Unit 2 - Genetics - Notes PDF

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Unit 2:- GeneticsIntroduction to Genetics and DNAGenetics:- The branch of biology dealing with hereditary and the variation of inherited characteristics. It also involves examining the processes of hereditary through the transfer of genetic material from one parent to offspring. Genetic material is ...

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Unit 2:- Genetics Introduction to Genetics and DNA Genetics:- The branch of biology dealing with hereditary and the variation of inherited characteristics. It also involves examining the processes of hereditary through the transfer of genetic material from one parent to offspring. Genetic material is passed in the form of Deoxyribonucleic Acid (DNA) which is packaged in the form of chromosomes. ● Genetic Material is a term used to describe all the material inside the organism that stores genetic information. Chromosome:- They are long thread-like strand, composed mostly of DNA. Chromosomes are usually found in the nucleus of all eukaryotic cells, but they vary widely between organisms of different species in their number, shape and size. ● The DNA is wrapped and coiled around proteins known as histones. Each wrap is called a nucleosome. ● Within the nucleus, the chromosomes form a mass of thread-like structures known as chromatin. ● Humans have 46 chromosomes, which makes 2 sets of 23. One set from the paternal side and one set from the maternal side ● Somatic cells (general body cells) are diploid and contain 46 chromosomes (one set). Gametes (sex cells) are haploid and contain 23 sets of chromosomes (one set) ● A Karyotype is a visual display of condensed chromosomes during mitosis which have been arranged in pairs according to chromosome number. ● Due to the consistent appearance of stained chromosomes, large abnormalities can be seen through a karyotype of an individual’s chromosomes. Deoxyribonucleic Acid (DNA):● DNA or Deoxyribonucleic Acid is a molecule/part of chromosomes they at carries genetic information in the nucleus which is necessary for cell structures and functions.

● Gene is a segment of a DNA molecule that codes for a particular trait or to make a particular protein; found at a specific location on a chromosome. ● These genes are mapped to different loci on a chromosome, with each chromosome carrying information for hundreds or thousands of years. ● The code for the instructions is found in the sequence of nucleotides: cytosine(C), adenine(A), guanine(G), thymine(T) and uracil(U). Uracil replaces Thymine in RNA. ○ A (adenine) is paired with T (thymine) and vice-versa. ○ G (guanine) is paired with C (cytosine) and vice-versa. ● DNA is double stranded with each strand having its own nucleotides. The two strands are held together by bonds between nucleotides. ● The pairing of nucleotides is known as complementary base pairing. ○ Sequences that pair together are called complementary sequences. ● The order of the nucleotides gives instructions on how to make proteins for the cell. Proteins are composed of amino acids in sequence. The sequence of amino acids is determined by the sequence of nucleotides. ● Each group of 3 nucleotides represents one amino acid. Each combination of three bases is known as a codon. The relationship between DNA and proteins is part of the central dogma of molecular biology.

Cell Reproduction:● All cells come from preexisting cells. ● These new cells are created through cell reproduction where a parent cell divides to produce two identical daughter cells. ● Cell reproduction is important for growth and repair ● Cells can receive important molecules from the outside environment through diffusion. Cells can eventually reach a point where they are too large to perform efficiently

Simple Diffusion and Osmosis:● Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration until particle concentration is equal throughout. ○ This occurs when there is a difference in concentration between two areas which will form a concentration gradient. ○ Concentrations will reach a dynamic equilibrium where particles move at equal rates in all directions. ● Osmosis is the net movement of water across a selectively permeable membrane. It is basically simple diffusion of water molecules. ● Osmosis occurs when there is a difference in solute concentration across a selectively permeable membrane. ● When the cell is too large, diffused materials take longer to reach all parts of the cell. ● Therefore, if the ratio of cell surface area to cell volume is too small, the cell can divide.

The Cell Cycle:-

● The cell cycle is mostly spent in interphase which is the growth phase of the cell. When it is ready, it can enter the cell division phase which consists of mitosis and cytokinesis.

Interphase:● Most of a cell’s life span is spent in interphase. This phase is for growth of the cell(G1 and G2) and replication of DNA (S) in preparation for mitosis. ● Replicated DNA (in the form of chromatin) is held together by centromeres and is not yet visible as individual chromosomes. These can be called sister chromatids.

