Chromosomes

Differences in Homologous Chromosomes

- Different origin (maternal, paternal) - Different alleles (forms of a gene)

Similarities Between Homologous Chromosomes

- Same genes at the same locus - Majority of the same DNA base sequence - Same length (in base pairs) - Same centromere position - Same staining pattern

Why karyotype?

- karyotyping can reveal changes in chromosome number, such as trisomy 21 (Down syndrome). - karyotypes can indicate chromosomal sex - karyograms are a source of diagnostic information for specific birth defects, genetic disorders, and cancers.

List three ways in which the types of chromosomes within a single cell are different.

- size; number of base pairs - which genes they carry - sequence of nitrogenous bases - location of the centromere - banding pattern when stained

Autoradiograph impact on advancing scientific knowledge

1. Circular vs. Linear Chromosomes - Could determine differences between the prokaryotes and eukaryotes. 2. Size of Chromosomes - Size and number of chromosomes can be compared between Species 3. Semiconservative replication - Could actually observe DNA replication 4. Understanding movement of DNA during mitosis and meiosis - Visualization of chromosomes during cell division clarified understanding of mitosis and meiosis.

Steps of Karyotyping Explain Step 1

1. Sample collection and cell culture - White cells from a blood sample or amniotic cells from a developing fetus are ideal. - Amniocentesis: amniotic fluid is removed from the uterus. Amniotic fluid is the fluid that surrounds and protects a baby during pregnancy. Tests baby cells. Takes 2 weeks for results. - Chorionic Villus Sampling: a small sample of cells (called chorionic villi) is taken from the placenta. 10-14 days for results.

Steps of Karyotyping

1. Sample collection and cell culture 2. Inhibition of mitosis with drugs 3. Separate the chromosomes 4. Staining 5. Photography 6. Karyotype Preparation

Steps of Karyotyping Explain Step 2

2. Inhibition of mitosis with drugs - The cells in the samples are grown in a lab ("cultured") and then treated with drugs which arrest (stop) the mitotic process at metaphase - Chromosomes become condensed and microscopically visible during mitosis metaphase, which is why they are stopped there. - Certain drugs (like colchicines) interfere with the spindle fiber apparatus and the chromosomes will remain paired at the metaphase plate; hence, these drugs are used to `freeze' the chromosomes in place at a time when they are the most visible.

State the minimum chromosome number in eukaryotes. Give examples of species with this.

2n=2. The "jack jumper ant" and Australian daisy both only have two chromosomes per cell.

Steps of Karyotyping Explain Step 3

3. Separate the chromosomes - A hypotonic salt solution is added to the cultured cells which causes the cells to take in water and in effect swell up and burst. - This process causes the chromosomes which are tightly paired at the metaphase plate to spill out of the cell and separate from one another. This allows each chromosome to be individually seen.

Steps of Karyotyping Explain Step 4

4. Staining - Chemicals are used to stain the DNA which makes up most of the chromosome composition; in this way the chromosomes are highly visible under the microscope.

Steps of Karyotyping Explain Step 5

5. Photography - With a camera fitted to a microscope, photographs are taken of several `mitotic plates' in which the chromosomes are visible and individually separated. - Enlargements are made of the images, and the chromosomes are digitally cut out.

Steps of Karyotyping Explain Step 6

6. Karyogram preparation - The individual chromosomes are paired up according to size and banding pattern. - The larger chromosomes (# 1-5) are arranged on the top row, and in order of descending size, four more rows are constructed with the paired chromosomes. - Once the 22 pairs of autosomes have been paired up, the remaining sex chromosomes (XX or XY) are positioned at the bottom right of the karyotype.

Describe the process of creating a karyogram.

A cell is "frozen" in metaphase by applying chemicals that disrupt the mitosis spindle. A hypotonic solution is then added; water enters the cell and bursts it to separate the chromosomes from one another. This allows them to be stained and seen under a microscope. A photograph of the chromosomes is taken for organization.

What is chromosome number and type?

A distinguishing characteristic of a species. Organisms with differing numbers of chromosomes are not likely able to interbreed; all the members of a species will have the same number of chromosomes.

