DNA Flashcards
Describe the process of MITOSIS
Mitosis is nuclear division plus cytokinesis, and produces two identical daughter cells. (PMAT)
(Interphase) - DNA replication, centrosome replication
Prophase - Chromatin condenses into chromosomes , centrosomes migrate to opposite poles of the nucleus, nucleolus dissipates, spindle network forms
Premetaphase - Nuclear membrane breaks down, microtubles invade the nuclear space, chromatids attach to microtubles
Metaphase- Chromosomes line up along equatorial plane
Anaphase - Sister chromatids separate and are pushed to the opposite poles of the cell
Telophase - Nuclear membrane reforms, chromosomes unfold into chromatin, cytokinesis begins (cell divides into two)
How many chromosomes do we have?
46, (22 pairs and a pair of sex chromosomes xx=female xy=male)
Describe the structure of chromosomes
DNA coils around nucleosomes, coils again into super coils and again into chromosomes. There is a centromere in the middle and p (short arm) and q (long arm).
Why is mitosis important?
Produces 2 genetically identical daughter cells - growth and replacement of dead cells
What causes down’s syndrome?
A chromosome abnormality - ie an extra genetic info on chromosome 21.
47, xy + 21
Where does meiosis occur? What is its function?
Meiosis occurs in the gametes for sexual reproduction. Meiosis mantains genetic diversity, it has 2 cell divisions and produces 4 haploid daughter cells.
Describe the process of meiosis
Interphase - replication of DNA
Phase 1 - cross-over (genes sort independently)
MEIOSIS I - produces 2 cells (metaphase, anaphase, telophase, cytokinesis) - ovulation in eggs, division comences at puberty in sperm
MEIOSIS II - produces 4 cells, only if fertilisation occurs in the eggs
(Interphase) - DNA replication, centrosome replication
Prophase - Chromatin condenses into chromosomes , centrosomes migrate to opposite poles of the nucleus, nucleolus dissipates, spindle network forms
Premetaphase - Nuclear membrane breaks down, microtubles invade the nuclear space, chromatids attach to microtubles
Metaphase- Chromosomes line up along equatorial plane
Anaphase - Sister chromatids separate and are pushed to the opposite poles of the cell
Telophase - Nuclear membrane reforms, chromosomes unfold into chromatin, cytokinesis begins (cell divides into two)
What is non-disjunction?
Nondisjunction during meiosis produces eggs or sperm cells that don’t have the normal number of chromosomes.
This occurs during anaphase of either meiosis I (this means two cells will be missing a chromosome, 22, and two cells will have an extra chromosome, 24) or meiosis II (two cells will have normal number of chromosomes, one cell will have one too many, 24, and the other will be missing one chromosome, 22)
What is trisomy?
Trisomy is the condition of having 3 copies of one chromosome type. It is designated as 2n + 1 because the cell has the normal two sets of each 23 types of chromosomes plus an extra copy of one chromosome.
Down syndrome occurs when there are 3 copies of chromosome 21, most trisomic conditions are lethal
What is monosomy?
Monosomy is the condition of having only 1 copy of a chromosome and is designated as 2n - 1.
Turners condition is when a female has just one sex chromosome, X
How many chromosomes has a diploid cell got?
A full set of chromosomes, 22 pairs plus sex chromsomes
How many chromosomes has a haploid cell got?
A single set of chromosomes, 23
What is gonadal/germline mosaicism?
Gonadal / germline mosaicism is when a person has two populations of cells in the gonads (testes or ovaries), one population of cells containing the usual genetic complement whilst the other contains a DNA mutation or chromosome anomaly.
If a sperm or an egg produced from the cells in the parent’s gonads containing the DNA mutation or chromosome anomaly is used to form a fetus, the child will have the genetic condition.
Therefore although the parent is healthy the child has the condition.
A sibling may also have the condition if the sperm or egg is from the patch of cells in the gonad which contains the genetic change. A child would not have the condition if formed from the cells in the gonad with the usual genetic pattern.
Gonadal/germline mosaicism is a likely explanation of the rare situations where a person without a dominant condition can have two children with the same autosomal dominant condition
Describe the structure of DNA
DNA is double stranded helix, present in the nucleus of cells.
It contains deoxyribose and phosphate, creating a diphosphate backbone. The 3’C of the deoxyribose bonds to the 5’C of the phosphate forming a 3’-5’ phosphodiester linkage.
