Genetics SL Flashcards
Gene
A sequence of DNA that encodes for a specific trait
Locus
The position of a gene on a particular chromosome
Allele
Alleles are alternative forms of a gene that code for the different variations of a specific trait
Gene mutation
A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait
Neutral mutations
Neutral mutations have no effect on the functioning of the specific feature (silent mutations)
Detrimental mutations
truncate the gene sequence (nonsense mutations) to block the normal function of a trait
Beneficial mutations
Beneficial mutations change the gene sequence (missense mutations) to create new variations of a trait
Cause of Sickle Cell Anaemia
Sickle cell anaemia results from a change to the 6th codon for the beta chain of haemoglobin
Polypeptide: The sixth amino acid for the beta chain of haemoglobin is changed from glutamic acid to valine (Glu to Val)
Consequence of Sickle Cell Anaemia
The amino acid change (Glu → Val) alters the structure of haemoglobin, causing it to form insoluble fibrous strands
The insoluble haemoglobin cannot carry oxygen as effectively, causing the individual to feel constantly tired
The formation of fibrous haemoglobin strands changes the shape of the red blood cell to a sickle shape
The sickle cells may form clots within the capillaries, blocking blood supply to vital organs and causing myriad health issues
The sickle cells are also destroyed more rapidly than normal cells, leading to a low red blood cell count (anaemia)
What is a genome?
The genome is the totality of genetic information of a cell, organism or organelle
What is The Human Genome Project (HGP) and what were the 4 outcomes?
The Human Genome Project (HGP) was an international cooperative venture established to sequence the human genome
The HGP showed that humans share the majority of their sequence, with short nucleotide polymorphisms contributing diversity
The completion of the Human Genome Project in 2003 lead to many outcomes:
Mapping – The number, location, size and sequence of human genes is now established
Screening – This has allowed for the production of specific gene probes to detect sufferers and carriers of genetic diseases
Medicine – The discovery of new proteins have lead to improved treatments (pharmacogenetics and rational drug design)
Ancestry – Comparisons with other genomes have provided insight into the origins, evolution and migratory patterns of man
Where is genetic material in prokaryotes
Genetic material is found free in the cytoplasm in a region called the nucleoid
What is the genetic material in prokaryotes?
The genetic material of a prokaryote consists of a single chromosome consisting of a circular DNA molecule (genophore)
The DNA of prokaryotic cells is naked – meaning it is not associated with proteins for additional packaging
(Prokaryotic cells may possess additional circular DNA molecules called plasmids)
What are plasmids?
Plasmids are small, circular DNA molecules that contain only a few genes and are capable of self-replication
Organisation of eukaryotic chromosomes
DNA is complexed with eight histone proteins (an octamer) to form a complex called a nucleosome
Nucleosomes are linked by an additional histone protein (H1 histone) to form a string of chromatosomes
These then coil to form a solenoid structure (~6 chromatosomes per turn) which is condensed to form a 30 nm fibre
These fibres then form loops, which are compressed and folded around a protein scaffold to form chromatin
Chromatin will then supercoil during cell division to form chromosomes that are visible (when stained) under microscope
What is a chromosone and describe its structure
Chromosomes are linear molecules of DNA that are compacted during cell division (mitosis or 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
What are homologous chromosomes?
Maternal and paternal chromosome pairs are called homologous chromosomes
Homologous chromosomes are chromosomes that share:
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)
Why must Homologous chromosomes be separated in gametes prior to reproduction?
Homologous chromosomes must be separated in gametes (via meiosis) prior to reproduction, in order to prevent chromosome numbers continually doubling with each generation
What is a diploid nucleus?
Nuclei possessing pairs of homologous chromosomes are diploid (symbolised by 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 nucleus?
Nuclei possessing only one set of chromosomes are haploid (symbolised by n)
These nuclei will possess a single gene copy (allele) for each trait
How is sex determined in humans?
