3.2: Chromosomes

*List differences in the genetic material of prokaryotes and eukaryotes.* Understanding: Prokaryotes have one chromosome consisting of a circular DNA molecule.

*Prokaryotic DNA* -Circular -One chromosome -Naked -Plasmids may be present -No intron sequences -Found in nucleoid region -One origin of DNA replication *Eukaryotic DNA* -Linear -Multiple chromosomes -Associated with histones -No plasmids -Intron sequences present -Contained in membrane bound nucleus -Multiple origins of DNA replication

*State the human cell haploid number.* Understanding: Haploid nuclei have one chromosomes of each pair.

*n=23* Where n: the number of chromosome types =: equals 23

*Define "diploid."* Understanding: Diploid nuclei have pairs of homologous chromosomes.

Diploid mean that the cell contains *two complete sets of the chromosomes*, one chromosome originating from each parent.

*Describe the use of a karyogram to diagnose Down syndrome.* Application: Use karyograms to deduce sex and diagnose Down Syndrome in humans.

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

*Describe the process of creating a karyogram.* Understanding: A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.

A cell is "frozen" in metaphase by the application of chemicals that disrupt the mitotic spindle. A hypotonic solution is added; water enters the cell causing it to swell and burst, separating the chromosomes from each other. The chromosomes are stained and viewed with a microscope. A photograph of the chromosomes is taken. The images of the chromosomes are then organized in a standard pattern.

*Define "karyogram."* Understanding: A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.

A karyogram is a *micro-photograph of all chromosomes of an individual* represented in a standard format.

*Define "sex chromosome."* Understanding: Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.

A sex chromosome is *a chromosome involved with determining the sex of an organism*, typically one of two kinds, X or Y in humans.

*Define "autosome."* Understanding: Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.

An autosome is *any chromosome that is not a sex chromosome.*

*Outline the advancement in knowledge gained from the development of autoradiography techniques.​* Nature of Science: Developments in research follow improvements in techniques- autoradiography was used to establish the length of DNA molecules in chromosomes.

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 *radioactively stained* chromosomes, bands in DNA electrophoresis gels, tissue samples and single cells.

*Outline how to search an online database [BLAST] for a given gene.* Skill: Use of databases to identify the focus of a human gene and its polypeptide product.

BLAST (Basic Local Alignment Search Tool) is to the nucleotide or protein sequence database as a search engine is to the internet. BLAST is a sequence alignment tool that allows you to identify an unknown sequence, map a sequence in a genome or get clues about related sequences. Using BLAST is like doing an experiment; you must optimize the experimental conditions to get good results. Too wide a search can take a long time and give you many random and meaningless matches. Too narrow a search will possibly miss significant matches.

*State an advantage of being diploid.​* Understanding: Diploid nuclei have pairs of homologous chromosomes.

Being diploid means there are two copies of each chromosome, and therefore two copies of each gene that the chromosome carries. So, *if one of the chromosomes carries a detrimental allele of a gene, there is a second copy of the gene whose allele may be able to counter the effects of the mutated version*. Essentially there is a "backup set of genes."

*Outline sex determination by sex chromosomes.* Understanding: Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.

Biological sex is usually 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 produce a female offspring) or a Y chromosome (to produce a male offspring).

*Describe Cairn's technique for producing images of DNA molecules from E. coli.* Application: Cairns' technique for measuring the length of DNA by autoradiography.

Cairn's *radioactively labeled DNA to produce images of the molecule*. This allows him to visualized and measure the length of DNA molecules.

*Outline conclusions drawn from the images produced using Cairn's autoradiography technique.* Application: Cairns' technique for measuring the length of DNA by autoradiography.

Cairn's was able to see that prokaryotic chromosomes are circular and measuring lengths of chromosomes. He also observed the DNA replication fork.

*List ways in which the types of chromosomes within a single cell are different.* Understanding: In a eukaryote species there are different chromosomes that carry different genes.

Chromosomes within a cell are different in: - size (as measured by the # of base pairs) - the genes they carry - the sequence of the nitrogenous bases - the location of the centromere - the banding pattern when stained

*Explain the benefit of a publicly accessible genome database.* Skill: Use of databases to identify the focus of a human gene and its polypeptide product.

Effort has been made to make human genome sequence information freely accessible to researchers around the globe. Data obtained from Human Genome Project funded research must be publicly available. The rationale is that our ability to *expeditiously and effectively increase our knowledge of genetics* depends on the ability of researchers to access current information.

*Describe the structure of eukaryotic DNA and associated histone proteins during interphase.* Understanding: Eukaryotic chromosomes are linear DNA molecules associated with histone proteins.

Eukaryotic DNA is *linear and associate with histone proteins* in a structure called the *nucleosome*. During interphase, the DNA is not super-coiled into chromosomes; it is in a loose form called *chromatin*.

*Outline the formation of a diploid cell from two haploid gametes.* Understanding: Diploid nuclei have pairs of homologous chromosomes.

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

*Define "haploid."* Understanding: Haploid nuclei have one chromosomes of each pair.

Haploid mean that the *cell contains only one set of chromosomes*; there are no homologous pairs.

*Define "homologous chromosome."* Understanding: Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.

Homologous chromosomes *a chromosome pair* (one from each parent).

*State a similarity between pairs of homologous chromosomes.* Understanding: Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.

Homologous chromosomes have *similar length, the same genes at the same locus, the majority of the same DNA base sequence, the same centromere position and will stain with the same pattern.*

*List the characteristics by which chromosomes are arranged on the karyogram.​* Understanding: A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.