It contains the bases - Adenine and Thymine, Guanine and Cytosine, these are joined to their pairs by hydrogen bonds.
Describe the structure of RNA
RNA has a single strand, it is present in mRNA, rRNA, and tRNA. Contains ribose sugar instead of deoxyribose
It contains the bases - Adenine and Uridine (swap T for U), Guanine and Cytosine, these are joined to their pairs by hydrogen bonds.
What is the function of mRNA?
mRNA template for protein synthesis, it produces in the nucleus, and escapes through the nuclear pore
What is the function of tRNA?
tRNA molecule carries the correct amino acid to ribosomes during translation
Why is DNA degenerate?
The genetic code is described as degenerate, or redundant, because a single amino acid may be coded for by more than one codon.
What is the function of rRNA?
rRNA is part of the ribosome (location of translation)
What is splicing?
RNA splicing occurs after transcription, it removes introns so just the exons remain
Give 3 examples of genetic mutations
Genetic mutations are occur a number of different ways
- duplication
- deletion
- mutation
- splice mutation
- nonsense mutation - introduction of STOP codon
- missense mutation (one nucleotide changes and this changes the amino acid coded for)
- expansion
What is a missense mutation
A missense mutation is a point mutation in which a single nucleotide change results in a codon that codes for a different amino acid. It is a type of nonsynonymous substitution (when aminoacid coded for changes)
What is a nonsense mutation?
A nonsense mutation is also a change in one DNA base pair that introduces a STOP codon. This type of mutation results in a shortened protein that may function improperly or not at all.
What is a Frameshift mutation?
A frameshift mutations can be insertions, deletions, and duplications. This type of mutation occurs when the addition or loss of DNA bases changes a gene’s reading frame. A reading frame consists of groups of 3 bases that each code for one amino acid. A frameshift mutation shifts the grouping of these bases and changes the code for amino acids. The resulting protein is usually nonfunctional.
What is a repeat expansion?
Nucleotide repeats are short DNA sequences that are repeated a number of times in a row. For example, a trinucleotide repeat is made up of 3-base-pair sequences, and a tetranucleotide repeat is made up of 4-base-pair sequences. A repeat expansion is a mutation that increases the number of times that the short DNA sequence is repeated. This type of mutation can cause the resulting protein to function improperly.
What is Allelic heterogeneity?
Allelic heterogeneity is the phenomenon in which different mutations at the same locus cause a similar phenotype.
eg. Cystic fibrosis
Locus heterogeneity
Locus heterogeneity is when mutations in different genes give rise to the same clinical condition
Give an example of
a) a genetic disease
b) multi-factorial disease
c) environmental disease
a) a genetic disease
These are rare, due purely to genetics eg Huntington’s or Down’s syndrome
b) multi-factorial disease
These are common in developed countries, eg diabetes, schizophrenia
c) environmental disease
These are most common in poor countries, eg. accidents, drugs, infections
Define the term genotype
The genetic makeup of a cell
Define the term phenotype
The appearance of an individual due to genotype and environmental factors
Define the term homezygous
When the alleles at a particular locus are the same
Define the term hetrozygous
When the alleles at a particular locus are the different
Define the term hemizygous
This is when there is only one allele at locus - when there is only one X chromosome on a male
What is a Chromosomal mutation
When a chromosomal mutation (structure or one too many etc) leads to an abnormality. eg, -duplication, deletion mutation
What is Single Nucleotide Polymorphism?
A Single Nucleotide Polymorphism is a DNA sequence variation occurring commonly within a population (e.g. 1%) in which a single nucleotide — A, T, C or G — in the genome (or other shared sequence) differs between members of a biological species or paired chromosomes.
This can result in CA repeats.
What is a wild type allele? What the opposite known as?
The allele that encodes the phenotype most common in a particular natural population is known as the wild type allele.
Any form of that allele other than the wild type is known as a mutant form of that allele, this is rarer and often is ‘affected.’
Define Polymorphism
Polymorphism - Frequent hereditary variations at a locus
Explain these different types of genetic disease
a) Chromosomal
b) Mendelian
c) non -traditional
- mitochondrial
- imprinting
- mosaicism
a) Chromosomal - when a mutation leads to an abnormaility
b) Mendelian - autosomal (non-sex chromosomes) dominance (presents in heterozygous state Dd)
automsomal recessive (presents in homozygous state dd)
sex-linked - disease caused by mutations on X chromosome, linked to sex allele from mother x chromosome
c) non -traditional
- mitochondrial - can only be inherieted from mother, children are energy deficient, there is a large variation/onset etc
-imprinting - only 1 out of the 2 alleles is active, the other is inactive. eg if the alelle from the father is ‘imprinted’ it does not manifest and only the mothers gene is expressed (rare in mammals)
-mosaicism - mutation in gonads, so parent is unaffected but the child has the mutated genes
What is X-linked inheritance? Give an example.
