CHROMOSOMES
Nature of science:
Developments in research follow improvements in techniques—autoradiography was used to establish the length of DNA molecules in chromosomes. (1.8)
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 sapiens and 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.
Developments in research follow improvements in techniques—autoradiography was used to establish the length of DNA molecules in chromosomes. (1.8)
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 sapiens and 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.
The chromosomes in prokaryotes
The nucleoid region of a bacterial cell contains a single, long continuous, circulated thread of DNA. Therefore this region is involved with the cell control and reproduction.
Prokaryote cells can reproduce using binary fission, whereas organisms such as plants and animals have to use sexual reproduction. Any time 2 parents are involved, the offspring will have pairs of chromosomes rather than a single chromosome. Because prokaryotes have only one parent they have only 1 chromosome.
Some prokaryotes also have plasmids but eukaryotes do not. Escherichia coli, like others prokaryotes have samlll loops of DNA tha are called extra copies of some of the genetic matieral of the organism. These loops are called plasmids. These smalls, circular DNA molecules are not connected to the main bacterial chromosome. The plasmids replicate independently of the chromosomal DNA. Plasmid DNA is not required by the cell under normal conditions, but it may help the cell adapt to unusual circumstance.
Prokaryote cells can reproduce using binary fission, whereas organisms such as plants and animals have to use sexual reproduction. Any time 2 parents are involved, the offspring will have pairs of chromosomes rather than a single chromosome. Because prokaryotes have only one parent they have only 1 chromosome.
Some prokaryotes also have plasmids but eukaryotes do not. Escherichia coli, like others prokaryotes have samlll loops of DNA tha are called extra copies of some of the genetic matieral of the organism. These loops are called plasmids. These smalls, circular DNA molecules are not connected to the main bacterial chromosome. The plasmids replicate independently of the chromosomal DNA. Plasmid DNA is not required by the cell under normal conditions, but it may help the cell adapt to unusual circumstance.
Eukaryotes chromosomes
The DNA of eukaryotes cells most often occurs in the form of chromosomes. Chromosomes carry information necessary for the cells 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 the next generation. When the cell’s not dividing the chromosoems are not visibles structures. During this phase the cells’s DNA is in form of chromatin. Chromatin is formed of strands of DNA and proteins called histones.
When looking at unfolded, DNA with an electron microscope you cans ee what looks like beads on a string. Each of the beads is a nucleosome. A nucleosome consists of 2 molecules of each of 4 different histones. DNA wrps twice around these eight protein molecules. The DNA attracted to the histones because DNA is negatively charged and the histones positively charged. Between the nucleosomes is a single string of DNA. There is often a fifth histone attached to the linking string o DNA near each nucleosome. This fifth histone leads to further wrapping.
When DNA is wrapped aroung the histones and then futher wrapped in even more elaborate structures, it is inaccessible to transcription enzymes. Therefore the wrapping of DNA regulates transcription.
Multiple chromosomes, eukaryotes have more than 1 chromosome. Most of htem have multiple pairs and each one will carry a different set of instruction for the cell.
Homologous chromosomes: the same genes but not always the same alleles
In a typical human cell, the 46 chromosomes can be grouped into 23 pairs called homologous chromosomes. Homologous means similar in shape and size and it means that the 2 chromosomes carry the same gene. The reason why there are 2 of each is because one is form mom and one is from dad. Although homologous chromosomes carry the same genes, they are not identical because alleles of the genes form each parent could be different.
Chromosomes only look like that when the cell is ready to divide. At this stage the two blue banded zones are part of the 2 connected sister chromatids forming a single chromosome attached at the centromere. Likewise the 2 red banded zone belong to its sister chromatids. Each chromatic includes the long arm as well as the short arm.
Chromosomes only look like that when the cell is ready to divide. At this stage the two blue banded zones are part of the 2 connected sister chromatids forming a single chromosome attached at the centromere. Likewise the 2 red banded zone belong to its sister chromatids. Each chromatic includes the long arm as well as the short arm.
Diploid and Haploid cells
The term diploid is used to describe a nucleus that has chromosomes organized into a pairs of homologous chromosomes. Most cells in the human body are diploid cells. And in such cells the nucleus contain a set of 23 chromosomes form the mom and 23 from the father. There is a category of cells that only contain 23 in total, the sex cells also called gametes. Because the chromosome 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. Generally speaking the vast majority of cells in sexually reproducing organisms are diploid, and only the gametes are haploid.
When an egg cell is fertilized by a sperm cell, a zygote is formed and the 2 haploid nuclei fuse together matching up their chromosomes into pairs. This means that in humans 2 haploid cells= 46, so diploid cells in humans have 23 pairs of chromosomes making a total of 46 chromosomes.
When an egg cell is fertilized by a sperm cell, a zygote is formed and the 2 haploid nuclei fuse together matching up their chromosomes into pairs. This means that in humans 2 haploid cells= 46, so diploid cells in humans have 23 pairs of chromosomes making a total of 46 chromosomes.
Chromosome number: a defining feature
As you can see, the number 46 for humans is very different compared with the number for a worm. One well studied worm, had 6 chromosomes, meaning its diploid number, 2n, is 6 and therefore its haploid number is 3. Although this is true for most of the cells, we have exceptions of haploid cells, when one chromosome is missing or an extra one. Generally speaking, the number of chromosomes is a characteristic feature of the cells of a specie.
Karyograms and karyotypes.
A karyogram is a representation of the chromosomes found in a cell arranged according to a standard format. The chromosomes are placed in order according to their size and shape. The shape depends mainly on the position of the centromere. A karyogram is used to show a person’s karyotype, which is the specific number and appearance of chromosomes in his or her cells.
Sex determination
The 23rd pair of chromosomes are called the sex chromosomes because they determine whether a person is male or female. The X chromosome is longer than the Y chromosome and contains many more genes. Unlike the other 22 pairs of chromosome, this is the only pair in which it is possible to find 2 chromosomes that are very different in size and shape.
In humans females there are 2 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 a girl
XX: girl
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
In humans females there are 2 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 a girl
XX: girl
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
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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.
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.