Flashcards in Genetics Deck (51)
What is the definition of a gene?
A heritable factor that consists of a specific length of DNA and influences a specific characteristic
What is the definition of an allele?
One specific form of a gene, differing from other alleles by one or a few bases only and occupying the same gene locus as other alleles of the same gene. They are formed by mutations in the nucleotide code.
What is the definition of a locus?
The position of a gene on a particular chromosome
What is a gene mutation?
A permanent change in the base sequence of DNA
What is the genome?
The whole of the genetic information of an organism.
This includes all genes as well as non-coding DNA sequences.
What are the causes of sickle cell anaemia? (3)
Sickle cell anaemia results from a change to the 6th codon for the beta chain of haemoglobin
1. DNA: The DNA sequence changes from GAG to GTG on the non-transcribed strand (CTC to CAC on the template strand)
2. mRNA: The mRNA sequence changes from GAG to GUG at the 6th codon position
3. Polypeptide: The sixth amino acid for the beta chain of haemoglobin is changed from glutamic acid to valine (Glu to Val)
What are the consequences of sickle cell anaemia? (5)
1. The amino acid change (Glu → Val) alters the structure of haemoglobin, causing it to form insoluble fibrous strands
2. The insoluble haemoglobin cannot carry oxygen as effectively, causing the individual to feel constantly tired
3. The formation of fibrous haemoglobin strands changes the shape of the red blood cell to a sickle shape
4. The sickle cells may form clots within the capillaries, blocking blood supply to vital organs and causing myriad health issues
5. The sickle cells are also destroyed more rapidly than normal cells, leading to a low red blood cell count (anaemia)
What was the human genome project?
The Human Genome Project was an international research effort to determine the sequence of the human genome and identify the genes that it contains. It was estimated that humans have between 21 000 – 23 000 genes
• The Human Genome Project showed that most of the genome does not code for proteins (originally labelled “junk DNA”)
• Some of these regions consist of areas that can affect gene expression or are highly repetitive sequences called satellite DNA
What are homologous chromosomes?
A pair of chromosomes (one from each parent) that are the same length and contain the same genes in the same location
How is DNA organized in eukaryotes?
The genetic material of eukaryotic cells consist of multiple linear molecules of DNA that are associated with histone proteins. This results in a greatly compacted structure, allowing for more efficient storage
(DNA is complexed with eight histone proteins to form a complex called a nucleosome, they are linked by an
an additional histone protein to form a string of chromatosomes which coil to form a solenoid structure.
These fibres then form loops, which are compressed and folded around a protein scaffold to form chromatin)
Where is DNA located in prokaryotes? (2)
1. Prokaryotes do not possess a nucleus. Instead genetic material is found free in a region of the cytoplasm called the nucleoid.
2. It consists of he genetic material of a prokaryote consists of a single loop chromosome. The DNA of prokaryotic cells are naked, meaning it is not associated with proteins (for additional packing)
What are plasmids? (4)
1. Plasmids are small separate (usually circular) DNA molecules that are sometimes present in prokaryotic cells.
2. They are not responsible for normal life processes and are often associated with antibiotic resistance
3. They can also be transferred from one bacterial cell to another through bacterial conjugation via sex pili
4. A plasmids can self-replicate and autonomously synthesise proteins, they are ideal vectors for gene manipulation in labs
What is a karyogram?
A diagram or photograph of the chromosomes present in a nucleus arranged in homologous pairs of descending length.
What are autosomes?
Sex is determined by sex chromosomes but autosomes are chromosomes that do not determine sex.
What is a karyotype?
A property of a cell/ the number and appearance of different chromosomes present in a cell
Why is karyotyping used prenatally? (2)
1. To determine the gender of the unborn child (via identification of the sex chromosomes)
2. To test for chromosomal abnormalities (e.g. aneuploidies or translocations)
What is Down Syndrome?
1. Down syndrome is a condition whereby the individual has three copies of chromosome 21 (i.e. trisomy 21)
2. It is caused by a non-disjunction event in one of the parental gametes
3. The extra genetic material causes mental and physical delays in the way the child develops
What is the importance of genome size?
Genome size can vary greatly between organisms and is not a valid indicator of genetic complexity. It simply refers to the total length of DNA in an organism
What is meiosis?
Meiosis is a reduction division of one diploid nucleus to form four haploid nuclei. Meiosis produces gametes.
What is autoradiography?
1. Autoradiography was created by John Cairns to measure the length of DNA molecules.
2. Cairns used autoradiography to visualize the chromosomes whilst uncoiled, allowing for more accurate indications of length.
3. By using tritiated uracil (3H-U), regions of active transcription can be identified within the uncoiled chromosome
Outline the stages of Meiosis I
P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells
Outline the stages of Meiosis II
P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells
What is crossing over? (3)
1. Crossing over involves the exchange of segments of DNA between homologous chromosomes during prophase I
2. The exchange of genetic material occurs between non-sister chromatids at points called chiasmata
3. As a consequence of this recombination all four chromatids will be genetically different
What is random orientation? (2)
1. When homologous chromosomes line up in metaphase I their orientation towards the poles is random
2. The orientation means that different combinations of maternal/paternal chromosome can be inherited when the bivalents separate in anaphase I
What is non-disjunction? (3)
1. A non-disjunction is an error in meiosis, where the chromosome pairs fail to split during cell division
2. It occurs in anaphase I where homologous pairs fail to split, or in anaphase II where the sister chromatids fail to split
3. As a result there will be too many or too few chromosomes in the final gamete cell (gamete would have 22 or 24 chromosomes - Down Syndrome)
What makes sexual reproduction produce variation? (3)
1.Crossing over (in prophase I)
2. Random assortment of chromosomes (in metaphase I)
3. Random fusion of gametes from different parents
What are the main differences between Metaphase I and II?
Replication prior to
Crossing over during prophase I
Homologous pairs line up randomly in metaphase I
Homologous pairs are separated into chromosomes in anaphase I
No crossing over during prophase II
Chromosomes line up randomly in metaphase II
Chromosomes are separated into chromatids during anaphase II
The alleles of an organism/symbolic representation
The characteristic or trait of an organism (Brown eyes or blue eyes, etc)