Describe the mechanism by which the SRY gene regulates embryonic gonad development.

A gene on the Y chromosome (SRY gene) causes embryonic gonads to develop as testes and secrete testosterone. The "sex determining region Y" (SRY) gene codes for a protein (also called SRY). The SRY protein is a transcription factor, which activates other genes that cause the development of seminiferous tubules in the embryonic gonads. This turns the gonad into a testes. Cells in the testes called Leydig cells then start secreting testosterone. The SRY gene is activated in weeks 6-8 of embryonic development. Since the gene is located only on the Y chromosome, the presence of the Y chromosome (the SRY gene) leads to development of male characteristics. Without the Y chromosome (and SRY gene) the embryo with develop as female.

Karyogram

A graphical photograph representation of the karyotype. An image of chromosomes arranged in a standard format.

Amniocentesis

A needle is inserted through the abdomen into the uterus. Amniotic fluid is withdrawn, which contains cells from the fetus that can be used in karyotyping. A risk is uterine contractions, miscarriage, and amniotic fluid leak.

What is each sex characteristic?

A spectrum. You can be anywhere along the spectrum and have a combination of traits.

Chorionic Villus Sampling

A thin tube is inserted into the uterus via the vagina and cervix. A small sample of placenta is removed, which contains fetal cells that can be used in karyotyping. A risk is bleeding.

How many chromosomes do all eukaryotic specifies have? Why?

All eukaryotic species have at least two chromosomes. This is because fusion of two haploid cells during fertilization leads to diploid zygote cell

What are the two procedures for obtaining fetal cells for production of a karyotype?

Amniocentesis and Chronic Villus Sampling

Autoradiograph

An image of things that have radioactivity. An image on an x-ray film or nuclear emulsion produced by the pattern of decay emissions (e.g., beta particles or gamma rays) from a distribution of a radioactive substance.

Urogenital Development

At 6 weeks, all human embryos have both Mullerian ducts (the precursors to the female reproductive system) and Wolffian ducts (the precursors to the male reproductive system).

Outline the advancement in knowledge gained from the development of autoradiography techniques.

Autoradiography is used to produce an image of a radioactive substance. The technique is used in cellular and molecular biology to visualize structures. For example, autoradiography can be used to visualize chromosomes, bands of DNA in electrophoresis gels, tissue samples, and single cells.

State an advantage of being diploid

Being diploid means yo have two copies of each chromosome and therefore have two copies of each gene that chromosome contains. So, if one of the chromosomes carries a deterimental allele of a gene, there is a second copy of the gene whose allele may be able to counter the affects of the negative version.

Outline sex determination by sex chromosomes.

Biological sex is determined by which sex chromosomes are present. XX = female XY = male The male parent determines the sex of the offspring by either passing on an X chromosome to make a female offspring or a Y chromosome to make a male offspring.

Describe Cairns' technique for producing images of DNA molecules from E. coli.

Cairns radioactively labeled DNA to product images of the molecule, this allowed humans to visualize and measure the length of DNA. - DNA was replicated many times in the presence of radioactive thymine in an incubator. As a result, the daughter DNA now contained radioactive thymine nucleotides. Digest the cell so that only the DNA remains. The radioactive DNA was left for about 2 months covered with a sheet of photographic film. the radioactive emissions from the DNA exposed the film (radioactive electrons react with photographic film), leading to an image of DNA (an autoradiograph). Cairns observed the circular DNA of a bacterial cell. Measure the chromosome length.

Outline conclusions drawn from the images produced by Cairns' autoradiography technique.

Cairns was able to see that prokaryotic chromosomes are circular. He also observed DNA replication occurring at the replication fork.

Species: Domestic Dog Scientific Name: Number of Chromosomes: Number of Genes:

Canis familiaris 78 20,000

List mechanisms by which a species' chromosome number can change. What might it lead to?

Chromosome Splitting and Chromosome Fusion It is a rare event in evolution that may lead to the formation of a new species.

List the characteristics by which chromosomes are arranged on the karyogram.

