How is DNA organized?
-its basic structure is the double helix
-this is then wound around proteins (called histones) to form chromatin. Under an electron microscope, it looks like beads on a chain. This is the form that DNA is stored in between cell divisions
-during cell division the DNA winds up more tightly and the chromatin coils on itself, looping and coiling to form thick rods called chromosomes, which are visible under the light microscope
What happens?
DNA is copied when it is uncondensed, then it condenses into chromosomes that have 2 halves (each a copy of the other). Each half is called a chromatid. Sister chromatids are identical. The point at which the DNA narrows and the chromatids are connected is called the centromere. Each chromosome has many genes, each gene defines a single characteristic.
The number and shape of chromosomes are species-specific. eg. Humans = 46 chromosomes, dogs = 78, pea = 14, fruit fly = 8
All sexually reproducing organisms have 2 sets of chromosomes, one from each parent (this is the diploid state). In humans a diploid cell has 46 chromosomes, half from the mother and half from the father (23). The chromosomes which carry the same kind of information are called homologous chromosomes.
Cell division
There are 2 types:
- mitosis (számtartó sejtosztodás): purpose is growth and repair, 2 identical daughter cells are produced
- meiosis (számfelező sejtosztodás): purpose is to produce gametes (sex cells) for reproduction, 4 genetically different cells are produced
The cell cycle describes the typical cycle of a somatic (body) cell that will go through mitosis:
During the first growth phase, the cell simply grows and carries out its normal functions. At a certain point, the cell enters the synthesis phase, where the DNA is replicated.
DNA replication refers to the creation of another DNA double helix using the first helix as a template. In order for this to occur:
1. The DNA double helix begins to unwind or unzip at one end to form a replication fork. Unwinding requires the help of an enzyme called a helicase.
2. Enzymes called DNA polymerases bind to the single strands of DNA. They then proceed to "read" the template strand (in the 5' to 3' direction) and add complementary nucleotides. Since the polymerase only travels in one direction, it will move more quickly along the leading strand, but on the lagging strand it will attach at the fork and move toward the end, until it meets up with a previously formed DNA strand fragment, then it will detach and reattach at the continuously unwinding replication fork. The fragments that are created in this way are called Okazaki fragments. They are "glued" together with the help of enzymes called ligases.
The end result is two semi-conservative daughter double helixes- meaning that each double helix contains one strand from the original and one strand that is new.
If you want to see a video: http://www.youtube.com/watch?v=teV62zrm2P0
Once DNA replication has occured, the nucleus then has 2 copies of all of its DNA and will continue to grow and carry out some normal functions, but it will also prepare for cell division, which is either mitosis or meiosis, depending on whether or not it is a cell that will simply copy itself, or a cell that is designed to produce gametes (eggs or sperm).
Mitosis is divided into 4 phases:
Prophase:
-chromatin condenses to chromosome
-nuclear envelope disintegrates and disappears
-spindle (magorsó) forms
Metaphase:
-chromosomes line up at the equator
Anaphase:
-chromatids are pulled to opposite poles of the cell
Telophase:
-cell plasma divides
-nuclear envelope reappears
(don't worry about the extra stages in the image below!!)
Image from: https://www.msu.edu/~robiemat/science.htm
Image from : http://imcurious.wikispaces.com/Midterm+Exam+2010+Review+P1
Meiosis occurs to produce haploid cells that will be gametes (sperm and eggs).
It is a division that reduces the chromosome number by half. It is divided into meiosis I and meiosis II
Meiosis I
Prophase I
-chromatin condenses to chromosomes
-chromosomes "find" their homologous pairs and crossing over occurs
Metaphase I
--nuclear membrane disappears
-homologous chromosomes line up at the equator and attach to spindle fibres
Anaphase I
- chromosomes pairs are split as they are pulled to opposite poles
Telophase I
- cell plasma divides
- nuclear membrane reforms
Short interphase, with no DNA replication
Meiosis II
Prophase II
-chromosomes condense
- nuclear membrane disappears
-spindle forms
Metaphase II
-chromosomes line up at the equator
Anaphase II
-chromatids are pulled to opposite poles of the cell
Telophase II
-cell plasma divides
-nuclear membrane forms
Image from: http://commons.wikimedia.org/wiki/File:Meiosis_diagram.jpg
So mitosis and meiosis share some characteristics, but are also unique in many ways. The following diagram presents a comparison of the two. Be sure to consider how they are similar and how they are different.
Image from: http://bioactive.mrkirkscience.com/09/ch9summary.html
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