The Cell Cycle:- Mitosis Living cells must make new cells continually to grow and reproduce. Identical genetic material is passed on to every new cell through mitosis. Mitosis is the continuous process whereby each cell can replicate its DNA and produce a new nucleus for a new cell. There are four steps:Prophase:-

● The chromatin condenses to become visible chromosomes ● Nucleolus and nuclear membrane start to disappear ● Spindle fibres form in animal cells from the centrioles at opposite ends of the cell. Metaphase:● The spindle fibres attached to the centromere of each double stranded chromosome ● Chromosomes are lined up along the middle of the cell (equator) Anaphase:● The spindle fibres pull the centromeres apart. ● One strand of each chromosome gets pulled to opposite ends of the cell (identical set on each side). Telophase:● ● ● ●

Each end of the cell has a complete set of chromosomes Nuclear membranes forms around each set Chromosomes unravel to chromatin Spindle fibers disappear and nucleolus start to reappear.

Cytokinesis:● Plant cells form a cell plate in the middle of the “parent cell” which forms a new cell wall between the two “daughter cells” ● In animal cells the, the membrane pinches in to divide the cytoplasm into two cells; the fold is called a cleavage furrow.

The Cell Cycle Meiosis:A human somatic (Body cell) cell is diploid (2n) and has 46 chromosomes, consisting of 22 pairs of autosomes (chromosomes not involved in sex determination) and 1 pair of sex chromosomes (XY or XX) Human gametes (egg and sperm cells) are haploid (n) meaning they contain only one set of 23 chromosomes. These gametes must be produced through a different process from mitosis which produces identical daughter cells.

● Meiosis is a special type of cell division that occurs in reproductive organs to produce reproductive cells known as gametes. ● Mitosis consists of two stages of cell division:- Meiosis I and Meiosis II that are similar to mitosis. ● Meiosis results in four haploid daughter cells while mitosis results in two diploid daughter cells. This is known as reductive division. ● Allows for genetic recombination to produce genetic variety in gametes. Prophase 1:● Homologous Chromosomes pair to form a tetrad consisting of two pairs of sister chromatids . ● Crossing over of the homologous chromosomes allows the exchange of genetic information between non-sister chromatids of a homologous pair. ● This recombination contributes to genetic variation. ● The tetrad forms a synapsis where the non-sister chromatids can cross over. ● This crossed over location is known as the chiasmata

Metaphase 1:● Spindle fibres attach to the centromere of each chromosome ● Homologous chromosomes are then aligned at the equator of the cells in homologous pairs ● One of the homologous chromosomes attached to one pole and the other set attached to the other pole ● The positioning is random, resulting in independent assortment, which increases genetic variability ○ Independent Assortment The combination of chromosomes in the daughter cells is random, based on the random positioning of

homologous chromosomes during metaphase I. Each daughter cell will have a full set, but the proportion of paternal vs. maternal chromosomes will vary. Anaphase 1:● Spindle fibres shorten and separate the homologous chromosomes to opposite ends of the cell ● Centromere does not split, sister chromatids remain attached Telophase:● Chromosomes begin to uncoil, spindle fibres disappear, nuclear membrane begins to form, separation of cytoplasm ● At the end of telophase I, each nucleus does not contain identical information. ● Does not occur in all cells Meiosis II:● The phases (prophase II, metaphase II, anaphase II, telophase II) contain the same steps as mitosis ● Cells begin with a haploid number of chromosomes that are already replicated, consisting of two sister chromatids

Gametogenesis:● At the end of meiosis, there will be 4 haploid gametes of varying genetic composition.

● The genetic variability is introduced through the process of crossing over (beginning in prophase I), and independent assortment (beginning in metaphase I). ● This process of egg and sperm production is known as gametogenesis. ● Production of male gametes in animals is known as Spermatogenesis. ● Production of female gametes in animals is known as oogenesis. ● During oogenesis, only one of the resulting haploid cells becomes a viable mature egg.

Mutations Before mitosis and meiosis can occur, DNA must be replicated. As this is occuring, mistakes often occur. If they are not fixed, it is a mutation. Mutations can be spontaneous mutations or induced mutations. Common causes of induced mutations include radiation and cigarette smoke. Depending on the mistake, cells can undergo controlled cell death or they may pass on that mistake to subsequent cells. Mutation can have a Positive, Negative, or Neutral effect. Point Mutations:A small change in the nitrogenous base sequence in DNA. Three major categories of point mutations are base-pair substitution, insertion and deletion. There could be silent mutations when less severe. They could also lead to a frameshift mutation which is usually more severe. If mutations occur in somatic cells, they are not passed on to offsprings. Only the individual is affected. If mutations occur in sex cells, they can be passed onto offspring. This could affect future generations as a genetic disorder. The effect will depend on the gene and the

other gametes. Chromosomal Mutations:Meiosis produces gametes which are haploid. This means that human egg and sperm cells will have 23 chromosomes. When an egg and sperm cell combine, they will form a zygote with 46 chromosomes. Some genetic conditions and disorders are caused by mistakes made during meiosis, which can result in an irregular number or size of chromosomes. Nondisjunction:- Error during meiosis that results in uneven chromosome numbers in gametes. Nondisjunction occurs when homologous chromosomes fail to separate during anaphase or when sister chromotation fail to separate during anaphase II. This can result in trisomy or monosomy. Diagnosis Karyotype can be arranged from white blood cells that have encouraged to enter mitosis. ● Prenatal Testing ○ Recommended for women over the age of 35 ○ Chorionic villus sampling (CVS) ○ Amniocentesis. ○ Multiple marker screening

