Topic 3: Genetics 3.2 Chromosomes Essential idea: Chromosomes carry genes in a linear sequence that is shared by members of a species.

Understandings 3.2.U1 Prokaryotes have one chromosome consisting of a circular DNA molecule. 3.2.U2 Some prokaryotes also have plasmids but eukaryotes do not. 3.2.U3 Eukaryote chromosomes are linear DNA molecules associated with histone proteins. 3.2.U4 In a eukaryote species there are different chromosomes that carry different genes. 3.2.U5 Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes. 3.2.U6 Diploid nuclei have pairs of homologous chromosomes.

3.2.U7 Haploid nuclei have one chromosome of each pair. 3.2.U8 The number of chromosomes is a characteristic feature of members of a species. 3.2.U9 A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length. 3.2.U10 Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex

Applications and Skills 3.2.A1 Application: Cairns’ technique for measuring the length of DNA molecules by autoradiography. 3.2.A2 Application: Comparison of genome size in T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiens and Paris japonica. Application: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes,Canis familiaris, Oryza sativa, Parascaris equorum. 3.2.A3 Application: Use of karyograms to deduce sex and diagnose Down syndrome in humans. 3.2.A4 Skill: Use of databases to identify the locus of a human gene and its polypeptide product.

Prokaryotes vs. Eukaryotes DNA Size Organelles Nucleus Endosymbiotic Theory Examples: Escherichia coli, Homo sapiens

Prokaryotic Genome Contain a circular strand of DNA. May contain smaller circular strands called plasmids.

True or False? Prokaryotic cells are always unicellular while eukaryotic cells are always multicellular. ?

Eukaryotic Cells Eukaryotic chromosomes are linear DNA molecules associated with histone proteins

Histone Proteins DNA (- charge) and Histones (+ charge) DNA wraps twice around these 8 histones Regulating transcription: When DNA is wrapped around histones and further wrapped in more elaborate structures, it is inaccessible to transcription enzymes Nucleosome 

Table 3.2 A comparison of eukaryote chromosomes and prokaryote chromosomes ProkaryoteEukaryote Number of chromosomes 2 or more ShapeLinear HistonesPresent Presence of plasmids Never Organized into pairsYes

Table 3.2 A comparison of eukaryote chromosomes and prokaryote chromosomes ProkaryoteEukaryote Number of chromosomes 12 or more ShapeLinear HistonesPresent Presence of plasmids Never Organized into pairsYes

Table 3.2 A comparison of eukaryote chromosomes and prokaryote chromosomes ProkaryoteEukaryote Number of chromosomes 12 or more ShapeCircularLinear HistonesNot presentPresent Presence of plasmids Never Organized into pairsYes

Table 3.2 A comparison of eukaryote chromosomes and prokaryote chromosomes ProkaryoteEukaryote Number of chromosomes 12 or more ShapeCircularLinear HistonesNot presentPresent Presence of plasmids SometimesNever Organized into pairsNoYes

An allele is one specific form of a gene that differs from other alleles, but occupies the same locus as other alleles of the same gene. The genome is the total genetic information of an organism (i.e. all the genes on all the chromosomes for that organism).

Homologous Chromosomes 23 pairs=46 chromosomes Draw and label homologous chromosomes with: centromeres, sister chromatids, and the locus of a possible allele

Diploid and Haploid N=number of sets of chromosomes that a nucleus can have How many sets of chromosomes do humans have? For humans: haploid, n=23; diploid, 2n=46

Table 3.3 A comparison of types of cells and chromosome numbers SpeciesHaploid=nDiploid=2n Human (Homo sapiens)46 Chimpanzee (Pan troglodytes) 48 Domestic dog (Canis familiaris) 39 Rice (Oryza sativa)24 Roundworm (Parascaris equorum) 1 Fill in the blanks. Be careful when writing scientific names.

Table 3.3 A comparison of types of cells and chromosome numbers SpeciesHaploid=nDiploid=2n Human (Homo sapiens)2346 Chimpanzee (Pan troglodytes) 2448 Domestic dog (Canis familiaris) 3978 Rice (Oryza sativa)1224 Roundworm (Parascaris equorum) 12

The Karyotype is unique to a species as all can differ in: Number of chromosomes Size of chromosomes Shape of chromosome (as seen during mitosis)

All of the cells in your body have 46 chromosomes (except your gametes). This is called the diploid number

Fruit flies have 8 chromosomes. (diploid number of 8)

What is the diploid number of gorilla chromosomes?

Text What is the diploid number of dog chromosomes?

What is the diploid number of frog chromosomes?

Human diploid or 2N number

In order for you to pass on your genetic information to your offspring, you must donate one set (23) of your chromosomes.

