DNA replication and Mitosis

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Structure of DNA

DNA is in the form of a double helix with backbones consisting of sugars (deoxyribose) and phosphate. These are attached to nucleotides; Adenine, Thymine, Cytosine and Guanine. They are known as base pairs and each have a complementary nucleotide, Adenine and Thymine and Cytosine and Guanine. The double helix keeps it shape by hydrogen bonds between the nucleotides. Between Adenine and Thymine there is a double bond also known as a pyrimidine while between Cytosine and Guanine there is a triple bond called purines. One strand of the DNA (leading strand) has 5' to 3' while the other (lagging strand) 3' to 5'.

Replication of DNA

Before a cell can divide, it must double it's DNA to ensure the new cells receive a complete set of DNA. The DNA is first unwounded and unzipped by an enzyme called helocase. The single strand binding protein keeps the two strands apart. Then polymerase adds the matching nucleotides to the leading strand with 5' to 3' and it is synthesised continuously. The lagging strand (3' to 5') is synthesised discontinuously and needs RNA polymerase and okozaki fragments to fill in the missing nucleotides. After the RNA primer is finished doing this, DNA polymeras and DNA ligase join the okozaki fragments to the lagging strand. The molecules then rewind themselves back to the double helix.

Difference between DNA, Chromosomes and Chromatin

The difference between the three is that DNA is the molecule which contains the biological instructions that make each species and they are organised in structures to form Chromosomes. If you were too add up all the chromosomes in a one cell you would have 2m of DNA and all of this has to fit into a nucleus with the diameter or 5-10um. The DNA wraps around a protein called histone resulting in a DNA protein complex called chromatin.

Mitosis

Mitosis is only a small part of the cell cycle. Within interphase the cell grows, synthesises and then grows again and mitosis occurs in the following steps.

INTERPHASE: The DNA in the nucleus is duplicated to ensure that the new daughter cells receive a complete set of DNA. The centrosomes (Cylinders next to the nucleus) are also replicated. This prepares the cell for mitosis.

PROPHASE: The DNA begins to condense and the centrosomes go to opposite ends of the cell, leaving a trail of protein called microtubules running from one end to the the other. The nuclear envelope breaks and the microtubules attach to kinetochores in the middle.

METAPHASE: The chromosomes line up in the centre and the microtubules meet in the centre.

ANAPHASE: Is where the motor protein pull the chromosome pairs apart and they break at the centrosome and move to opposite ends of the cell.

TELOPHASE: The chromosomes begin to unravel and become threadlike, and from this two nuclei form and the two new cells become separated. Now that they are completely two new cells the cycle begins again.

Mitosis vs Meiosis

(one major difference)

In mitosis, the cells form two daughter cells using the process of IPMAT which only divides the cell once therefore the cells are asexual. The cells that undergo mitosis reproduce for growth and repair. The chromosome number stays the same in mitosis and therefore it produces two diploid cells containing 2n chromosomes (46). The process of meiosis produces four daughter cells using the process IPMAMAT which divides the cell twice therefore showing the cells are sexual. Cells that undergo meiosis reproduce for genetic diversity through sexual reproduction. The chromosome number is halved and therefore produces 4 haploid cells containing n chromosomes (23). When the sperm and ovum join together it produces 46 chromosomes to form a diploid cell.