Sex is determined by a pair of chromosomes called the sex chromosomes (or heterosomes)
Females possess two copies of a large X chromosome (XX)
Males possess one copy of an X chromosome and one copy of a much shorter Y chromosome (XY)
With is a Autosome?
A chromosone which doesnt determin sex
What is a karyotype?
Karyotypes are the number and types of chromosomes in a eukaryotic cell
How do you make a karyogram?
1.Harvesting cells (usually from a foetus or white blood cells of adults)
2.Chemically inducing cell division, then arresting mitosis while the chromosomes are condensed
3. The chromosomes are stained and photographed to generate a visual profile and the chromosomes are arranged into homologous pairs according to size
What. is a karyogram
The chromosomes are stained and photographed to generate a visual profile that is known as a karyogram
The chromosomes of an organism are arranged into homologous pairs according to size
What is down syndrome? What. causes it? And what is the effect?
Down syndrome is a condition whereby the individual has three copies of chromosome 21 (i.e. trisomy 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
One of the parental gametes had two copies of chromosome 21 as a result of non-disjunction
The other parental gamete was normal and had a single copy of chromosome 21
When the two gametes fused during fertilisation, the resulting zygote had three copies of chromosome 21
Why may karyotyping be used prenatally?
Karyotyping will typically occur prenatally and is used to:
Determine the gender of the unborn child (via identification of the sex chromosomes)
Test for chromosomal abnormalities (e.g. aneuploidies or translocations)
What is Autoradiography? and how is it preformed? (EXTRA)
Measuring the length of DNA molecule
1.Cells are grown in a solution containing radioactive thymidine (tritiated thymidine – 3H-T)
2.Thymidine is incorporated into the chromosomal DNA of the cell,
3.The chromosomes are isolated by gently lysing the cells and fixing the chromosomes to a photographic surface
4.The surface is then immersed in a radioactively-sensitive emulsion containing silver bromide (AgBr)
5.The radiation released from the tritiated thymidine converts the Ag+ ions in silver bromide into insoluble metal grains
6.Excess silver bromide is washed away, leaving the silver grains to appear as small black dots
7.When the photographic film is developed, the chromosomal DNA can be visualised with an electron microscope
Why do infertile offspring such as mules come to be?
Chromosome number is a characteristic feature of members of a particular species
Organisms with different diploid numbers are unlikely to be able to interbreed (cannot form homologous pairs in zygotes)
In cases where different species do interbreed, offspring are usually infertile (cannot form functional gametes)
What is meoisis?
Meiosis is the process by which sex cells (gametes) are made in the reproductive organs
It involves the reduction division of a diploid germline cell into four genetically distinct haploid nuclei
Process of Meoisis HL
Meiosis I
The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated
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
Meiosis II
The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)
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
The final outcome of meiosis is the production of four haploid daughter cells
What are sister chromatids and how do they form?
During Interphase DNA is replicated (in the S phase) to produce two genetically identical copies
The two identical DNA molecules are identified as sister chromatids, and are held together by a single centromere
The sister chromatids are separated during meiosis II, following the separation of homologous chromosomes in meiosis I
How does crossing over occur?
In prophase I, homologous chromosomes undergo a process called synapsis, whereby they pair up to form a bivalent (or tetrad)
The homologous chromosomes are held together at points called chiasmata (singular: chiasma)
Crossing over of genetic material between non-sister chromatids can occur at these chiasmata
As a result of this exchange of genetic material, new gene combinations are formed on chromatids (recombination)
Once chiasmata are formed, the homologous chromosomes condense as bivalents and then are separated in meiosis
If crossing over occurs then all four haploid daughter cells will be genetically distinct (sister chromatids are no longer identical)
How do different chromosone combinations form?
