Introductory Biology

6.1 Chromosomes

Sex was invented by bacterias. Bacterias keep their structure, function and reproduction simple and reproduce every 20-30 minutes. Their DNA is a single circular loop. These prokaryotic bacteria reproduce asexually by going through the binary fission process. During binary fission: DNA is copied and then divide into two cells (forming new cell wall).

On the other hand, eukaryotic cells have a complex structure, function and reproduction system and usually have sex to reproduce. Their DNA have several separate chromosomes/lines and the DNA are into long linear groups. The DNA can be thought as a text book containing many information by using letters (DNA has only four letters). The bacterium only have one volume and humans have 1000 volumes of text book information in their DNA. Us humans clone ourselves all the time.  There are two ways the eukaryotic cells can reproduce; by mitosis and meiosis and both these processes first copy the DNA and then distributed between daughter cells. Mitosis occurs asexually for development, growth and repair. Meiosis is sexually performed and produce more gametes, eggs and sperms Meiosis takes a set of chromosomes and changes from diploid to haploid and make more somatic (sperm and egg) cells. Haploid gametes are one set of chromosomes/ sperm and egg.

Before cell division, eukaryotic cells form chromosomes. There is no relationship between the number of chromosomes and the complexity. A single chromosome may contain hundreds of genes and chromosomes control their own movement. Genes are parts of DNA coding for the production of proteins/RNA. Chromosomes are mainly DNA and we have about 20 trillion meters of DNA inside of us. In order for DNA to stay small to fit inside our body, it is coiled up by raping around the protein. After they are copied, a chromosome has two twin chromatids. Those chromatids are combined together at the centromere. The proteins are the cause of the DNA to coil up. There are several levels of supercoiling futher  which condense the chromosome. These are common chromosome Features: banding patterns, positions of the centromere, presence of satellites, and the length of the chromatids.

Human genome have two sets of 23 chromosomes (each chromosomes have a thousand genes = 23 thousand genes) and 46 chromosomes which all differ in its shape, size and set of genes. Homologous chromosomes are the two members of a pair and are identical (size, shape and set of genes). One from each parent and so the specifics of genes differ. Diploid are the double set of homologous chromosomes. A haploid (single set of chromosomes) is made after meiosis, which produces eggs and sperms.  Homozygous: genes with the same information and heterozygous: genes with different information.

Chromosomes determine the genetic traits. Autosomes are the chromosomes that DO NOT determine gender and sex chromosomes determine gender/other traits. There are different systems according to the species. For humans, its the Y that makes the guy and double X makes females. Grasshoppers make a male with one x and a female with double x.

The chromosome number result from non-disjunction as homologous chromosomes fail to separate during meiosis. Down syndrome is resulted when chromosome number 21 has three chromosomes. The extra chromosomes make extra proteins which influence the variety of aspects the baby acts. Non-disjunction is during meiosis when the chromosomes do not quite separate. When a women reaches 37, the percentage of getting a child with down syndrome increases. The baby cannot survive (abortion) when there is an extra chromosome in  any number except for numbers 13, 18 and 21. Karyotype is the picture of chromosomes of a dividing cell organized by size. Amniocentesis is one method to detect nearly 300 chromosome disorders and collects fetal cells from the amniotic fluid. The chorinoic villus sampling takes a sample of fetal cells from the placenta. Deletion, duplication, inversion, reciprocal translocation alternate the chromosomes structure. Deletion is when the segments of DNA are lost. Duplication is when the segments of DNA are copied. Inversion is the process where the segments of DNA are reversed and translocation is when the segments of DNA are moved. With the slight change with the number of chromosomes, it can cause a huge effect on the baby inside the mother’s stomach.


One thought on “6.1 Chromosomes

  1. Yurika,

    Great job of summarizing the concepts in 6.1 on chromosomes. Your blog is very thorough and well-organized. I like the way you’ve used italics to highlight the key terms.

    Mr. F.

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