X-chromosome inactivation

GENE 105 October 24, 2006 Continuation of Sex-linked traits and sex chromosomes X-chromosome inactivation Females have 2 X chromosomes and thus 2 ...
Author: Laura Dennis
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GENE 105

October 24, 2006

Continuation of Sex-linked traits and sex chromosomes

X-chromosome inactivation

Females have 2 X chromosomes and thus 2 copies of each X-lined gene. Males have 1 X chromosome and one copy of each gene on the X. In mammals, the process of X-inactivation helps balance the dosage of X-linked genes in females. The process of X-inactivation occurs during embryonic development and results in the repression (turning off) of the genes located on one of the two Xchromosomes in female cells. It is believed that X-inactivation occurs randomly that is either X chromosome (maternal or paternal-inherited) has an equal chance of being inactivated. (Barr body - inactive X looks much smaller than active X)

X-inactivation can expose X-linked recessive traits in a heterozygous female. X-inactivation gives rise to tortoiseshell and calico cats. • A gene that confers coat color is present on the X-chromosome. The dominant allele gives rise to black-brown fur color and the recessive allele causes yellowish-orange fur.

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Because X-inactivation is random in cats, a female cat that is heterozygous for this X-linked coat color gene will have patches of black-brown fur and yellowish-orange fur depending on which X chromosome is inactivated.



If X-inactivation occurs early in development, the patches will be large and if it occurs later in development, then the patches will be smaller.

Nucleic Acids

Reading DNA Structure Lewis Chapter 9 pp 171-179 Cartoon Guide 116-124 Nucleotide - the basic unit of nucleic acid is called a nucleotide and it contains a sugar containing a phosphate group (PO4) and nitrogenous base. The base can be A, adenine, G, guanine, C, cytosine, T, thymine, or U, uracil.



DNA - deoxyribonucleic acid or DNA is made of deoxynucleotides. A deoxynucleotide is one deoxyribose sugar, one phosphate group, and one nitrogenous base (purine or pyrimidine). Deoxyribose sugar, phosphate groups and the bases A, C, G, T are found in DNA.



RNA - Ribose sugar, phosphate groups, and the bases A, C, G, and U are found in ribonucleic acid or RNA. RNA is found in the cytoplasm and nucleus.

Historical Review of DNA 1928 - Fred Griffith - worked with Diplococcus pneumoniae, which cause pneumonia in mice. Griffith noticed two types of bacteria, (1) virulent and (2) harmless, that can be distinguished by how they grow on agar plates, and how deadly they are when used to infect mice.

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• The virulent strains form smooth shiny colonies on agar plates and cause

death when injected into mice • The harmless bacteria form rough bumpy colonies on agar plates and do not harm mice. • Griffith discovered that if he boiled the virulent bacteria (called heatkilled), they were no longer virulent. But if he mixed the harmless bacteria with the heat-killed virulent bacteria solution and injected this mixture into mice, the mice died. A substance in the heat-killed bacteria solution caused the harmless bacteria to become virulent! He called the substance the transforming principle because it was able to transform harmless D. pneumoniae into a virulent form. 1944 - Oswald T. Avery C. M. MacLeod, M. McCarty treated the heat-killed virulent bacteria solution with enzymes, DNases to break down DNA, or proteases to breakdown proteins. While the boiled solution of dead virulent bacteria treated with proteases was able to transform the harmless bacteria, the solution treated with DNases was not. Their results strongly suggested that DNA is the hereditary material. 1953 - Alfred Hershey and Martha Chase confirmed that DNA is the genetic material. They labeled bacterial viruses with radioactive sulfur to mark the proteins or with radioactive phosphate to label the DNA. They found that upon infection of bacteria with the viruses with the labeled DNA, the radioactivity was transferred into the bacteria. Infection with viruses with containing radioactive proteins did not lead to the transfer of radioactive proteins into the bacteria. These experiments proved that DNA was the genetic material. DNA Structure – the following bits of information were used to figure out the structure of DNA 1. 1950 - Erwin Chargaff showed that in DNA, the numbers of adenines (A) and thymines (T) are always equal, as are the numbers of guanines (G) and cytosines (C). 2. 1953 - Rosalind Franklin used x-ray diffraction to determine the structure of DNA. She passed x-rays through DNA in the form of a fiber and the shadow produced suggested that DNA has a helical structure.

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Based on the findings of Chargaff and Franklin, Jim Watson and Francis Crick proposed the model that DNA is comprised of two helically intertwined chains held together by hydrogen bonds between purines and pyrimidines.

Nucleotides join through covalent bonds between the phosphate group at the 5’ carbon of the deoxyribose sugar and the 3’ hydroxyl group of the deoxyribose sugar. These long chains of sugar-phosphate bonds form the sugar-phosphate backbone.

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HYDROGEN BONDS - weak attractions that occur between a hydrogen molecule in one molecule and a non-hydrogen on another molecule. Base pairs • A pairs with T through two hydrogen bonds • C pairs with G through three hydrogen bonds Two complementary DNA strands form an antiparallel double helix with one strand running 5’ to 3’ and the complementary strand running 3’ to 5’. Directionality is determined by the polarity of the sugar-phosphate backbone.

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DNA is stored in the nucleus as chromatin. Chromatin consists of double stranded DNA wrapped around highly basic histone proteins (this is called a nucleosome). Nucleosomal DNA is associated with other DNA-binding proteins as well.

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