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Chromosome

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Chromosomes are large organic molecules that contain most or all of the genetic information of the cell or virus and a great many genes. This threadlike structure is found in the cells of every organism and is responsible for passing of characteristics to offspring is the process of heredity.

The structure of the chromosome consist of two long strands of Deoxyribonucleic Acid (DNA) that are bound together, and twisted into the shape of a helix. The DNA helix is organized additionally by becoming wrapped around proteins called histones forming a DNA-protein complex known as a nucleosome. During cell division, the nucleosomes coil further into a dense, visible (stainable) mass of DNA most people think of as a chromosome.

In a eukaryotic cell the chromosome is contained within the organelle known as the nucleus. Before the synthesis of DNA each of the chromosomes have only one double stranded DNA molecule: after the DNA has been replicated the two DNA molecules are called sister chromatids.(Purves, 196).

Variations Among Organisms

The number of genes per chromosome, the number of chromosomes, and the length of chromosomes varies considerably among the various organisms on Earth. Humans have 24 different chromosomes (22 pairs of autosomes plus 2 sex chromosomes), 46 total chromosomes with about 1000 genes each (30 to 38 thousand genes per cell). The human chromosomes range in size from 50 x 106 nucleotides in length, which is equal to 1.7 cm if fully extended - up to 250 x 106 nucleotides in the largest (which would extend 8.5 cm)..[1] The DNA in a single human cell would extend over 2 meters if attached end-to-end. However, during cell division (mitosis) the chromosomes condense into structures about 5µm long, which are then visible when stained under a standard compound microscope.

Animals No. Chromosomes Plants No. Chromosomes
Human 46 Petunia 14
Bovine 60 Apple 34
Feline 38 Tomato 24
Canine 78 Alfalfa 16
Goat 60 Potato 48
Sheep 54 Corn 20
Pig 38 Soybean 40
Bat 44 Barley 14
Horse 64 Pepper 24
Donkey 62 Lettuce 18
Comparative Sequence Sizes (Bases)
Yeast chromosome 3 350 Thousand
Escherichia coli (bacterium) genome 4.6 Million
Largest yeast chromosome now mapped 5.8 Million
Entire yeast genome 15 Million
Smallest human chromosome (Y) 50 Million
Largest human chromosome (1) 250 Million
Entire human genome 3 Billion

[2]

Chromosomes in Eukaryotes

In the Eukaryotic cell the nucleus has many large linear chromosomes. The chromosomes contained within this nucleus all have a centromere, which is a special sequence or structure in a part of the chromosome. Chromatin is a protein that is made up of DNA that is found with in the nucleus. This is what packages the chromosomes. While the chromatin goes through a phase called interphase the chromosome is not yet seen. The eukaryotic cell goes through a stage known as mitosis or meiosis. During this stage the cell turns into a form that is easier to move and is smaller. Because of the size of the cell the chromosome is then visible. When the microtubules form a spindle within the cell it can attach to the chromosome. [1]

Chromosomes in Prokaryotes

Structure of the Chromosome

Centromere.png

The chromosome is made up of mostly deoxyribonucleic acid and proteins. Nucleotides are the bases for how the DNA structure is formed. A gene occupies much of the space within the chromosome. Because of the way that the chromosome is shaped even when the DNA is confined the chromosome can still go through with transcription. RNA is the molecule that tell the DNA what type of proteins will be produced in the different areas of the chromosome. Chromosomes are what make the DNA replicate and reproduce making each of its offspring have half of the parent cells genes. [2]

Chromatin

Chromatin is the visible (stainable) mass of DNA and protein that is located within the nucleus. The DNA double helix combines with histone proteins and becomes densely packed to form chromatin. Before the cell goes through cell division the chromatin aggregates, which makes the structure that is most frequently known as the chromosome. (Purves, 70)

During mitosis the chromatin is more coiled because its nucleosomes (DNA histone complex) begin to coil. The chromatin continues to fold until the chromosomes begins to separate into distinct bodies. (Purves, 169-171) In some eukaryotes, chromatin is needed in order for transcription. This process is called chromatin remodeling.

