Essential idea: Chromosomes carry genes in a linear sequence that is shared by members of a species.
Nature of science: Developments in research follow improvements in techniques—autoradiography was used to establish the length of DNA molecules in chromosomes.
Understandings: • Prokaryotes have one chromosome consisting of a circular DNA molecule. • Some prokaryotes also have plasmids but eukaryotes do not. • Eukaryote chromosomes are linear DNA molecules associated with histone proteins. • In a eukaryote species there are different chromosomes that carry different genes. • Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes. • Diploid nuclei have pairs of homologous chromosomes. • Haploid nuclei have one chromosome of each pair. • The number of chromosomes is a characteristic feature of members of a species. • A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length. • Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.
Applications and skills: • Application: Cairns’ technique for measuring the length of DNA molecules by autoradiography. • Application: Comparison of genome size in T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiensand Paris japonica. • Application: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum. • Application: Use of karyograms to deduce sex and diagnose Down syndrome in humans. • Skill: Use of databases to identify the locus of a human gene and its polypeptide product.
International-mindedness: • Sequencing of the rice genome involved cooperation between biologists in 10 countries.
Aims: • Aim 6: Staining root tip squashes and microscope examination of chromosomes is recommended but not obligatory. • Aim 7: Use of databases to identify gene loci and protein products of genes.
Guidance: • The terms karyotype and karyogram have different meanings. Karyotype is a property of a cell—the number and type of chromosomes present in the nucleus, not a photograph or diagram of them. • Genome size is the total length of DNA in an organism. The examples of genome and chromosome number have been selected to allow points of interest to be raised. • 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.
Understandings: Prokaryotes have one chromosome consisting of a circular DNA molecule.
As you can see in the diagram, the DNA above called the nucleoid region is circular DNA which, unlike eukaryotes, is not associated with any histone proteins
There is one copy of each gene except when the cell and its DNA are replicating
Understandings: Some prokaryotes also have plasmids but eukaryotes do not.
Plasmids are small separate (usually circular) DNA molecules located in some prokaryotic cells
Plasmids are also naked (not associated with proteins) and are not needed for daily life processes in the cell.
The genes in plasmids are often associated with antibiotic resistant and can be transferred from one bacterial cell to another.
Plasmids are readily used by scientists to artificially transfer genes from one species to another (ie. Gene for human insulin)
Understandings: Eukaryote chromosomes are linear DNA molecules associated with histone proteins.
Eukaryotic chromosomes are linear and are made up of DNA and histone proteins.
Histones are globular shaped protein in which the DNA is wrapped around.
DNA wrapped around 8 histone proteins is called a nucleosome.
The DNA wraps twice around the histone protein core.
Another histone protein is attached to the outside of the DNA strand. This helps maintain the colloidal structure of the nucleosome.
DNA, because of its negative charge is attracted to the positive charge on the amino acids of the histone proteins.
Understandings: In a eukaryote species there are different chromosomes that carry different genes.
Eukaryotic chromosomes are linear chromosomes that vary in length and in position of the centromere that holds the sister chromatids together
In humans there are 23 types of chromosomes. There are 22 pairs of autosomes. The 23rd pair are the sex chromosomes. Males have an X and a Y chromosome and females have two X chromosomes
Each chromosome carries a specific sequence of genes along the linear DNA molecule. The position where the gene is located is called the locus
All eukaryotic species contain at least two different chromosomes, but most contain more than only two
Understandings: Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.
Homologous chromosomes are chromosomes within each cell that carry the same genes
One chromosome came from an individual’s mother and one from the father
They have the same shape and size
These chromosomes pair up during meiosis
Even though these chromosomes carry the same genes, they could have different alleles
Understandings: Diploid nuclei have pairs of homologous chromosomes.
Diploid nuclei have two copies of each type of chromosome. One chromosome comes from the mother and one from the father.
Haploid gametes (sperm and egg) fuse during sexual reproduction which produces zygote with a diploid nucleus
This cell will then divide by mitosis to produce numerous cells, all with a diploid nucleus
Each nucleus has two copies of each gene, accept the sex chromosomes
Understandings: Haploid nuclei have one chromosome of each pair.
Haploid nuclei have one copy of each chromosome or one full set of the chromosomes in that particular species eg. Human 23 chromosomes
These are called gametes, which are sperm and egg
Human sperm and eggs each contain 23 chromosomes
Understandings: The number of chromosomes is a characteristic feature of members of a species.
Chromosome number is a characteristic feature of that species.
Chromosome number does not indicate how complicated an organism might be
Organisms with different numbers of chromosomes would unlikely be able to interbreed
Chromosome number tends to remain unchanged over millions of years of evolution; however, sometimes through evolution chromosomes can fuse together or split to change the number of chromosomes an organism contains
Understandings: A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.
In karyotyping, chromosomes are arranged in pairs according to their size and structure with the largest at chromosome pair 1 and the smallest at chromosome 22.
Chromosomes are stained during mitosis (generally in metaphase) in order to see the chromosomes, and a micrograph is taken of the stained chromosomes
This stained image of the chromosomes is called a Karyogram
The 23rd pair are the sex chromosomes. Females have two X chromosomes and males have one X chromosome and one Y chromosome.
Understandings: Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.
The X and Y chromosome determine the sex of an individual
The X chromosome is quite large in comparison to the Y chromosome and has a centromere that is located near the center or middle of the chromosome
The Y chromosome is relatively small with its centromere located near the end of the chromosome
If an individual has two X chromosomes they will be a female and if they have an X and a Y chromosome they will be a male
All other chromosomes are called autosomes and do not affect the sex of an individual
The X chromosome has many genes located on it essential to human development, while the Y chromosome has a small number of genes (some of these are shared with the X chromosome). The rest of the genes on the Y chromosome are only necessary for male development
Application: Cairns’ technique for measuring the length of DNA molecules by autoradiography.
Semi-conservative mode of replication of bacterial chromosome was also demonstrated by-J. Cairns
Using the technique of autoradiography Cairns first supplied the cells with suitable radioactive material like titrated thymidine (H3-TdR)
H3 is heavy isotope of hydrogen and it replaces normal hydrogen in thymidine to give rise to titrated thymidine).
This used because this will selectively label only DNA and will not label RNA, since the thymine base is absent in RNA. The titrated thymidine gets incorporated into DNA and replaces ordinary thymidine.
The cellular material is then sectioned or else the cells may be broken down to release the intact bacterial chromosomes on slides. These slides are then covered by photographic emulsion and stored in dark.
During this storage the particles emitted by titrated thymidine will expose the film, which can be developed. This photograph will then show the regions of the presence of tritium and thus indirectly show the presence of labelled DNA.
The results showed that autoradiographs from this replicating material prepared at regular known intervals demonstrated semi-conservative mode of replication.
In the figure below, one of the two strands in the daughter DNA molecules is derived from the parent molecule and the other is newly synthesized. In θ shaped figure, which is obtained in the second cycle of replication in presence of label, two arcs in the split region would never be equally labelled. For instance, one arc would be twice as heavily labelled as the other arc. This is what was actually observed by Cairns. The observations thus clearly supported the semi-conservative nature of replication.
Application: Comparison of genome size in T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiens andParis japonica. Organism Genome Size (base pairs) Description
T2 Phage
E. coli
D. melanogaster
H. sapiens
P. japonica Application: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.
Application: Use of karyograms to deduce sex and diagnose Down syndrome in humans.x
Skill: Use of databases to identify the locus of a human gene and its polypeptide (protein) product.
Type the gene name into the search of the website below and use the information to complete the chart below.