Topic 3 - Genetics Flashcards
Define “gene”.
A gene is a sequence of DNA nucleotides that codes for an RNA or protein that in turn influences a trait/characteristic.
Define “gene locus.”
A gene locus is the location of a gene on a chromosome. Each chromosome carries many genes.
Define “allele.”
An allele is a version/variation of a gene. Most genes come in a variety of different forms.
For analogy: if there is an ice cream gene , then the alleles would be vanilla, chocolate and mint.
Describe an example of a gene with multiple alleles.
Nearly all genes have multiple alleles (multiple versions). For example, in humans the ABO blood type is controlled by a single gene, the isoagglutinogen gene (I for short). The I gene has three common alleles:
I^A: codes for antigen type A
I^B: codes for antigen type B
i: codes for no antigen
State similarities between alleles of the same gene.
Alleles of the same gene are found at the same locus on homologous chromosomes, have mostly the same nucleotide sequence and code for the same general type of protein (for examples the A and B alleles for blood type both code for a membrane embedded protein).
State the difference between alleles of the same gene.
Alleles of the same gene are slightly different from each other in the sequence of nucleotides. They can vary by just one base (i.e. A –>T), called a single nucleotide polymorphism (SNP) or by the insertion or deletion of a base.
State the source of new alleles of a gene.
New alleles (versions) of a gene are formed through random mutation (changes) in the DNA sequence of the gene. Most new mutations arise due to errors in DNA replication.
Describe a base substitution mutation.
Substitution mutations replace one base with another. The new allele that results from the mutation might result in:
Missense - cause one amino acid in the protein coded for by the gene to change
Silent - have no effect on the protein coded for by the gene
Nonsense - code for an incomplete, non-functioning polypeptide for form.
Define “genome.”
The genome is the complete set of genes and genetic material present in a cell or organism.
State the size in base pairs of the human genome.
The human genome is composed of about 3.2 billion base pairs divided amongst nucleus chromosomes and mitochondrial DNA.
Define “sequence” in relation to genes and/or genomes.
Sequence (noun): the order of the nitrogenous bases in a gene or genome. “The sequence of the gene is ATCCGTA.”
Sequence (verb): the process of determining the order of the nitrogenous bases in a gene of genome. “We are going to sequence the gene to test for a genetic disease.”
State the aim of the Human Genome Project.
The main aims of the Human Genome Project were to determine the sequence of the ≈ 3.2 billion base pairs and identify the location of the ≈ 20-25 thousand genes in the human genome.
Outline outcomes of the Human Genome Project.
The Human Genome Project:
-determined the sequence of the base pairs in the sample humans
- identified the location of many genes on chromosomes
- identified human genetic variations (SNPs)
-improved detection of genetic diseases
-developed new technologies, medical treatments and research techniques
-spurred international collaboration
Define “sickle cell anemia.”
Sickle cell anemia is a group of disorders that affects hemoglobin, the molecule in red blood cells that delivers oxygen to cells throughout the body. People with this disorder have atypical hemoglobin molecules, which distort red blood cells into a sickle, or crescent, shape.
Define “substitution mutation.”
A substitution is a mutation that exchanges one base for another (i.e., switching an A to a C). Such a substitution could: change a codon to one that encodes a different amino acid and cause a change in the protein produced.
State the cause of sickle cell anemia, including the name of differences in the Hb alleles.
Sickle cell anemia is caused by single base substitution mutation in the gene coding for one of the polypeptide chains in hemoglobin. In the mutation, the sequence GAG (on the sense strand of DNA) is mutated to GTG. This results in a codon that codes for the amino acid VAL instead of GLU.
Outline the consequences of the sickle cell mutation on the impacted individual.
Sickle cells are destroyed rapidly in the bodies of people with the disease, causing anemia, a condition in which there aren’t enough healthy red blood cells to carry adequate oxygen to the body’s tissues. Anemia results in fatigue and weakness.
The sickle cells also block the flow of blood through vessels, resulting in lung tissue damage that causes acute chest syndrome, pain episodes and stroke. It also causes damage to the spleen, kidneys and liver.
