Understandings:
NATURE OF SCIENCE
Developments in research follow improvements in techniques: autoradiography was used to establish the length of DNA molecules in chromosomes.
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 sapiens, and Paris japonica.
● Application: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis
familiaris, Oryza sativa, and 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
Guidance
- Eukaryote chromosomes
The DNA of eukaryotic cells most often occurs in the form of chromosomes. Chromosomes carry information necessary for the cell to exist. This allows the organism, whether unicellular or multicellular, to survive. DNA is the genetic material of the cell. It enables certain traits to be passed on to the next generation. When the cell is not dividing, the chromosomes are not visible structures. During this phase, the cell’s DNA is in the form of chromatin. Chromatin is formed of strands of DNA and proteins called histones.
When looking at unfolded DNA with an electron microscope, you can see what looks like beads on a string. Each of the beads is a nucleosome. A nucleosome consists of two molecules of each of four different histones. The DNA wraps twice around these eight protein molecules. The DNA is attracted to the histones because DNA is negatively charged and the histones are positively charged. Between the nucleosomes is a single string of DNA. There is often a fifth type of histone attached to the linking string of DNA near each nucleosome. This fifth histone leads to further wrapping (packaging) of the DNA molecule and eventually to the highly condensed or supercoiled chromosomes.
When DNA is wrapped around the histones and then further wrapped in even more elaborate structures, it is inaccessible to transcription enzymes. Therefore, the wrapping or packaging of DNA regulates the transcription process. This allows only certain areas of the DNA molecule to be involved in protein synthesis.
- Multiple chromosomes
Eukaryotes have more than one chromosome. Most eukaryotes have multiple pairs of chromosomes, and each chromosome will carry a different set of instructions for the cell.
- Diploid and haploid cells
The term diploid is used to describe a nucleus that has chromosomes organized into pairs of homologous chromosomes. Most cells in the human body are diploid cells, and in such cells the nucleus contains a set of 23 chromosomes from the mother and 23 from the father. There is a category of cells that only contain 23 chromosomes in total: the sex cells, also called gametes. Because the chromosomes in sperm and egg cells do not come in pairs, but rather only have a single chromosome from each pair, they are said to be haploid. The adult form of animal cells is rarely haploid, but there are exceptions, for example male bee, wasp, and ant cells are haploid. Generally speaking, the vast majority of cells in sexually reproducing organisms are diploid, and only the gametes are haploid.
The variable n represents the haploid number, and it refers to the number of sets of chromosomes that a nucleus can have. For a human egg cell, n = 23. When an egg cell is fertilized by a sperm cell (a sperm is also haploid and therefore contains 23 chromosomes), a zygote is formed and the two haploid nuclei fuse together, matching up their chromosomes into pairs. Hence humans generally have a total of 23 + 23 = 46 chromosomes. This means that in humans, 2n = 46, so diploid cells in humans have 23 pairs of chromosomes making a total of 46 chromosomes.
Easily messed up definitions:
- Genome: the whole genetic information of an organism
- Gene: a heritable factor that consists of the length of DNA and controls a specific characteristic
- Allele: one specific form of a gene, differing from other alleles by one or a few bases only.
• Humans have diploid cells
–Diploid (2n) = two sets of each chromosome (2n = 46, where n is 23)
–Haploid (n) = one set of each chromosome
• Arranged in pairs known as homologous chromosomes = chromosomes pairs that are of the same length and staining pattern and possess genes for the same characters at the same length
- Karyotypes
•Organization of chromosomes of a cell according to size and structure
•Each human cell has 23 pairs of chromosomes
(22 pairs of autosomes, 1 pair of sex chromosome)
•Can detect gene mutations = change in base sequence of a gene (new alleles are formed from gene mutations)
- Sex determination
The 23rd pair of chromosomes are called the sex chromosomes because they determine whether a person is a male or a female. The X chromosome is longer than the Y chromosome, and contains many more genes. Unlike the other 22 pairs of chromosomes, this is the only pair in which it is possible to find two chromosomes that are very different in size and shape.
In human females there are two X chromosomes. When women produce gametes, each egg will contain one X chromosome. Human males have one X chromosome and one Y chromosome. When males produce sperm cells, half of them contain one X chromosome and half contain one Y chromosome. As a result, when an egg cell meets a sperm cell during fertilization, there is always a 50% chance that the child will be a boy and a 50% chance that the child will be a girl
• XX = female
• XY = male.
The chances remain the same no matter how many boys or girls the family already has.
Any chromosome that is not a sex chromosome is called an autosome, or autosomal chromosome. Humans have 22 pairs of autosomes and one pair of sex chromosomes
- 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.
