3.2 Chromosomes Flashcards
Whats the difference in chromosome in prokaryotes and eukaryotes?
Prokaryotes have circular chromosomes and eukaryotes have linear chromosomes
What is a genophore?
The genetic material of a prokaryote
What does it mean if the DNA of prokaryotic cells is naked?
meaning it is not associated with proteins for additional packaging
What are additional circular DNA molecules in prokaryotic cells called?
Plasmids
What are plasmids?
Plasmids are small, circular DNA molecules that contain only a few genes and are capable of self-replication
- Plasmids are present in some prokaryotic cells, but are not naturally present in eukaryotic cells
What is bacterial conjugation?
Bacterial cells may exchange plasmids via their sex pili.
- Copies of plasmids to be transferred from one cell to another, allowing spread through a population
- Allows bacteria to evolve new features within a generation
Why are DNA packaged with histone proteins?
= Results in a greatly compacted structure, allowing for more efficient storage
What is the process of Autoradiography?
- Cells are grown in a solution containing radioactive thymidine (tritiated thymidine – 3H-T)
- The tritiated thymidine is incorporated into the chromosomal DNA of the cell (3H-T is used as thymidine is not present in RNA)
- The chromosomes are isolated by gently lysing the cells and fixing the chromosomes to a photographic surface
- The surface is then immersed in a radioactively-sensitive emulsion containing silver bromide (AgBr)
- The radiation released from the tritiated thymidine converts the Ag+ ions in silver bromide into insoluble metal grains
- Following a period of exposure, excess silver bromide is washed away, leaving the silver grains to appear as small black dots
- When the photographic film is developed, the chromosomal DNA can be visualised with an electron microscope
What did John Cairns pioneer?
- a technique for measuring the length of DNA molecules by autoradiography
- Cairns used autoradiography to visualise the chromosomes whilst uncoiled, allowing for more accurate indications of length
- By using tritiated uracil (3H-U), regions of active transcription can be identified within the uncoiled chromosome
How did autoradiography help measure the length of DNA molecules?
- Previously, chromosome length could only be measured while condensed during mitosis (very inaccurate due to supercoiling)
- Now regions of active transcription can be identified within the uncoiled chromosome
What key events were discovered during chromosomal replication by John Cairns and his use of autoradiography?
- DNA replication involves formation of a replication bubble (and prokaryotic replication involves a single origin of replication)
- DNA replication is bi-directional (it occurs independently at both ends of the replication bubble)
What is the constriction point on each chromosome called?
Centromere, which divides the chromosomes into two sections (arms)
- The shorter section is designated the p arm and the longer section is designated the q arm
What is the relationship between the homologous chromosomes and sequence of genes and alleles of those gene?
Homologous chromosomes carry the same sequence of genes but not necessarily the
same alleles of those genes
What do homologous chromosomes share with each other?
- The same structural features (e.g. same size, same banding patterns, same centromere positions)
- The same genes at the same loci positions (while the genes are the same, alleles may be different)
What are homologous chromosomes?
- maternal and paternal chromosome pairs
Sexually reproducing organisms inherit their genetic sequences from both parents = organisms will possess two copies of each chromosome (one of maternal origin ; one of paternal origin)
When are homologous chromosomes seperated?
In gametes via meiosis prior to reproduction, in order to prevent chromosome numbers continually doubling with each generation.
Anaphase - spindle fibres contract and pull homologous chromosome to opposite poles
What is a diploid?
Nuclei possessing pairs of homologous chromosomes (2n)
- These nuclei will possess two gene copies (alleles) for each trait
- All somatic (body) cells in the organism will be diploid, with new diploid cells created via mitosis
- Diploid cells are present in most animals and many plants
What is a haploid?
Haploid: Half of the number of chromosomes
What happens when two haploid gametes fuse?
The resulting diploid cell (zygote) can grow and develop into a new organism
Why is the father responsible for determining the sex of the offspring?
- Only the absence of a Y chromosome, female sex organs will develop
- If the male sperm contains an X chromosome, the growing embryo will develop into a girl
- If the male sperm contains a Y chromosome, the growing embryo will develop into a boy
- In all cases the female egg will contain an X chromosome (as the mother is XX)
What are autosomes and heterosomes?
Heterosomes: sex chromosome
Autosomes: The remaining chromosomes in the organism that do not determine sex
What are karyotypes?
are the number and types of chromosomes in a eukaryotic cell – they are determined via a process that involves:
- Harvesting cells (usually from a foetus or white blood cells of adults)
- Chemically inducing cell division, then arresting mitosis while the chromosomes are condensed
- The stage during which mitosis is halted will determine whether chromosomes appear with sister chromatids or not
What is stained in a karyogram and why?
Chromosomes to generate a visual profile. They are then arranged into homologous pairs according to size (with sex chromosomes shown last)
What is Karyotyping used for?
- Determine the gender of the unborn child (via identification of the sex chromosomes)
- Test for chromosomal abnormalities (e.g. aneuploidies or translocations)
How can Down syndrome be detected through an karyogram? What is Down syndrome caused by?
Down syndrome is a condition whereby the individual has three copies of chromosome 21
- It is caused by a non-disjunction event in one of the parental gametes
- The extra genetic material causes mental and physical delays in the way the child develops
What are organisms with different diploid numbers unable to do? What happens when different species do interbreed?
Interbreed
- In cases where different species do interbreed, offspring are usually infertile (cannot form functional gametes)
- For instance, a horse (diploid = 64) and a donkey (diploid = 62) may produce an infertile mule (non-diploid = 63)
What are the 3 general rules of genome size?
- Viruses and bacteria tend to have very small genomes
- Prokaryotes typically have smaller genomes than eukaryotes
- Sizes of plant genomes can vary dramatically due to the capacity for plant species to self-fertilise and become polyploid
Why is DNA replicated before meiosis? for what?
DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids
What are autosomes and heterosomes?
Heterosomes: sex chromosome
Autosomes: The remaining chromosomes in the organism that do not determine sex
What are karyotypes?
are the number and types of chromosomes in a eukaryotic cell – they are determined via a process that involves:
- Harvesting cells (usually from a foetus or white blood cells of adults)
- Chemically inducing cell division, then arresting mitosis while the chromosomes are condensed
- The stage during which mitosis is halted will determine whether chromosomes appear with sister chromatids or not
What is stained in a karyogram and why?
Chromosomes to generate a visual profile. They are then arranged into homologous pairs according to size (with sex chromosomes shown last)
What is Karyotyping used for?
- Determine the gender of the unborn child (via identification of the sex chromosomes)
- Test for chromosomal abnormalities (e.g. aneuploidies or translocations)
How can Down syndrome be detected through an karyogram? What is Down syndrome caused by?
Down syndrome is a condition whereby the individual has three copies of chromosome 21
- It is caused by a non-disjunction event in one of the parental gametes
- The extra genetic material causes mental and physical delays in the way the child develops
What are organisms with different diploid numbers unable to do? What happens when different species do interbreed?
Interbreed
- In cases where different species do interbreed, offspring are usually infertile (cannot form functional gametes)
- For instance, a horse (diploid = 64) and a donkey (diploid = 62) may produce an infertile mule (non-diploid = 63)
What are the 3 general rules of genome size?
- Viruses and bacteria tend to have very small genomes
- Prokaryotes typically have smaller genomes than eukaryotes
- Sizes of plant genomes can vary dramatically due to the capacity for plant species to self-fertilise and become polyploid