Karyotyping A complete set of chromosomes is called a karyotype. Each chromosome has genes specific for that chromosome making it identifiable. Karyotyping is arranging the chromosomes in pairs according to their structure. The chromosomes are arranged depending upon: – Their length – The position of their centromere Karyotyping can be used to detect chromosome aberrations in foetuses. – eg: An amniocentesis to check for Downs syndrome (47 Chromosomes)

Karyotype vs. karyogram A karyogram is used to show a person’s karyotype (which is the specific number and appearance of the chromosomes in his or her cells). Imagine you are a scientist… how could you get someone’s karyotype?

What is the % chance that a baby will be a boy?

What is the % chance that a baby will be a girl?

What is the % chance that your future child will be a boy?

Autosomes vs. Sex Chromosomes

Human Chromosome: Female

Karyotype Lab Tips: #1 is longest, #22 is shortest Chromosomes have centromeres that are near the ends- not in the middle! If you have a male, the #23 chromosomes will not look alike (one is labeled X and the other Y) Use the dark/light banding patterns to help you match! Disorders: XXY- Kleinfelter’s syndrome Extra 13- Patau Syndrome Extra 18- Edward’s Syndrome Extra 21- Down’s Syndrome Missing sex chromosome- Turner’s Syndrome

The chromosomes of eukaryote cells (such as those from plants and animals) are complex in their structure compared to those of prokaryotes. The illustration below shows a chromosome during the early stage of meiosis. Here it exists as a chromosome consisting of two chromatids. A non-dividing cell would have chromosomes with the 'equivalent' of a single chromatid only. The chromosome consists of a protein coated strand which coils in three ways during the time when the cell prepares to divide

Banded chromosome: This light microscope photo is a view ofthe polytene chromosomes in a salivary gland cell of a sandfly. Itshows a banding pattern that is thought to correspond to groups ofgenes. Regions of chromosome puffing are thought to occur wherethe genes are being transcribed into mRNA (see SEM on right).  A polytene chromosome viewed with a scanning electron microscope(SEM). The arrows indicate localized regions of the chromosome that are uncoiling to expose their genes (puffing) to allow transcription of those regions. Polytene chromosomes are a special type of chromosome consisting of a large bundle of chromatids bound tightly together.

Karyotyping: the chromosome complement of a cell (and the organism) Male Karyotype Female Karyotype

Term Clarification The two DNA molecules formed by DNA replication prior to cell division are considered to be sister chromatids until the splitting of the centromere at the start of anaphase. After this, they are individual chromosomes. Sister Chromatids Chromosomes

Human Chromosomes Human Genome consists of 46 chromosomes 23 pairs with each pair called a HOMOLOGUE Homologues code for the same genes but maybe different versions of the gene called ALLELES (e.g eye colour gene comes in many forms such as blue, green brown etc)

Downs Syndrome

Down’s Syndrome (trisomy 21) heart defects, lung problems, intellect deficient

Patau’s Syndrome (trisomy 13) Eye, brain and circulation defects - rarely live few months

Klinefelter’s Syndrome under developed male sex organs, some breast development

47 XYY males Taller than average, lower intellect

Monosomy X Female, short, not sexually mature.

Find Databases for Genomes Compare the genome size for the following: T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiens Paris japonica

John Cairns Although Meselson and Stahl showed that DNA replication was semiconservative, their results did not provide any information on chromosome replication -- questions such as the number of places that replication started (origins) and the direction of replication remained to be answered.

John Cairns provided a very elegant demonstration of E. coli chromosomal replication in He radioactively labelled the chromosome by growing his cultures in a medium containing 3H-thymidine. The nucleoside base was incorporated uniformly into the bacterial chromosome. He then isolated the chromosomes by lysing the cells very very gently and placed them on an electron micrograph (EM) grid which he exposed to X-ray film for two months.

Cairns showed that: The E. coli chromosome is circular. This had been suspected from genetic mapping evidence but direct physical confirmation was a nice demonstration that the genetic results were valid. There is a single origin of replication in E. coli.. However, Cairns result were also open to some interpretation: His results did not allow one to discriminate between unidirectional or bidirectional modes of replication. His results were consistent with either model. The location of the origin of replication could not be determined as such. Although, it was clear that each chromosome had a single origin of replication, this did not establish whether every chromosome had exactly the same origin of replication.

The Cairns’ Technique This technique also showed a way of visualising chromosomes: It is impossible to get an accurate length of chromosomes during supercoiling in mitosis. This allows chromosomes to be seen when they are uncoiled so as to measure their size.

Diploid/Haploid Revision: Explain Mitoisis and give the result of this process in terms of number of chromosomes in the daughter cells Diploid cells have the full complement of chromosome (i.e an homologous pair for every chromosome and 2 sex chromosomes) 46 chromsomes (23 pairs) Given the shorthand ‘2N’ Haploid cells have only one of each of the pairs of chromosomes 23 chromosomes Given the shorthand ‘N’

Activity What is the common name and compare of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.

How Do We Get a Haploid Cell? Via a process called meiosis: This will be explained in the next topic