NATURE OF SCIENCE
Developments in research follow improvements in techniques: autoradiography was used to establish the length of DNA molecules in chromosomes.
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 sapiens, and Paris japonica.
● Application: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis
familiaris, Oryza sativa, and 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
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
- Eukaryote chromosomes
The DNA of eukaryotic cells most often occurs in the form of chromosomes. Chromosomes carry information necessary for the cell to exist. This allows the organism, whether unicellular or multicellular, to survive. DNA is the genetic material of the cell. It enables certain traits to be passed on to the next generation. When the cell is not dividing, the chromosomes are not visible structures. During this phase, the cell’s DNA is in the form of chromatin. Chromatin is formed of strands of DNA and proteins called histones.
When looking at unfolded DNA with an electron microscope, you can see what looks like beads on a string. Each of the beads is a nucleosome. A nucleosome consists of two molecules of each of four different histones. The DNA wraps twice around these eight protein molecules. The DNA is attracted to the histones because DNA is negatively charged and the histones are positively charged. Between the nucleosomes is a single string of DNA. There is often a fifth type of histone attached to the linking string of DNA near each nucleosome. This fifth histone leads to further wrapping (packaging) of the DNA molecule and eventually to the highly condensed or supercoiled chromosomes.
When DNA is wrapped around the histones and then further wrapped in even more elaborate structures, it is inaccessible to transcription enzymes. Therefore, the wrapping or packaging of DNA regulates the transcription process. This allows only certain areas of the DNA molecule to be involved in protein synthesis.
- Multiple chromosomes
Eukaryotes have more than one chromosome. Most eukaryotes have multiple pairs of chromosomes, and each chromosome will carry a different set of instructions for the cell.
- Diploid and haploid cells
The term diploid is used to describe a nucleus that has chromosomes organized into pairs of homologous chromosomes. Most cells in the human body are diploid cells, and in such cells the nucleus contains a set of 23 chromosomes from the mother and 23 from the father. There is a category of cells that only contain 23 chromosomes in total: the sex cells, also called gametes. Because the chromosomes in sperm and egg cells do not come in pairs, but rather only have a single chromosome from each pair, they are said to be haploid. The adult form of animal cells is rarely haploid, but there are exceptions, for example male bee, wasp, and ant cells are haploid. Generally speaking, the vast majority of cells in sexually reproducing organisms are diploid, and only the gametes are haploid.
The variable n represents the haploid number, and it refers to the number of sets of chromosomes that a nucleus can have. For a human egg cell, n = 23. When an egg cell is fertilized by a sperm cell (a sperm is also haploid and therefore contains 23 chromosomes), a zygote is formed and the two haploid nuclei fuse together, matching up their chromosomes into pairs. Hence humans generally have a total of 23 + 23 = 46 chromosomes. This means that in humans, 2n = 46, so diploid cells in humans have 23 pairs of chromosomes making a total of 46 chromosomes.
Easily messed up definitions:
- Genome: the whole genetic information of an organism
- Gene: a heritable factor that consists of the length of DNA and controls a specific characteristic
- Allele: one specific form of a gene, differing from other alleles by one or a few bases only.
• Humans have diploid cells
–Diploid (2n) = two sets of each chromosome (2n = 46, where n is 23)
–Haploid (n) = one set of each chromosome
• Arranged in pairs known as homologous chromosomes = chromosomes pairs that are of the same length and staining pattern and possess genes for the same characters at the same length
- Karyotypes
•Organization of chromosomes of a cell according to size and structure
•Each human cell has 23 pairs of chromosomes
(22 pairs of autosomes, 1 pair of sex chromosome)
•Can detect gene mutations = change in base sequence of a gene (new alleles are formed from gene mutations)
- Sex determination
The 23rd pair of chromosomes are called the sex chromosomes because they determine whether a person is a male or a female. The X chromosome is longer than the Y chromosome, and contains many more genes. Unlike the other 22 pairs of chromosomes, this is the only pair in which it is possible to find two chromosomes that are very different in size and shape.
In human females there are two X chromosomes. When women produce gametes, each egg will contain one X chromosome. Human males have one X chromosome and one Y chromosome. When males produce sperm cells, half of them contain one X chromosome and half contain one Y chromosome. As a result, when an egg cell meets a sperm cell during fertilization, there is always a 50% chance that the child will be a boy and a 50% chance that the child will be a girl
• XX = female
• XY = male.
The chances remain the same no matter how many boys or girls the family already has.
Any chromosome that is not a sex chromosome is called an autosome, or autosomal chromosome. Humans have 22 pairs of autosomes and one pair of sex chromosomes