Images of the chromosomes are arranged by size (largest to smallest, sex chromosomes always last) and paired based on banding pattern and centromere position.

*List similarities in the genetic material of prokaryotes and eukaryotes.* Understanding: Prokaryotes have one chromosome consisting of a circular DNA molecule.

In both prokaryotic and eukaryotic cells: -The *DNA is double helix* made of two anti-parallel strands of nucleotides linked by hydrogen bonding between complementary base pairs. -The *replication of DNA is semi-conservative* and depends on complementary base pairing. -*DNA is the genetic code* for creating proteins through transcription and translation.

*Describe the structure and function of nucleoid DNA.​* Understanding: Prokaryotes have one chromosome consisting of a circular DNA molecule.

In prokaryotic cells, the *main DNA of the cell is collectively called the nucleoid*. Unlike in eukaryotic cells, the *nucleoid DNA is not enclosed in a membrane*. The nucleoid DNA is a double helix that *forms a circular loop* and *is not wrapped around histone proteins* (termed "naked.")

*List applications of karyogram analysis (karyotyping).​* Understanding: A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.

Karyotypes are used to prenatally identify the sex of the fetus and/or abnormal chromosome numbers (for example Down syndrome due to extra chromosome 21). The results of a karyogram analysis may lead to a decision to abort the fetus or to prepare for consequences of abnormality in offspring.

*Outline the procurement of fetal cells from which chromosomes are isolated for arrangement on a karyogram.​* Understanding: A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.

Karyotyping requires cells that are in metaphases, so the chromosomes are condensed and visible in the cells. In prenatal testing, fetal cells can be collected from chorionic villus sampling or by amniocentesis.

*Define the term "naked" in relation to prokaryotic DNA.​* Understanding: Prokaryotes have one chromosome consisting of a circular DNA molecule.

Naked means that the *DNA does not wrap around histone proteins*.

*State why chromosome number and type is a distinguishing characteristic of a species.* Understanding: The number of chromosomes is a characteristic feature of member of a species.

Organisms with differing numbers of chromosomes are usually not able to interbreed, maintaining the same number of chromosomes with the species.

*Describe the structure and function of plasmid DNA.​* Understanding: Some prokaryotes also have plasmids but eukaryotes do not.

Plasmids are *extra pieces of DNA found only in prokaryotic cells*. Like nucleoid DNA, plasmid DNA is *circular and naked* however plasmids are much *smaller* than the main nucleoid DNA and plasmids *replicate independently* of the nucleoid DNA. Plasmids are not found in all prokaryotic cells, *can be shared* between bacteria and often contain *genes for antibiotic resistance*.

*State the human cell diploid number.* Understanding: Diploid nuclei have pairs of homologous chromosomes.

Somatic human cells have a diploid number of *2n=46* Where 2: there are two of each of the... n: number of chromosome types =: for a total of 46= the number of chromosomes

*Outline the structure and function of the two human sex chromosomes.* Understanding: Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.

The *X chromosome* is the larger of the two sex chromosomes (a length of about 156 million bp and 1805 genes). The *Y chromosome* is much smaller (a length of 57 million bp and about 460 genes)

*List example haploid cells.​* Understanding: Haploid nuclei have one chromosomes of each pair.

The *eggs and sperm* of humans are haploid.

*Explain why chromatin DNA in interphase is said to look like "beads on a string."* Understanding: Eukaryotic chromosomes are linear DNA molecules associated with histone proteins.

The base unit of chromatin is the *nucleosome, a structure composed of DNA wrapped around histone proteins*. A chain of nucleosomes gives the appearance of "beads on a string."

*State a difference between pairs of homologous chromosomes.* Understanding: Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.

The genes and the position of the genes on each homologous chromosome are the same, however the *genes may be different alleles* (therefor slight differences in sequence of the gene).

*State the minimum chromosome number in eukaryotes.* Application: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascarsis equorum.

The minimum chromosome number in eukaryotes is *2n=2*

*Explain why the chromosome number of a species does not indicate the number of genes in the species.* Application: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascarsis equorum.

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, it's possible to have large chromosomes with relatively few genes or smaller chromosomes that are packed full of genes!

*Explain why the typical number of chromosomes in a species is always an even number.* Application: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascarsis equorum.

The reason why *most* eukaryotic organisms have an even number of chromosomes is because of *sexual reproduction,* in which each parent gives one set of chromosomes, resulting in an even number in the offspring.

*State the number of nuclear chromosome types in humans.* Understanding: In a eukaryote species there are different chromosomes that carry different genes.

There are *24 types of human chromosomes.* There are 22 autosomes and 2 types of sex chromosomes.

*Describe the relationship between the genome size of a species and the species complexity in structure, physiology and behavior.​* Application: Comparison of genome size in T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiens, Paris japonica.

There is a great *variety of genome sizes*. In general, *eukaryotes have larger genomes than prokaryotes*. However, the *size of the genome and the number of genes do not appear to correlate to a species "complexity."*

*Deduce the sex of an individual given a karyogram [female].* Application: Use karyograms to deduce sex and diagnose Down Syndrome in humans.

To determine sex from a karyogram, examine the last pair of chromosomes. *XX= female*

*Deduce the sex of an individual given a karyogram [male].* Application: Use karyograms to deduce sex and diagnose Down Syndrome in humans.

To determine sex from a karyogram, examine the last pair of chromosomes. *XY= male*