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Chromosomes are structures found in the nucleus of cells Chromosomes carry all of our genes, and therefore all of our genetic information Humans have.

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Presentation on theme: "Chromosomes are structures found in the nucleus of cells Chromosomes carry all of our genes, and therefore all of our genetic information Humans have."— Presentation transcript:

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2 Chromosomes are structures found in the nucleus of cells Chromosomes carry all of our genes, and therefore all of our genetic information Humans have 46 chromosomes, or 23 pairs, to carry our approximately 25,000 genes The first 22 pairs are called autosomes The 23 rd pair are the sex chromosomes or gonosomes this pair will either be XX or XY

3  During mitosis, the 23 pairs of human chromosomes begin condensing during prophase, and reach their most condensed state during metaphase  A karyotype analysis usually involves blocking cells in mitosis and staining the condensed chromosomes with Giemsa dye.  The dye stains regions of chromosomes that are rich in the base pairs Adenine (A) and Thymine (T) producing a dark band

4  A karyotype is a pattern or picture of chromosomes. The chromosomes are paired and arranged according to size  Each chromosome is paired with its homologous chromosome  its exact match in size and structure, though the homologous chromosomes may carry different alleles of the same gene  Then the chromosome pairs are labeled  The autosomes are numbered 1 through 22 according to size  The sex chromosomes are a different story ▪ The X and X or the X and Y are paired, then placed at the end, even though they are not necessarily the smallest chromosomes ▪ These chromosomes do not receive a number - just XX or XY

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6  A karyotype is an actual photograph of the chromosomes from one cell.  Karyotypes are usually done using blood cells, fetal skin cells (from amniotic fluid or the placenta) and occasionally bone marrow cells  It takes at least one week to complete a karyotype

7 1. Sample Collection 2. Separating cells 3. Growing Cells 4. Synchronizing Cells 5. Releasing Chromosomes from their cells 6. Staining Chromosomes 7. Analysis a. Counting & Sorting of chromosomes b. Looking at Chromosomes Structure

8  In newborns, a blood sample which contains red bloods cells, white blood cells, serum and other fluids is collected.  A karyotype will be done on the white blood cells which are actively dividing (a state known as mitosis).

9  During pregnancy, the sample can either be amniotic fluid collected during an amniocentesis ▪ The amniotic fluid contains fetal skin cells which are used to generate a karyotype  or a piece of the placenta collected during a chorionic villi sampling test (CVS).  The important thing is to choose a cell type that is actively dividing, some cells, like nerve cells, divide infrequently or never!

10  In order to analyze chromosomes, the sample must contain cells that are actively dividing (or in mitosis).  In blood, the white blood cells are actively dividing cells.  Sometimes adult cells are treated with chemicals such as phytohemaglutenin (PHA) to induce mitotis  Most fetal cells are actively dividing.  Once the sample reaches the cytogenetics lab, the non-divided cells are separated from the dividing cells using special chemicals.

11  In order to have enough cells to analyze, the dividing cells are grown in special media or a cell culture.  This media contains chemicals and hormones that enable the cells to divide and multiply.  This process of “culturing” the cells can take 3 to 4 days for blood cells, and up to a week for fetal cells

12  The best mitotic phase for chromosome analysis is prometaphase or metaphase  In order to get all the cells to this specific stage of cell division, the cells are treated with a chemical which stops cell division at the point where the chromosomes are the most compact.  cholicine

13  In order to see these compact chromosomes under a microscope, the chromosomes have to be out of the white blood cells.  This is done by treating the white blood cells with a hypotonic solution that causes  Chromosomes to spread apart for easier viewing  And causes cells to burst  This is done while the cells are on a microscopic slide. The leftover debris from the white blood cells is washed away, and the chromosomes are fixed to the slide.

14  Chromosomes are naturally colorless.  In order to be able to tell one chromosome from another, a special dye called Giemsa dye is applied to the chromosomes on the slide.  Giemsa dye stains regions of chromosomes that are rich in the bases adenine (A) and thymine (T).  When stained, the chromosomes look like strings with light and dark bands.  Each chromosome has a specific pattern of light and dark bands which enables cytogeneticist to tell one chromosome from another.

15  1 st use a protease to digest any protins  Giemsa reagent consists of a mixture od dyes including  Aminophenothiazine dyes(basic)  Eosin (acidic)  Produces light & dark bands  Patterns of banding change with the stage of mitosis  There may be as many as 1,000 bands in early prophase, and as few as 300 on a given chromosome in metaphase

16 LIGHT BANDS  GC rich  Most genes are in light bands  Light bands represent relatively open (uncoiled) regions of DNA making transcription easier DARK BANDSS  AT rich  Highly coiled regions of DNA  Contains few genes  Most genes in these regions are tissue specific, and so not transcribed in all cells

17  Once chromosomes are stained, the slide is put under the microscope and the analysis of the chromosomes begins.  A picture is taken of the chromosomes and at the end of the analysis  This is known as a “metaphase spread”  Take a picture of the spread using a special microscope with attached camera

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19  When we draw chromosomes, we typically draw them in the shape of the letter X  What does this represent (ie, label the important parts of the X structure)  Why, in the above metaphase spread are the chromosomes NOT in the X shape, but rather linear?