There are two types of chromatin:

  • Euchromatin- contains DNA that is to be transcribed to RNA.
  • Heterochromatin- The genes are inactive and is not transcribed. (Purves, 294)

Histones

Histones are located within the chromosome. The histones are used in order to from nucleosomes. Inside of the nucleosomes are eight histone molecules, 146 base pairs of DNA, and histone (H1) (Purves, 171) in a nucleosome a histone is positively charged and DNA is negatively charged. When the histones and the DNA combine and the histone reduces its charge it will the give off the DNA. Acetylation is used to catalyze the histone to disaggregate the nucleosome (Purves, 294). A histone is any one of a group of basic proteins forming the core of a nucleosome, the structural unit of a eukaryotic chromosome (Purves, G-12).

Sexual Reproduction

Main Article: Sexual reproduction

During sexual reproduction each offspring receives a gamete from their parent. A parent gives their offspring chromosomes, which are the gamete. Each of the gametes have a set of chromosome. During sexual reproduction the gametes or egg and sperm combine in order to make on cell. This one cell is known as a zygote. The zygote now contains two sets of chromosome. Somatic cells are those that inhibit multicellular organisms that are not sued for reproduction these cells have two sets of chromosomes. One chromosome of these somatic cells are from their parent. These members appear to be the same size and appearance, which makes them homologous. Their genetic info are not identical to the two chromosomes, the number of chromosomes within the gamete is noted by n. the cell is called haploid. The fertilization is the fusion of two haploid gametes. The zygote is then the result. There is now 2 chromosomes and the zygote, which is call diploid. (Purves, 175)

Heredity

Sex chromosomes.gif

Heredity is the passing of characteristics throughout the human life to their offspring. Chromosomes use heredity to pass on the characteristics from a mom and a dad to their offspring. Genes are what are carried throughout the cell in order to pass on the right characteristics from one generation to the next. What makes up the genes are that the chemicals known as deoxyribonucleic acid or DNA. These long chains of deoxyribonucleic acid are found within the chromosome. Heredity is what determines the characteristics of your body and what you inherit from your parents. This one of the processes that are used in the process of the chromosome. (World Book)

DNA

Deoxyribonucleic acid is a chain-like molecule that can be found in every living cell on earth. There are sections within the DNA that are what determine the passing of characteristic throughout a families life. DNA is found in the nucleus of most cells. They are found in threadlike structures, which are called chromosomes. DNA directs many things within the cell like the formation, growth, and the reproduction of a cell. Through the process of cell division the chromosome will give an equal amount of chromosomes to each of the new cell so that the DNA will have remnants from the original cell. this is how the DNA and the characteristics from the parent organisms get passed on. (World Book)

Chromosome Fusion

Evolutionists have concluded that a type of chromosome fusion from ape-like ancestors into humans happened roughly 2 million years ago. This is determined to be irrefutable proof to some as clear-cut evidence of common ancestry evolution.[3][4] Creationists would agree that indeed a chromosomal fusion event did indeed create human chromosome #2. It is because the chimp specie has two fewer chromosomes then a human evolutionists consider this scientific finding a strong piece of evidence to vindicate their philosophical view of a specific origin of life.

This line of argument for common ancestry aligns with that used for pseudogenes and generally junk DNA. Pseudogenes emerge within similar loci of apes and humans, or what are called parallel intron gains and are considered evidence of Darwinian assumptions.

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References

  1. Molecular Biology of the Cell 3rd edition. 1994. by Bruce Alberts, et. al., Chapter 8 "The Cell Nucleus"
  2. Primer on Molecular Genetics by the Human Genome Research Project.
  3. Origin of Human Chromosome 2 Revisited
  4. Human Chromosome 2 is a fusion of two ancestral chromosomes By Alec MacAndrew
  • Purves, William K. et al. Life: The Science of Biology. Gordensville, VA. 2004.
  • Chromosome MSN Encarta
  • Chromosomes Wikipedia

External links