Cell Biology help needed please?

Janet

First I shall answer PART 2>:

The maternal-effect genes, including bicoid and nanos, are required during oogenesis. The transcripts or protein products of these genes are found in the egg at fertilization, and form morphogen gradients. The maternal-effect genes encode transcription factors that regulate the expression of the gap genes. The gap genes roughly subdivide the embryo along the anterior/posterior axis. The gap genes encode transcription factors that regulate the expression of the pair-rule genes. The pair-rule genes divide the embryo into pairs of segments. The pair-rule genes encode transcription factors that regulate the expression of the segment polarity genes. The segment polarity genes set the anterior/posterior axis of each segment. The gap genes, pair-rule genes and segment polarity genes are together called the segmentation genes, because they are involved in segment patterning.

Coming to Part 1:

In 1953, Alma Howard and S. R. Pelc were the first to designate the phases of interphase as G1, S and G2. They did this by measuring the incorporation of radioactive (32P) into the growing root tips of broad beans (Vicia faba). They proved that about 87% of the 30 hour cell cycle in broad beans is spent in interphase and about 20% of the cell cycle is spent in DNA synthesis. The length of a cell cycle varies tremendously, depending on the type of cell, the type of plant, its health, age temperature, an many other factors.

The interphase in the stage between two divisions, beings the longest. In a typical human cell, 89 out of 90 hours are spend in interphase. During this phase metabolic activities are high and no division occurs and hence also called resting phase.

G1 phase

G1 phase or gap 1 phase or first growth period is the first stage after division. One the most important process it undergoes is the synthesis of nucleotide that are use of the next round of DNA replication. In algae the cell cycle also is brief for several hours. Short cell cycle also occur in growing embryos and roots. On the other hand cell cycle may be from day to months in plants that grow slowly. The G1 phase also varies in accordance to the cell cycle. In general, G1 is the longest part of the cell cycle. If can last even for years. The nerve cells remain permanently in G1 phase. Generally this phase constitutes 25-50% of the total interphase.

During this phase the daughter cells grows in size. mRNA, rRNA and tRNA and proteins are synthesized.

If a cell undergoes cell cycle arrest it does not enter in G, S and G phase and insted enters Go phase Where it begin to differentiate and nature.

S PHASE

'Making chromatin is the major takes of the this phase. 'S' stand for 'Synthesis of DNA : During this phase DNA molecule replicates and the period last for 35-40% of the interphase Genes that are formed of nucleotides replicate at this stage. In fast thousand of genes held together in a linear sequence in a chromosome in form of bead. The heads are nucleosomes and are basic structural unit of The chromosome. The nucleosomes contain negatively charged DNA and positively charged histones (proteins). During the S phase, linking pieces of DNA as well as genes are replicated and new histone molecules couples with new DNA are formed. This the entre chromosome is duplicated. 1500 the chromosome with its two chromatids are held together by a centromere. This each chromosome is a double chromosome and twice as large as it was before S phase.

G2 Phase

In a few types of cell, G2 may be quite long and during much of it the cell physiology is similar to that in G1 . Following S phase , the cell progresses into G2 (gap 2 or premitosis) phase during which cells prepare for cell division. This phase usually lasts only about 3 to 5 hours. the alpha and beta tubulin necessary for the spindle microtubules is synthesized, and the cell is believed to produce proteins necessary for processing chromosomes and breaking down the nuclear envelop. Incultured animal tissue, if a cell whose nucleus is dividing is forced to use with one in G1 , the second cell's nucleus also begins the first steps of nuclear division. This is evidence that during G2 , the first cell produced factors necessary to start nuclear division and that these factors are located in the cytoplasm.G2 is the second growth phase where the nucleus increases in volume cell onganelles are doubled and spindle are formed for undergoing cell division.

G1, S and G2 and constitute the interphase portion of the cell cycle. After G2 cell division occurs.

DIVISION PHASE OF THE CELL CYCLE

It includes the M phase where mitosis occurs. This is of short duration of about 45 minutes in a typical human cell cycle of duration of 90 hours. A cell division involves two processes (1) division of the nucleus(Karyokinesis) and (2) division of the cytoplasm (cytoplasm).

G0 or Differential Phase

After division the daughter cells again start ( or enter) G1 phase. Here then gro

1) Ribosome, ER (protein synthesis) and Golgi (protein maturation, glycosylation and secretion) 2) Mitochindria 3) Plasma membrane (for cilia projection), cilia and flagella (?) 4) Lysosome, peroxisome, vacuole (digestive and ROS-neutralizing structures) 5) Chloroplasts. Not too sure about a few, but I think the general idea's about right. Hope this helps :)