Chromosomes and Life Cycles
Chromosome Structure
All cells contain a nucleus that has a membrane around it. Inside the
nucleus are chromosomes. The chromosomes are made from a molecule
called deoxyribonucleic acid - which is commonly known as DNA. DNA is
a very long molecule, so long in fact that the DNA from just one
person is long enough to stretch all the way to the moon. Reading this
it would be hard to understand how DNA could fit into our cells, but
this problem is overcome due to it being highly coiled to make it much
shorter, and is also divided into lengths called chromosomes.
DNA has two jobs to do - 1) It must store information, this would
account for why the DNA molecule has to be so long, as it has to store
a phenomenal amount, and 2) It must be able to copy itself so that
this information can be passed on to the next generation. It is
important that the DNA can make an exact copy of itself with as few
mistakes as possible so that problems will not occur due to faulty
genes in the next generation.
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A simplified picture of DNA is shown above.
The DNA is divided into sections called genes. Each gene contains all
the information for a single instruction. This instruction is how to
make a single protein, which is produced in the cells cytoplasm. The
body uses the different proteins produced by the genes in many
different ways. Every cell in your body contains about 40,000
different genes. All this may be more easily understood by reading the
next statement,
'If all the chromosomes in a nucleus are represented by a book, and
each chromosome is represented by a chapter, then each gene would
represent a single letter.'
From species to species the number of chromosomes varies. In humans
there are normally 46 chromosomes. However on rare occasions there can
be more than this, but it always leads to health problems. An extra
Trisomy 13 or Patau Syndrome” Trisomy 13 is a genetic disorder found in babies. It is also called Patau syndrome in honor of the physician who first described it, Krause Palau. Trisomy 13 is a genetic disorder in which there is three copies of chromosomes on Chromosome 13. Patau first described the syndrome and its involvement with trisomy in 1960. It is sometimes called Bartholin-Patau syndrome, named in part for Thomas Bartholin, a French physician who described an infant with the syndrome in 1656.
As part of the cell cycle, mitosis is the nuclear division of replicated chromosomes by the disconnection of the replicated chromosomes to form two genetically identical daughter nuclei. Escorted by mitosis is commonly the process of cytokinesis. The cytokinesis process entails a dividing cell splitting into two, resulting in the subdivision of the cytoplasm into two cellular suites.
DNA (Deoxyribonucleic acid) is a molecule found in in the nucleus of all cells in the body which carries our genetic information. DNA is found in the form of chromosomes, with a total of 23 pairs in the human body1. DNA holds the genetic coding for all our characteristics, i.e. our eye colour, body shape, and how we interact with others on a daily basis.
Cell division is extremely important; cells must divide in order to maintain an efficient volume to surface area ratio, allow organisms to grow and develop, and repair any damaged tissue. Cells are able to do all this through two processes: meiosis and mitosis. Without these processes, humans would not be able to do many of the basic functions we are so accustomed to, including growing, healing even the smallest cuts, and even reproducing! However, meiosis and mitosis, although both procedures for cell division, are very different.
1. DNA is a nucleic acid that carries the genetic information in the cell and is capable of self-replication and synthesis of RNA. DNA consists of two long chains of nucleotides twisted into a double helix and joined by hydrogen bonds between the complementary bases adenine and thymine or cytosine and guanine. The sequence of nucleotides determines individual hereditary characteristics.
Precise chromosomal DNA replication during S phase of the cell cycle is a crucial factor in the proper maintenance of the genome from generation to generation. The current “once-per-cell-cycle” model of eukaryotic chromosome duplication describes a highly coordinated process by which temporally regulated replicon clusters are sequentially activated and subsequently united to form two semi-conserved copies of the genome. Replicon clusters, or replication domains, are comprised of individual replication units that are synchronously activated at predetermined points during S phase. Bi-directional replication within each replicon is initiated at periodic AT-rich origins along each chromosome. Origins are not characterized by any specific nucleotide sequence, but rather the spatial arrangement of origin replication complexes (ORCs). Given the duration of the S phase and replication fork rate, adjacent origins must be appropriately spaced to ensure the complete replication of each replicon. Chromatin arrangement by the nuclear matrix may be the underpinning factor responsible for ORC positioning. The six subunit ORC binds to origins of replication in an ATP-dependent manner during late telophase and early G1. In yeast, each replication domain simply contains a single ORC binding site. However, more complex origins are characterized by an initiation zone where DNA synthesis may begin at numerous locations. A single round of DNA synthesis at each activated origin is achieved by “lic...
All cells contain a complex structure known as deoxyribonucleic acid (DNA). DNA is a chemical that determines how we are. The multiple combinations of its components are what makes a difference in each person. Long molecules of DNA are organized into chromosomes, which are grouped into 23 pairs. Then the chromosomes are broken down into short segments of DNA known as genes.
The short story by Jack Ritchie, titled “The Absence of Emily” clearly exemplifies the very reason the gender literary lense is used. The story portrays a fluidity between the power of men and women, depicts interaction between the two, and reveals the power each holds throughout the writing. It is clear that the simple difference of an X and Y chromosome severely shapes a life. Yet it is also clear the neither gender holds an absolute power over the other.
DNA – the very molecule that defines who we are. It is still fascinating that a molecule that is so small that is not visible to the naked eye determines not just our physical appearance but also our mental wellbeing. Over 60 years, the discovery of the double helix DNA had impacted various fields relating to Biology and Chemistry, contributing to the advancement of technology and subsequently mankind too.
Trisomy 21 is a chromosomal disorder. “It is caused by an extra chromosome 21.” (http://www.geneticcounselling.eu) This is due to nondisjunction. During meiosis a complication occurs which results in an extra chromosome. An offspring does not inherit Trisomy 21 (Down Syndrome), but other forms of the disorder can be passed along from a parent to a child.
“The effect of protein synthesis inhibition on the entry of messenger RNA into the cytoplasm of sea urchin embryos”, Hogan and Gross. J. Cell Biol. 49(3):692-701.
The DNA molecule is shaped like a twisted ladder. James Watson and Francis Crick solved the structure of DNA.Scientists, like Rosalind Franklin also helped to make this discovery.The model is called a double helix because two long strands twist around each other like a twisted ladder.
DNA (deoxyribonucleic acid) is a self-replicating molecule or material present in nearly all living organisms as the main constituent in chromosomes. It encodes the genetic instructions used in the development and functioning of all known living organisms and many viruses. Simply put, DNA contains the instructions needed for an organism to develop, survive and reproduce. The discovery and use of DNA has seen many changes and made great progress over many years. James Watson was a pioneer molecular biologist who is credited, along with Francis Crick and Maurice Wilkins, with discovering the double helix structure of the DNA molecule. The three won the Nobel Prize in Medicine in 1962 for their work (Bagley, 2013). Scientist use the term “double helix” to describe DNA’s winding, two-stranded chemical structure. This shape looks much like a twisted ladder and gives the DNA the power to pass along biological instructions with great precision.
The process of meiosis is different from many other forms of cell division. In meiosis the daughter cells only have half the number of chromosomes of the parent. What that means is that the cells produced in meiosis are not exact copies of the original cell. Meiosis is fragmented down into many stages. Every cell in the course of meiosis includes the cell growing, dividing, and spitting, and dividing again in order to create the four cells as the end product.
Meiosis is a specialized form of nuclear division in which there two successive nuclear divisions (meiosis I and II) without any chromosome replication between them. Each division can be divided into 4 phases similar to those of mitosis (pro-, meta-, ana- and telophase). Meiosis occurs during the formation of gametes in animals.