Chromosomes are photographed and arranged by size (largest to smallest), banding pattern, and centromere position.

Sex Determination in Mammals

Chromosomes to genes to gonads to hormones to genital phenotype to brain to behavior

Chromosome Fusion

Decreases the number of chromosomes. Two chromosomes fuse in the centromere region. Results in 1 chromosome when there used to be 2. Sometimes two different chromosomes grab onto each other and then fail to separate. Scientists have observed humans and other mammals with fused chromosomes.

Diploid Define: Notation: Function: Example Cells: Human Number:

Define: The cell or organism contains 2 complete sets of chromosomes (two copies of each chromosome), one from each parent. Notation: 2n Function: Two copies of each chromosome allows for a "backup" if one is damaged. Example Cells: Any body/somatic cell Human Number: 2n=46

Haploid Define: Notation: Function: Example Cells: Human Number:

Define:Haploid means that the cell or organism contains only one set of chromosomes (one copy of each chromosome); there are no chromosome pairs. Notation: n Function: Carry 1/2 the genetic information to the next generation. Example Cells: Gametes; sperm and ovum Human Number: n=23

Describe the use of a karyogram to diagnose Down syndrome

Down syndrome is caused by a nondisjunction of chromosome #21, resulting in three chromosome #21, which can be observed in a karyogram.

Outline the formation of a diploid cell from two haploid gametes

Gametes (egg and sperm) are haploid, which means they have one complete set of chromosomes. When the gametes fuse during fertilization, the two sets of chromosomes (one from the egg and one from the sperm) combine to create a diploid zygote.

Species: Human Scientific Name: Number of Chromosomes: Number of Genes:

Homo sapiens 46 23,000

Explain the relationship between the number of human and chimpanzee chromosomes.

Humans have 46 chromosomes and chimpanzees have 48 chromosomes. There is evidence that two chimp chromosomes fused to create 1 human chromosome (chromosome #2 in humans). Evidence of chromosome fusing can be seen when comparing human and chimpanzee chromosomes. Chimps have two chromosomes that appear to have fused to be one single chromosome in humans. The centromeres, genes and banding patterns of the two chimp chromosomes match with those of the one human chromosome. Early in human evolution (about 6 million years ago), these two chromosomes from a common ancestor of chimps and humans fused to form a single chromosome that has been inherited by all humans since. The chromosome has the same genes in the same order as in the two chimp chromosomes aligned.

State the number of nuclear chromosome types in a human cell

In the nucleus of a human cell there are 23 types of chromosomes. Males would have an additional type, the Y chromosome. There are 22 autosomes and 2 types of sex chromosomes.

Chromosome Splitting

Increases the number of chromosomes. One chromosome with genes A, B, C, D, and E will duplicate the centromere sequence, splitting at the centromeres because they are pulled to opposite poles during meiosis. Results in 2 chromosomes when there used to be 1.

The Spectrum of Overlapping Sex Characteristics

Male: 1. Gonads (testicles) 2. Penis 3. XY Chromosomes 4. Male hormones 5. Male phenotype 6. Male internal reproductive structures Female: 1. Gonads (ovaries) 2. Vagina/Vulva 3. XX Chromosomes 4. Female hormones 5. Female phenotype 6. Female internal reproductive structures Each corresponding number represents a spectrum; you can have any combination of traits and be anywhere along each spectrum.

Outline the structure and function of the Y-chromosome.

Much smaller than the X chromosome; a length of 57 million base pairs and about 460 genes. The short arm has 10 million base pairs.

Are all chromosomes in each species the same size?

No, the size varies from species to species.

Differentiation

One of the first genes that scientists identified as playing a role in sexual development is the sex determining region Y (SRY) gene/ It affects a key control point for the differentiation fo the immature gonads into testes. Absence of the SRY gene is one type of sex verification test for athletes to compete in women's events. The SRY gene is located typically on the Y chromosome.

Species: Asian Rice Scientific Name: Number of Chromosomes: Number of Genes:

Oryza sativa 24 38,000

Species: Chimpanzee Scientific Name: Number of Chromosomes: Number of Genes:

Pan troglodytes 48 22,000

Species: Round Worm Scientific Name: Number of Chromosomes: Number of Genes:

Parascaris equorum 2 20,000

What did Cairns' results show?