X-linked inheritance is a mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be expressed in males. It is inherited from the mother, and because females have two X chromosomes the disease does not manifest.
An female carrier will pass the disease to 50% of her sons, and 50% of her daughters will be carriers.
Eg. the main example is duchenne muscular dystrophy.
This affects males, they have delayed walking/running as children due to 70% deletion in dystrophin gene. This leads to progressive weakness and rep failure. They cannot reproduce, have raised creatine kinase enzyme.
What is duchenne muscular dystrophy?
Duchenne muscular dystrophy is the main example of X-linked inheritance.
This affects males, they have delayed walking/running as children due to 70% deletion in dystrophin gene, coding for an abnormal dystrophin protein. This leads to progressive weakness and rep failure. They cannot reproduce, and have raised creatine kinase enzyme.
1/3 of cases are due to new mutation, gonadal mosacism
What is sickle cell anaemia?
Sickle cell anaemia is an autosomal recessive disorder. Haemoglobin (2 alpha chains and 2 beta chains), in sickle cell anemia the two beta chains are not formed correctly. Normally RBCs are disc shaped and flexible, in sickle cell anaemia they become rigid and crescent shaped. (and therefore carry a reduced amount of O2.
Symptoms: Pain areas, often in joints or sudden in the chest
-dizziness and fatigue due to low oxygen in the body
Urinary: inability to make concentrated or dilute urine or blood in urine
Also common: abnormal breakdown of red blood cells, delayed development, inflamed fingers or toes, pallor, shortness of breath, or yellow skin and eyes
Describe the process of DNA transcription
1) Double helix unwinds at replication fork
2) DNA helicase breaks the H breaks exposing the bases
3) RNA polymerase attaches forming H bonds between original strand and the new strand via complementary base paring (T swaps for U)
4) Reaches STOP codon
4) mRNA strand is formed
4) Leaves nuclear membrane through pore
5) Splicing - Introns are removed leaving just exons
Describe the process of translation
Translation
1. Occurs at ribosomes in the cytoplasm
2. Initation - tRNA binds to small subunit of ribosome
-aminoacyl-tRNA synthetase links amino acids to tRNA
-Initiation factors form an initiation complex
-This positions tRNA opposite the start site codon on the mRNA
-The large subunit binds, enclosing the mRNA
3. Elongation - A ribsosome contains 2 binding sites for tRNA
site 1 - holds tRNA linked to protein chain
site 2 - contains the next amino acid to join
tRNA carries amino acids to ribosome to join via complementary base pairing
Amino acids join via peptide bond
Ribosome moves along mRNA forming polypeptide chain
Ribosome reaches STOP codon and link between tRNA and chain in broken, protein released from ribosome
What is an initiation complex?
When RNA polymerase binds to the promotor region on the gene with transcription factors, this occurs before translation.
What is a linked SNP?
A single nucleotide diffference in DNA sequences that is present in at least 1% of the population, they do not affect protein function but can affect drug responses and the risk of disease.
What is the structure and function of sugars?
Cn(H20)2
Typical carbohydrates are composed of strings or chains of monosaccharides - that is, chains of
individual sugars. A monosaccharide (mono = one, saccharide = sugar) is the smallest
carbohydrate unit.
What is the structure and function of nucelotides?
- Nucleotides are the subunits that are linked to form the nucleic acids ribonucleic acid (RNA) and deoxyribonucleic acid (DNA)
- three parts: a five-carbon sugar (ribose or deoxyribode), a phosphate, and a nitrogen-rich structure called a nitrogenous base.
- pyrimidines (cytosine, thymine, and uracil) are smaller, having only one ring structure. The larger purines (adenine and guanine) have two rings.
What is the structure and function of amino acids?
- carbon (the alpha carbon)
- hydrogen atom (H)
- Carboxyl group (-COOH)
- Amino group (-NH2)
- “variable” group or “R” group
They often exist as zwitterions, this is when the H from the carboxyl group joins the amino group so they is a +ve end and a -ve end. This changes depending on Ph.
What is the structure and function of lipids?