Explain the consequence of the mutation causing sickle-cell anemia in relation to the processes of transcription and translation.
With sickle cell, the sequence GAG (on the sense strand of DNA) is mutated to GTG. When the mutated sickle cell gene is transcribed, the mRNA codon becomes GUG rather than GAG. During translation, the mutated codon will code for the wrong amino acid to join the polypeptide (where there should be a glutamic acid a valine is inserted instead). As a result, the polypeptide will fold into an incorrect shape, resulting in a distorted hemoglobin molecule that in turn alters the red blood cell shape and reduces its ability to carry oxygen.
State the number of genes in the human genome.
There are an estimated 20,000-25,000 genes in the human genome.
Describe the relationship between the number of genes in a species and the species complexity in structure, physiology and/or behavior.
In general, eukaryotes have more genes than prokaryotes. However, within plants and animals there is little correlation between complexity and the number of genes.
Explain which gene types are often used to assess the differences in the base sequences of a gene between two species.
Genes that are present in the species being studied must be selected. For example, the COX1 gene (which codes for a protein involved in cellular respiration) is present in the majority of eukaryotic species so it is a good choice for comparing sequences between species. Additionally, the gene has been sequenced for many species and is therefor accessible in genome databases.
Outline the use of a computer software tool to create an alignment of the gene sequences between different species.
A sequence alignment is a way of arranging DNA sequences so that similarities and differences between the sequences of different species can be identified. Computer software programs are able to complete alignments quickly and accurately.
Summarize the information that can be determined given gene sequence alignment data.
Sequence alignment data can be used to measure evolutionary relationships between species. The more similar two sequences, the more closely related two species are.
Outline the technological improvement that sped the DNA sequencing process.
The largest advancement in gene sequencing was the automation of the process with computer-assisted technology. What used to take humans hours or days can now be done by a computer much more rapidly, more accurately and for less money.
Describe the structure and function of nucleoid DNA.
In prokaryotic cells, the main DNA of the cell is collectively called the nucleoid. Unlike in eukaryotic cells, the nucleoid DNA is not enclosed in a membrane. The nucleoid DNA is a double helix that forms a circular loop and is not wrapped around histone proteins (termed “naked.”)
List differences in the genetic material of prokaryotes and eukaryotes.
Prokaryotic DNA
-Circular
-One chromosome
-Naked
-Plasmids may be present
-No intron sequences
-Found in nucleoid region
-One origin of DNA replication
Eukaryotic DNA
-Linear
-Multiple chromosomes
-Associated with histones
-No plasmids
-Intron sequences present
-Contained in membrane bound nucleus
-Multiple origins of DNA replication
Define the term “naked” in relation to prokaryotic DNA.
Naked means that the DNA does not wrap around histone proteins.
List similarities in the genetic material of prokaryotes and eukaryotes.
In both prokaryotic and eukaryotic cells:
-The DNA is double helix made of two anti-parallel strands of nucleotides linked by hydrogen bonding between complementary base pairs.
-The replication of DNA is semi-conservative and depends on complementary base pairing.
-DNA is the genetic code for creating proteins through transcription and translation.
Describe the structure and function of plasmid DNA.
Plasmids are extra pieces of DNA found only in prokaryotic cells. Like nucleoid DNA, plasmid DNA is circular and naked however plasmids are much smaller than the main nucleoid DNA and plasmids replicate independently of the nucleoid DNA. Plasmids are not found in all prokaryotic cells, can be shared between bacteria and often contain genes for antibiotic resistance.
Describe the structure of eukaryotic DNA and associated histone proteins during interphase.
Eukaryotic DNA is linear and associate with histone proteins in a structure called the nucleosome. During interphase, the DNA is not super-coiled into chromosomes; it is in a loose form called chromatin.
Explain why chromatin DNA in interphase is said to look like “beads on a string.”