20  The first step of the analysis is counting the chromosomes.  Most humans have 46 chromosomes.  People with Down syndrome have 47 chromosomes.  It is also possible for people to have missing chromosomes or more than one extra chromosome.  By looking at just the number of chromosomes, it is possible to diagnose different conditions including Down syndrome, Turners, Kleinfelters

21  Sort the chromosomes by  comparing chromosome length,  the placement of centromeres (the areas where the two chromatids are joined),  the location and sizes of G-bands.  The chromosomes pairs are numbered from largest (number 1) to smallest (number 22).  There are 22 pairs of chromosomes, called autosomes, which match up exactly.  2 sex chromosomes ▪ two X‘s is a female ▪ X and a Y is a male.

22  In addition to looking at the total number of chromosomes and the sex chromosomes, the cytogeneticist will also look at the structure of the specific chromosomes to make sure that there is no missing or additional material, no structural abnormalities like  Translocations  Deletions  Duplications

23  Unfortunately in high school we aren’t allowed to stick needles in our students and/or work with human bodily fluids  Possible spread of infectious diseas  Parents complain  We will practice karyotyping from the analysis standpoint

24  The analysis involves comparing chromosomes for their  Length  placement of centromeres (areas where the two chromatids are joined)  location and sizes of G-bands.

25  Each chromosome has 2 arms separated by a constriction point called the centromere  Metacentric: term used to describe chromosomes with centromere at or near the center of the chromosome  Submetacentric: term used to describe chromosomes with centromere slightly displaced from the center

26  Very submetacentric: term used to describe chromosomes with centromere halfway between the middle and the tip of the chromosome  Acrocentric: term used to describe chromosomes with centromere very near one end

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28  Telocentric: term used to describe chromosomes with centromere at the very tip of one end  These are absent in human chromosomes

29  Notice, the centromere divides the chromosome into 2 separate regions  In general  The shorter chromosome arm is designated p for petite  The longer arm is designated the q arm, because the letter q comes after the letter p in the alphabet

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32  Open your worksheet titled Key to Human Chromosomes  It is a dichotomous key  Start by cutting chromosomes out of your metaphase spread  Divide all chromosomes into one of the 2 class sizes  Class 1 = 1 st 12 chromosome pairs (24), and X chromosomes ▪ So class 1 will contain 25 chromosomes for a boy and 26 chromosomes for a girl  Class 2 = the rest of the smaller chromosomes

33  Normal female  46, XX  Normal Male  46, XY  Telomeres, centromeres, and a number of prominent bands areused as landmarks on a chromosome

34  A section of a chromosome between 2 landmarks is called a region  Regions are numbered 1,2,3 in both directions starting with the centromere  Bands within a region are numbered according to the same rule  Example: the 1 st band in the 2 nd region of the p arm of chromosome #1 is indicated as follows  1p21

35  14 refers to chromosome # 14  q refers to the q arm (the longer arm) of chromosome 14  3 refers to region 3 of the q arm of chromosome 14  And 2 refers to the 2 nd band in the 3 rd region of the q arm in chromosome 14

36  There may also be sub-bands  Decimal points are used to indicate what sub- band  Example: 14q32.3  Refers to the 3 rd (last) sub-band within the 2 nd band of region 3 in the q arm of chromosome 14

37  45, X  45 chromosomes, 1 X chromosome  47, XXY  45 Chromosomes, two X chromsomes one Y chromosome

38  Use a + or – sign  before the symbol to indicate additional or missing whole chromosomes  After a symbol to indicate additional or missing segments of a chromosome  47, XY, +21  an additional 21 st chromosome  46,XX, 1q+  female karyotype with 46 chromosomes showing an increase in the length of the long arm of chromosome 1

39  Question marks are used to indicate uncertainty  45, XY, -?8 ▪ 45 chromosomes ▪ XY is male ▪ Missing a chromosome, probably #8  There is special nomenclature for inversions, deletions, translocations, insertions, etc.; but its is beyond the scope of this class

40  C-Banding: stains constitutive heterochromatin ▪ Regions of chromosomes that are highly compacted containing highly repetitive DNA  Results from selective removal of the chromosomes except from regions of the C-Bands  Used for determining presence or absence of a centromere, as well as for studying abnormalities of the centromeric region