During metaphase I, homologous chromosomes line up at the equator as bivalents in one of two arrangements:
Maternal copy left / paternal copy right OR paternal copy left / maternal copy right
The orientation of pairs of homologous chromosomes is random, as is the subsequent assortment of chromosomes into gametes
The final gametes will differ depending on whether they got the maternal or paternal copy of a chromosome following anaphase I
As this random assortment will occur for each homologous pair, the number of possible gamete combinations are dependent on the number of homologous pairs
Gamete combinations = 2n (where n represents the haploid number)
Why must gametes be haploid?
In order to reproduce, organisms need to make gametes that are haploid (one copy of each chromosome)
Fertilisation of two haploid gametes (egg + sperm) will result in the formation of a diploid zygote that can grow via mitosis
If chromosome number was not halved in gametes, total chromosome numbers would double each generation (polyploidy)
What are the sources of genetic variation?
The three main sources of genetic variation arising from sexual reproduction are:
Crossing over (in prophase I)
Random assortment of chromosomes (in metaphase I)
Random fusion of gametes from different parents
What is non-disjunction?
Non-disjunction refers to the chromosomes failing to separate correctly, resulting in gametes with one extra, or one missing, chromosome (aneuploidy)
How my non-disjunction occur?
Failure of homologues to separate in Anaphase I (resulting in four affected daughter cells)
Failure of sister chromatids to separate in Anaphase II (resulting in only two daughter cells being affected)
Why do chances of having a baby with genetic abnormalities increase with age?
Studies show that the chances of non-disjunction increase as the age of the parents increase
The risk of chromosomal abnormalities in offspring increase significantly after a maternal age of 30
There is a higher incidence of chromosomal errors in offspring as a result of non-disjunction in meiosis I
What is Chorionic villi sampling?
Chorionic villi sampling involves removing a sample of the chorionic villus (placental tissue) via a tube inserted through the cervix
It can be done at ~11 weeks of pregnancy with a slight risk of inducing miscarriage (~1%)
What is Amniocentesis ?
Amniocentesis involves the extraction of a small amount of amniotic fluid (contains fetal cells) with a needle
It is usually conducted later than CVS (~16 weeks of pregnancy) with a slightly lower risk of miscarriage (~0.5%)
How did Gregor Mendel develope the principles of inheritance by performing experiments on pea plants
First, he crossed different varieties of purebred pea plants, then collected and grew the seeds to determine their characteristics
Next, he crossed the offspring with each other (self-fertilization) and grew their seeds to similarly determine their characteristics
These crosses were performed many times to establish reliable data trends (over 5,000 crosses were performed)
As a result of these experiments, Mendel discovered the following things:
When he crossed two different purebred varieties together the results were not a blend – only one feature would be expressed
E.g. When purebred tall and short pea plants were crossed, all offspring developed into tall growing plants
When Mendel self-fertilised the offspring, the resulting progeny expressed the two different traits in a ratio of ~ 3:1
E.g. When the tall growing progeny were crossed, tall and short pea plants were produced in a ratio of ~ 3:1
What conclusions did Mendel draw?
-Organisms have discrete factors that determine its features (these ‘factors’ are now recognised as genes)
-Furthermore, organisms possess two versions of each factor (these ‘versions’ are now recognised as alleles)
-Each gamete contains only one version of each factor (sex cells are now recognised to be haploid)
-Parents contribute equally to the inheritance of offspring as a result of the fusion between randomly selected egg and sperm
-For each factor, one version is dominant over another and will be completely expressed if present
Law of Segregation
When gametes form, alleles are separated so that each gamete carries only one allele for each gene
Law of Independent Assortment:
The segregation of alleles for one gene occurs independently to that of any other gene, however, this does not hold true for genes located on the same chromosome
Principle of Dominance
Recessive alleles will be masked by dominant alleles, but some genes show co-dominance or incomplete dominance
How do gametes contain only one allele of a gene?
During meiosis I, homologous chromosomes are separated into different nuclei prior to cell division
As homologous chromosomes carry the same genes, segregation of the chromosomes also separates the allele pairs
Consequently, as gametes contain only one copy of each chromosome they therefore carry only one allele of each gene
