3.2 Chromosomes Flashcards
Prokaryotic chromosomes
and plasmids
- A single chromosome that consists of a circular DNA molecule
- The area where the chromosome is found is called the nucleoid
- It contains the entire genome
- Prokaryotic DNA is called ‘naked’ DNA because it is not organised around histone proteins
- Lack of proteins allow chromosomes to supercoil into a compact size
Plasmids:
- Some prokaryotes also have plasmids (which carry a few genes and are capable of self replication)
- They are not found in eukaryotic cells
- They allow the sharing of genetic material between bacterial cells
- Helps bacteria resist antibacterial drugs
Plasmids
- Only in prokaryotes
- Contain one or several genes often related to a specific function like digesting a particular food or resisting an otherwise deadly antibiotic
- Plasmids can be easily exchanged between bacteria (via their sex pili - process called conjugation), providing a mechanism by which antibiotic resistance spreads
Eukaryotic chromosomes
- Linear DNA molecules wrapped around proteins called histones
- Chromosomes are found in the nucleus
Homologous chromosomes
Carry the same sequence of genes, but not necessarily the same alleles of those genes.
Homologous chromosomes have the same structural features (size, centromere positions) and the same genes at the same loci positions (the alleles may be different)
- Because sexually reproducing organisms inherit their genetic sequences from both parents, these organisms witll possess two copies of each chromosome. These maternal and paternal chromosome pairs are called homologous chromosomes.
The human genome
- 23 pairs of chromosomes
- One of the 23 pairs are the sex chromosomes
Sex chromosomes (or heterosomes)
- Determines the sex of the organism
- XY: male
- XX: female
Autosomes
- Chromosomes that do not determine sex
Diploid cells
- Nucleus possess pairs of homologous chromosomes
- These nuclei have two gene copies (alleles) for each trait
- All somatic (body) cells in the organism will be diploid
- Nucleus contains two copies of each chromosome (23 homologous pairs for a total of 46 chromosomes)
Haploid cells
- Contains only one copy of each chromosome
- These nuclei have a single gene copy (allele) for each trait
- Gametes/sex cells
- Haploid cells are produced from diploid cells by meiosis
Sperm cells
- Contain 22 autosomes and one copy of the sex chromosome, either X or Y
- Determines the sex of the offspring
Egg cells
- Contain 22 autosomes and one copy of an X chromosome
Karyogram
Shows the chromosomes in an organism in homologous pairs of decreasing length
- Used to determine the sex of an individual or to find missing or extra chromosomes
Karyotype
- A property of a cell –> the number and type of chromosomes present in the nucleus
Down syndrome
- Caused by trisomy of chromosome 21
- It is caused by a non-disjunction (failiure of chromosomes to separate) event in one of the parental gametes
- Symptoms: heart problems, learning disabilities, slower growth leading to smaller stature, diminished muscle tone
Eukaryotic chromosomes during interphase and mitosis
Druing interphase the chromosomes are in a less organized way: chromatin (the DNA + histones)
Druing the first stage of mitosis (and meiosis) the chromosomes condense to form the sister chromatids (X shape)
Structure of a chromosome
X shaped - when compacted during mitosis/meiosis
- Each chromosome has a constriction point called a centromere, which divides the chromosome into two sections (or ‘arms’)
- The shorter section is designated the p arm and the longer section is designated the q arm
- Eukaryotic species possess multiple chromosomes that may differ in both their size and the position of their centromere
- Each chromosome will carry specific genes and the position of a particular gene on a chromosome is called the locus
What method to use to show the bands of each chromosome?
Staining
- Shows where there are rich areas of adenine and thymine base pairs, this is where we see the stain the darkest.
Why are homologous chromosomes separated in gametes during meiosis?
Homologous chromosomes must be separated in gametes (via meiosis) prior to reproduction, in order to prevent chromosome numbers continually doubling with each generation.
How to determine the karyogram of a species
- Cells are harvested
- Made to replicate through chemical means
- Halted in the mitosis stage when chromosomes are condensed
- The chromosomes are stained, photographed and arranged according to the karyogram model
Cairns’ technique for measuring the length of DNA molecules by autoradiography.
Application
- Cairns grew prokaryotes in radioactive thymine containing tritium
- The radioactive chromosomes are then placed on a photographic plate and placed in silver bromide solution
- The silver bromide solution reacts with the radioactive chromosomes and causes the silver in the solution to turn into an insoluble substance which can then be viewed through an electron microscope.
- The visual through the electron microscope shows the insoluble silver as little black dots
- With a clear visual of uncoiled chromosomes, the length cam be measured
Are the number of chromosomes a characteristic feature of members of a particular species?
Yes. This means that species with with different diploid numbers will not produce viable offspring.
Exception: horse and donkey = mule (which can not reproduce
Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.
Application
Humans (Homo sapiens): 46
Chimpanzee (Pan trogolodytes): 48
Dog (Canis familiaris): 78
Rice (Oryza sativa): 24
Roundworm (Parascaris equorum): 4
There is little evidence that a greater number of chromosomes means the organisms will be more complex
Comparison of genome size in T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiens and Paris japonica.
Application
Virus (T2 phage): 170,000 bp
Bacteria (Escherichia coli): 4.6 bp
Fruit fly (Drosophilia melanogaster): 130 million bp
Humans (Homo sapiens): 3.2 billion bp
Canopy plant (Paris japonica): 150 billion bp
Genome size is not a valid indicator of genetic complexity
However there are trends in genome size that can be found:
- Viruses and bacteria tend to have very small genomes. Prokaryotes typically have smaller genomes than eukaryotes
Genome size
Genome size is the total length of DNA in an organism