The base unit of chromatin is the nucleosome, a structure composed of DNA wrapped around histone proteins. A chain of nucleosomes gives the appearance of “beads on a string.”
List ways in which the types of chromosomes within a single cell are different.
Chromosomes within a cell are different in:
- size (as measured by the # of base pairs)
- the genes they carry
- the sequence of the nitrogenous bases
- the location of the centromere
- the banding pattern when stained
State the number of nuclear chromosome types in humans.
There are 24 types of human chromosomes. There are 22 autosomes and 2 types of sex chromosomes.
Define “homologous chromosome.”
Homologous chromosomes a chromosome pair (one from each parent).
State a similarity between pairs of homologous chromosomes.
Homologous chromosomes have similar length, the same genes at the same locus, the majority of the same DNA base sequence, the same centromere position and will stain with the same pattern.
State a difference between pairs of homologous chromosomes.
The genes and the position of the genes on each homologous chromosome are the same, however the genes may be different alleles (therefor slight differences in sequence of the gene).
Define “diploid.”
Diploid mean that the cell contains two complete sets of the chromosomes, one chromosome originating from each parent.
State the human cell diploid number.
Somatic human cells have a diploid number of
2n=46
Where
2: there are two of each of the…
n: number of chromosome types
=: for a total of
46= the number of chromosomes
Outline the formation of a diploid cell from two haploid gametes.
Gametes (egg and sperm) are haploid. When gametes fuse during fertilization, the two sets of chromosomes (one from the egg and one from the sperm) combine to create a diploid zygote.
State an advantage of being diploid.
Being diploid means there are two copies of each chromosome, and therefore two copies of each gene that the chromosome carries. So, if one of the chromosomes carries a detrimental allele of a gene, there is a second copy of the gene whose allele may be able to counter the effects of the mutated version. Essentially there is a “backup set of genes.”
Define “haploid.”
Haploid mean that the cell contains only one set of chromosomes; there are no homologous pairs.
State the human cell haploid number.
n=23
Where
n: the number of chromosome types
=: equals
23
List example haploid cells.
The eggs and sperm of humans are haploid.
State why chromosome number and type is a distinguishing characteristic of a species.
Organisms with differing numbers of chromosomes are usually not able to interbreed, maintaining the same number of chromosomes with the species.
Define “karyogram.”
A karyogram is a micro-photograph of all chromosomes of an individual represented in a standard format.
Describe the process of creating a karyogram.
A cell is “frozen” in metaphase by the application of chemicals that disrupt the mitotic spindle. A hypotonic solution is added; water enters the cell causing it to swell and burst, separating the chromosomes from each other. The chromosomes are stained and viewed with a microscope. A photograph of the chromosomes is taken. The images of the chromosomes are then organized in a standard pattern.
List the characteristics by which chromosomes are arranged on the karyogram.
Images of the chromosomes are arranged by size (largest to smallest, sex chromosomes always last) and paired based on banding pattern and centromere position.
Outline the procurement of fetal cells from which chromosomes are isolated for arrangement on a karyogram.
Karyotyping requires cells that are in metaphases, so the chromosomes are condensed and visible in the cells. In prenatal testing, fetal cells can be collected from chorionic villus sampling or by amniocentesis.
List applications of karyogram analysis (karyotyping).
Karyotypes are used to prenatally identify the sex of the fetus and/or abnormal chromosome numbers (for example Down syndrome due to extra chromosome 21).
The results of a karyogram analysis may lead to a decision to abort the fetus or to prepare for consequences of abnormality in offspring.
Outline the structure and function of the two human sex chromosomes.
The X chromosome is the larger of the two sex chromosomes (a length of about 156 million bp and 1805 genes).
The Y chromosome is much smaller (a length of 57 million bp and about 460 genes)
Define “autosome.”
An autosome is any chromosome that is not a sex chromosome.
Define “sex chromosome.”
A sex chromosome is a chromosome involved with determining the sex of an organism, typically one of two kinds, X or Y in humans.
Outline sex determination by sex chromosomes.
Biological sex is usually determined by which sex chromosomes are present.