Prokaryotic cells have circular DNA DNA replication is semiconservative Prokaryotic cells have a single origin of replication

SRY Gene

Sex-determining Region Y Expression of SRY initiates testicular development of the undifferentiated gonads. The absence of SRY expressoin allows the default, female state, to develop.

What happened to Maria Martinez?

She was an olympic athlete disqualified from competition for failing sex test. This happened because she was born with testes which produce testosterone, but has no receptors for testosterone. Anatomically, she has undescended testes, and external female genitalia, but no uterus or ovaries. Her gender is female, and her sex is intersex.

Sex Chromosomes

The 23rd pair of chromosomes that determine the sex of an individual.

What causes testes-promoting genes versus ovary-promoting genes?

The SRY gene produces TDF. Activation of this makes testes-promoting genes. Repression of this makes ovary-promoting genes.

Karyotype

The characteristics of the chromosomes of an individual (number, type, shape). The description of the chromosomes as seen in the karyogram.

What does a karyogram show?

The chromosomes of an organism in homologous paris of decreasing length.

Autosomes

The first 22 homologous pairs of human chromosomes that do not influence the sex of an individual.

Outline the structure and function of the X-chromosome.

The larger of the two sex chromosomes; a length of 156 million base pairs and containing 1,805 genes. The short arm has 61 million base pairs.

Variation

The most common rule in biology for survival as a species.

Explain why the chromosome number of a species does not indicate the number of genes in the species.

The number of chromosomes does not indicate the number of genes. It's possible to have one large chromosome with many genes, or many smaller chromosomes with fewer genes. Likewise, its possible to have large chromosomes with relatively few genes or smaller chromosomes that are packed full of genes.

What does chromosome number in a species indicate?

The number of chromosomes in a species has no specific significance nor does it indicate any relationship between two species.

Karyotyping

The process of determining the karyotype by creating a karyogram.

Kinetochore

The protein in the centromere to which the spindle fibers attach.

Explain why the typical number of chromosomes in a eukaryotic species is an even number.

The reason why most eukaryotic organisms have an even number of chromosomes is sexual reproduction, in which each parent gives one set of chromosomes, resulting in an even number in the offspring. For example: parent 1: n=13 parent 2: n=13 Their offspring: 2n=26

Deduce the sex of an individual given a karyogram.

The sex from a karyogram is determined by the last pair (23rd in humans) of chromosomes. XX = female XY = male

What drugs are used in step 2 (inhibition of mitosis with drugs) of karyotyping?

These same types of drugs are commonly used as 'chemotherapeutic agents' to stop rapidly-dividing cells characteristic of most cancers.

What is similar about chromosomes within members of a species?

They have the same number of chromosomes.

What tool is used in amniocentesis? What is done with the extracted material? Fluid: Cells: Cell Culture:

Tool: Centrifuge Fluid: composition analysis. Cells: Karyotype, sex determination, biochemical, and recombinant DNA studies. Cell culture: biochemical studies, chromosomal analysis. Analysis using recombinant DNA methods.

Homologous Chromosomes

Two chromosomes, one of paternal origin, and one of maternal origin. They carry the same sequence of genes, but not necessarily the same alleles of those genes. One chromosome, from dad, is one chromatid unreplicated DNA with associated histones that duplicates during S phase of the cell cycle to create sister chromatids from the father. The same process occurs with one chromosome from the mother. The sister chromatids, one from each parent, are a homologous pair.

Early Embryo

Until about 2 months after fertlization, the immature gonads of human embryos are bipotential, meaning that they can develop into either ovaries or testes. The ultimate identity of the gonads results from the effects and interactions of many genes.

Genes for a Female

XX or XY with a mutated or missing SRY gene

Genes for Male

XY with SRY gene or XX with SRY gene

Sex includes...

structural and functional characteristics determined by sex chromosomes.

Gender includes...

the behavioral, cultural, and psychological traits typically associated with one sex.