41  Silver Staining for NOR  Ribosomal DNA, which codes for ribosomal RNA, occurs in multiple copies on the short arms of all acrocentric chromosomes  Silver stain results in the deposition of silver grains on the active nucleolar organizer regions (NORs)  In other words, NORs which were actively transcribing their rRNA’s in the preceding interphase, contain the proteins to which thre silver grains bind

42  Fluorescence In Situ Hybridization  Involves precise annealing of ssDNA probes to complementary target sequences  Can be visualized with a fluorescent microscope by using a probe directly labeled with a fluorophore  Can be used to find  If a specific gene is present (and or duplicated)  And if it is in the correct region of the correct chromosome

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45  Whole Chromosome Painting  Same idea as FISH, but you use a mixture of many fluorophore labeled probes  Must also use unlabeled blocking DNA to supress highly repetitive regions common to many different chromosomes  Allows for  Chromosome enumeration  Identification of chromosomal structural rearrangements

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47  Particularly useful in solid tumor and linical genetic research because it has the ability to aid in the analysis of complex and cryptic translocations  Also helpful in research that is evaluating the extent of chromosomal breakage and translocations following exposure to mutagenic chemicals or radiation

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49  A karyotype allows a cyto-geneticist or lab technician to examine the chromosomes and see if there is anything extra or missing, or if the structure of the chromosomes is grossly different than usual

50  Trisomy 21 is the presence of 3 chromosome 21’s.  Trisomy 21 causes the condition commonly known as Down syndrome  The extra chromosome leads to the specific characteristics of Down syndrome, some of which are very familiar  not all individuals with Down syndrome will show the exact same characteristics there is a great deal of variability

51  Individuals with Down syndrome have a typical facial appearance  All have some degree of mental retardation, but for most it is mild or moderate  Can learn to read, write, do some math, and live day-to to-day with minimal assistance from others  Others require a lot of attention and care  There are several possible health concerns, including  heart problems, hearing loss, feeding concerns, and others  Individuals with Down syndrome are now living longer than in the past – into their 50’s and 60’s  We are now discovering that those individuals who survive to this age are at very high risk of developing Alzheimer Alzheimer’s disease

52  Trisomy 13 is the presence of 3 chromosome 13’s, and causes the condition sometimes known as Patau syndrome  Trisomy 13 is a very serious condition with only about 5% of babies with the disorder survive past their first year  Most pregnancies involving Trisomy 13 end in miscarriage  All individuals with Trisomy 13 have severe mental retardation  Other characteristics commonly seen in people with Trisomy 13 include  a small head  extra fingers and/or toes  and a cleft lip or cleft palate  Trouble breathing, especially in their sleep  seizures

53  Trisomy 18 is the presence of 3 chromosome 18’s, & causes the condition sometimes known as Edwards syndrome  Only about 10% of babies with the disorder survive past their first year.  A majority of babies who survive are female.  Most pregnancies involving Trisomy 18 end in miscarriage

54  Children with Trisomy 18 usually have breathing problems, difficulty eating, and many have seizures. Some have serious heart conditions.  All individuals with Trisomy 18 have severe mental retardation.Most babies with Trisomy 18 are very small and have certain recognizable facial features. They also tend to overlap their fingers in a very distinct pattern

55  An individual with the genotype 47, XXY is male The person has 47 chromosomes just like someone with Trisomy 21, 13, or 18, but does not have a typical “trisomy,” as he has two of one chromosome and one of another  One could say, though, that such an individual has a sex chromosome trisomy  The extra X chromosome leads to features of the condition commonly known as Klinefelter syndrome

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57  Affects about 1 in 1000 males  Most males are taller than average and may have a different distribution of body fat (e.g. more than usual in the hips or chest)  Also tend to have sparse facial and body hair  Some have a degree of mental retardation, but many have normal intelligence  The most common feature is infertility ▪ It is estimated about 2% of infertile men have Klinefelter Syndrome  Since the characteristics of the syndrome are not always obvious, many males with Klinefelter will never be diagnosed

58  An individual with the genotype 45, X is phenotypically female  The person has 45 chromosomes instead of the usual 46 Instead of a trisomy, this would be called a monosomy X  One copy of an X chromosome = monosomy  Monosomy X is the only monosomyknown to be compatible with life  Having only one copy of an X chromosome leads to the features of the condition known as Turner syndrome

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60  Affects about 1 in 5000 newborn females  Physical features  Swelling of the hands and feet  webbing of the neck  broad chest  May also have features which affect their health, including  heart disease  a“horseshoe-shaped”kidney  Individuals with Turner syndrome will not typically have mental retardation, but may have specific learning disabilities

61  Why would the presence (or absence) of a chromosome lead to certain characteristics in a person?  Why wouldn’t the physical, mental, and behavioral features be exactly the same for two people with the same extra (or missing) chromosome?  Why are babies with trisomy 21 more likely to survive than babies with trisomy 13 or 18?  Why do you think monosomy X (Turner syndrome) is the only monosomy in which the individual survives?


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