Lipids fall into two categories. One is based on glycerol, and the other is steroids.
-Most dietary and storage fats are triglycerides, these are formed by glycerol and three carboxylic acids, which we call fatty acids. These form esters via dehydration.
Saturated fats - triglycerides that have no double bonds in their carboxylic acid chains.
Unsaturated fats - triglycerides that have double bonds in their carboxylic acid chains
Trans fats - are triglycerides that have trans double bonds in their carboxylic acid chains.
What is the structure and function of proteins?
x
Give an example of a monosaccharide
Monosaccharides - single molecules, usually with 5 or 6 carbons
- pentoses - sugars with 5 carbons, including arabinose, xylose, ribose
- hexoses - sugars with 6 carbons, including glucose, fructose, galactose, mannose
Give an example of a disaccharide
Disaccharides - sugars containing 2 monosaccharides
- sucrose - glucose + fructose
- maltose - glucose + glucose
- lactose - glucose + galactose
Give an example of a Polysaccharides
Polysaccharides - combinations of a large number of monosaccharides into complex threedimensional
forms. Examples include:
-glycogen -a 1-4 links, b1-6 links,
Produced by the liver and stored inliver and muscles. Glycogen serves as a glucose reserve for animals.
-starch - three dimensional strings of glucose stored in plants. amylose a1-4, and amylopectin 1,6
Explain the structure of a protein
Primary - order of amino acids
Secondary - a-helixes and b-pleated sheets
Tertiary - electro static, hydrophobicity, h-bonds, covalent bonds (globular or fibulous)
Quaternary - several polypeptides forming a confirmational shape, non covalent bonds
What is an isoenzyme?
An enzyme that catalyses the same reaction as another enzyme but has a different structure
What is a coenzyme
A non-protein compound that helps enzymes to catalyze a reaction by binding with the protein to create an active enzyme (eg electron transport chains)
What is the function of the smooth endoplasmic reticulum?
The smooth endoplasmic reticulum is the site of membrane lipid synthesis, it also processes synthesised proteins.
(highly folded flattened sheets)
What is the function of of the rough endoplasmic reticulum?
The rough endoplasmic reticulum is the site of protein synthesis. Ribosomes on the rough endoplasmic reticulum are called ‘membrane bound’ and are responsible for the assembly of many proteins. (highly folded flattened sheets)
What is the function of the cytoskeleton?
The cytoskeleton is made of filamentous proteins that brace the internal structure.
What is the function of the golgi apparatus?
The golgi apparatus is formed from parallel stacks of membrane, it processes macromolecules by adding sugars, it also is involved in the proteolysis of peptides into active forms, and the sorting on macromolecules.
What is the function of the nucleolous
Site of rRNA formation, this results in incomplete ribosomes.
What is the function of mitochondria in the cell?
Mitochondria are the site of oxidative phosphorylation. They have a double folded membrane and their own DNA.
What is a chiral centre?
A chiral centre is when a carbon has four different chemical groups attached to it. This means it can form two optically active forms, most living forms have isomer D.
What are the stages of the cell cycle?
Mitosis, G1, S, G2
In what phase of the cell cycle does DNA replication take place?
S phase
What is Mendel’s first law?
The principle of segregation (First Law):
The two members of a gene pair (alleles) segregate (separate) from each other in the formation of gametes. Half the gametes carry one allele, and the other half carry the other allele.
What is Mendel’s second law?
The principle of independent assortment (Second Law): Genes for different traits assort independently of one another in the formation of gametes.
What is lyonization?
X-inactivation (also called lyonization) is a process by which one of the copies of the X chromosome present in female mammals is inactivated.
The inactive X chromosome is silenced by its being packaged in such a way that it has a transcriptionally inactive structure called heterochromatin.
-about the time of gastrulation in the epiblast
What is the Knudson hypothesis?
The Knudson hypothesis, also known as the two-hit hypothesis or multiple-hit hypothesis, is the hypothesis that cancer is the result of accumulated mutations to a cell’s DNA.
It was later found that carcinogenesis (the development of cancer) depended both on the activation of proto-oncogenes (genes that stimulate cell proliferation) and on the deactivation of tumor suppressor genes (TSG),
Describe the process of DNA replication
- DNA helicase unwinds
- DNA polymerase
4) Enzyme works from 5’ to 3’ so replicates the leading 5’ strand continually
5) The lagging strand is replicated in okazaki fragments, these are later linked together by DNA ligase
