes also have to fuse in a process of fertilisation, this fertilisation takes place in the embryo sac within the ovule.

the pollen grain germinates on the style and a pollen tube grows down through the style towards the ovary, with its growth controlled by the tube nucleus.

the pollen grain contains two nuclei , the tube nucleus and the generative nucleus.

on germination of the pollen, the generative nucleus divides to form two haploid gamete nuclei which move down the pollen tube.

the tube grows through a small pore into the embryo sac and the two male gamete nuclei enter the sac.

one fuses with the egg cell to form a diploid zygote and the other fuses with two nuclei in the embryo sac called polar nuclei to form a triploid cell.

the diploid zygote divides to form the embryo, and the triploid cekk divides to form the seed's storage tissue, endosperm.

the diploid cell divides giving rise to numerous specialised cells, creating the huge variety of structures within a body.

the cell cycle is the pattern of events that causes a fertilised egg to develop into a coplete new body of an induvidual.

the cell cycle is divided into two main stages, interphase and division.

interphase is a time of organised activity where by the cell sythesises new cell componenets such as organelles and membranes, and new DNA.

the formation of new cellular proteins occurs throughout interphase, whereas DNA sythesis occurs during the S phase only.

the length of interphase varies depending on the role of the cell.

In the develping human embryo there is no  interphase for the first few divisions - as the zygot already contains the material needed to make the first 16 or so cells.

in these first few divisions the embryo divides without growing in size, producing smaller cells with each cell cycle. this makes the embryonic cell cycle much faster than those of any other body cells.

the S and G2 phase remain relatively constant in duration. Whereas the G1 phase is more variable , so can take days, weeks, months and even years to complete this stage.

the interphase nucleus is a fairly uniform, featureless structure with one or two dark patches which are nucleoli .

ribosomes are formed in the nucleioli and are made of protein and rRNA (ribosomal RNA) . the rest of the nucleus contains chromosomes.

during interphase the induvidual chromosomes are unravelled . this allows access to the genetic material enabling new proteins to be synthesised .

in preparation for cell divison , the cell sythesises extra cytoplasmic proteins and organelles . the cell must also produce copies of DNA for the 2 new cells.

after interphase is over the cell contains enough cytoplasm, organelles and DNA to form 2 new cells. the next stage is to share out both the DNA and the contents of the cytoplasm so that each new cell can function independantly.

The DNA is separated in nuclear divison (mitosis). cytoplasmic divison follows this.

cell divison is a continuous process, where a single cell with double the usual amounts of cell contents becomes 2 new cells.

There are 4 stages of nuclear divison (mitosis) called prophase, metaphase,anaphase and telophase.

prohase occurs after interphase once there is enough cell contents for two new cells to be produced.

during prophase the chromosomes condense , becoming shorter and thicker with each chromosome visible as two strands called chromatids which have two identicle strands.

these are effectively two chromosomes joined at one region called the centromere.

during prophase microtubules from the cytoplasm form a three dimensional stcture called the spindle.

the centrioles move around the nuclear envelope and position themselves at opposite sides of the cell.

these form the two poles of the cell that are involved in the organisation of the spindle fibres . the spindle fibres form between the poles ans the widest part of the spindle fibres is called the equator.

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.

the next stage of mitosis is called anaphase.

at this stage the centromeres split and the spindle fibres shorten pulling the two halfs of the centromeres in opposite directions.

one chromotid of each centromere is pulled to each of the poles.

anaphase ends when the chromotids reach both ends of the cell and the spindle fibres break down.

the last stage of mitosis is called telophase.

this is effectively the reverse of prophase.

the chromosomes unravel and the nuclear envelope reforms so that the two sets of genetic infomation become enclosed in seperate nuclei.

after nuclear divison cytoplasmic divison occurs, this is the last stage.

the cell surface membrane constricts around the centre of the cell, it is thought that proteins actin and myosin which are responsibe for the muscle contraction may also be responsible for cytoplasmic divison.

finally two new cells are formed.

then the cell cycle returns back to interphase for the entire process to undergo once again.