XX = female
XY = male
The male parent determines the sex of the offspring by either passing on an X chromosome (to produce a female offspring) or a Y chromosome (to produce a male offspring).
Outline conclusions drawn from the images produced using Cairn’s autoradiography technique.
Cairn’s was able to see that prokaryotic chromosomes are circular and measuring lengths of chromosomes.
He also observed the DNA replication fork.
Describe Cairn’s technique for producing images of DNA molecules from E. coli.
Cairn’s radioactively labeled DNA to produce images of the molecule. This allows him to visualized and measure the length of DNA molecules.
Describe the relationship between the genome size of a species and the species complexity in structure, physiology and behavior.
There is a great variety of genome sizes. In general, eukaryotes have larger genomes than prokaryotes. However, the size of the genome and the number of genes do not appear to correlate to a species “complexity.”
State the minimum chromosome number in eukaryotes.
The minimum chromosome number in eukaryotes is 2n=2
Explain why the typical number of chromosomes in a species is always an even number.
The reason why most eukaryotic organisms have an even number of chromosomes is because of sexual reproduction, in which each parent gives one set of chromosomes, resulting in an even number in the offspring.
Explain why the chromosome number of a species does not indicate the number of genes in the species.
The number of chromosomes does not indicate the number of genes. It’s possible to have one large chromosome with many genes or many smaller chromosomes with fewer genes. Likewise, it’s possible to have large chromosomes with relatively few genes or smaller chromosomes that are packed full of genes!
Deduce the sex of an individual given a karyogram [female].
To determine sex from a karyogram, examine the last pair of chromosomes.
XX= female
Deduce the sex of an individual given a karyogram [male].
To determine sex from a karyogram, examine the last pair of chromosomes.
XY= male
Describe the use of a karyogram to diagnose Down syndrome.
Down Syndrome is caused by a nondisjunction of chromosome #21, resulting in three chromosome #21, which can be observed in a karyogram.
Explain the benefit of a publicly accessible genome database.
Effort has been made to make human genome sequence information freely
accessible to researchers around the globe. Data obtained from Human Genome Project funded research must be publicly available. The rationale is that our ability to expeditiously and effectively increase our knowledge of genetics
depends on the ability of researchers to access current information.
Outline how to search an online database [BLAST] for a given gene.
BLAST (Basic Local Alignment Search Tool) is to the nucleotide or protein sequence database as a search engine is to the internet. BLAST is a sequence alignment tool that allows you to identify an unknown sequence, map a sequence in a genome or get clues about related sequences.
Using BLAST is like doing an experiment; you must optimize the experimental conditions to get good results. Too wide a search can take a long time and give you many random and meaningless matches. Too narrow a search will possibly miss significant matches.
Outline the advancement in knowledge gained from the development of autoradiography techniques.
Autoradiography is used to produce an image of a radioactive substance. The technique is used in cellular and molecular biology to visualize structures. For example, autoradiography can be used to visualize radioactively stained chromosomes, bands in DNA electrophoresis gels, tissue samples and single cells.
Define “meiosis.”
Meiosis is a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes.
Compare divisions of meiosis I and meiosis II.
Meiosis I
Reductive division (diploid to haploid)
Results in two haploid cells
Chromosomes remain replicated (X)
Crossing over occurs
Proceeded by interphase with DNA replication
Meiosis II
Non-reductive division (haploid to haploid)
Results in four haploid cells
Chromatids of a chromosome separate (X to / and )
No crossing over occurs
Proceeded by interkinesis, without DNA replication
Compare sexual and asexual life cycles.
The life cycle is the period of time that an organism passes through until producing offspring of its own.
Sexual
Two parents
Meisosis
results in increased genetic variation
Asexual
One parent
Fission, mitosis or budding
Requires less energy
Explain why meiosis must occur as part of a sexual life cycle.
The sexual life cycle combines genetic information from two parents. In order to maintain the correct number of chromosomes in the offspring, the parents must undergo meiosis to create gametes with half of the genetic information.