Down Syndrome:Abnormality:- An extra chromosome number 21 (trisomy 21) Frequency:● Approximately 1 in 800 live births ● Increases with age to around 1 in 40 live births for women in their forties Characteristics:● Round face, short height, large forehead ● Varying levels of developmental and intellectual disabilities

Turner Syndrome:Abnormality:- Only one X chromosome present (XO)

Frequency:- Approximately 1:2500 female births. Characteristics:● Female in appearance but often do not undergo puberty and are unable to conceive ● Short stature, along with minor physical traits ● Normal intelligence though developmental delays and behavioural problems do occur

Triple X Syndrome:Abnormality:-Three X chromosomes (XXX) Frequency:- 1 in 1000 female births. Characteristics:● Females with typically no unusual features ● Normal sexual development and are able to conceive ● Increased risk of learning disabilities ● Seizures or kidney abnormalities affect about 10 percent

Klinefelter Syndrome:Abnormality:- Two X chromosome and one Y chromosome (XXY) Frequency:- 1 in 500 male births. Characteristics:● Reduced testosterone that could lead to delayed or incomplete puberty and feminine body characteristics but with varying severity ● Usually sterile

Patau Syndrome:Abnormality:- An extra chromosome 13 (Trisomy 13) Frequency:- 1 in 25000 live births. Characteristics:● Severe intellectual disability and physical abnormalities ● Heart defects, brain/spinal cord abnormalities, small eyes, extra fingers or toes, cleft lip, weak muscle tone

● Death within first days or weeks of life. Only 10% survival past first year

Edward’s Syndrome:Abnormality:- An extra chromosome 18 (Trisomy 18) Frequency:- 1 in 6000 live births Characteristics:● Low birth weight, heart defects and other organ abnormalities ● Small, abnormally shaped head, clenched fist with overlapping fingers ● Death before birth or within first month. 5-10% survive past first year but with severe intellectual disability

Other Chromosomal Abnormalities:Aside from abnormal chromosome numbers, other abnormalities involving chromosome structure are also possible. The four main categories are deletion, duplication, inversion, and translocation.

Deletion:● Portion of a chromosome is missing ● Can be caused by viruses, irradiation, or chemicals ● Can result in mental and physical abnormalities. ● Example – cri-du-chat

Duplication:● Repetition of a gene sequence one or more times within one or more chromosomes ● Repeats are often common but at a certain point can affect the functioning of the generation ● Example – fragile X syndrome

Inversion:● Gene segment is inserted in reverse order ● Can have many effects through altered gene activity

Translocation:- Part of one chromosome switches places with part of another chromosome

Mendelian Genetics:Gregor Mendel:● ● ● ●

He was considered to be “the father of genetics” Mendel was a monk who started his work in inheritance in 1853. His work is the basis for genetic inheritance today. Three key factors that contributed to his success were:○ Appropriate organism choice ○ Appropriate design and execution of experiment ○ Proper data analysis.

Mendel worked with pea plants (Pisum sativum) which were key to his success. They were readily available, easy to grow and mature quickly, have contained sexual organs, and have a variety of easily identifiable traits. Mendel picked characteristics that had two possible variations.

Purebred Plants:● A purebred organism for a specific trait will only produced that trait when

mated with another purebred organism of the same trait. ● A hybrid is the offspring of two different pure-breeding plants. ● Trait is a particular version of a characteristic that is inherited, such as hair colour or blood type.

Monohybrid Cross:- A cross designed to study the inheritance of only one trait. ● A monohybrid is the offspring of true breeding plants that differ in only one characteristics. ● Therefore the offspring are only a hybrid for one characteristic. A cross is the successful breeding of two organisms from distinct genetic lines. ● P generation refers to the parent plants used in a cross. ● F1 Generation refers to the offspring of a P-Generation cross. Offspring of a cross between the F1 generation is the F2 generation.

Principle of Dominance ● Allele is a specific form of a gene ● Homozygous describes an individual that carries two of the same alleles for a given characteristics. ● Heterozygous describes an individual that carries two different alleles for a given characteristics. ● Genotype is the genetic makeup of an individual. Phenotype is an individual’s outward appearance with respect to a specific characteristics. ● Dominant Allele:- The allele that if present is always expressed. ● Recessive allele:- The allele that is expressed only if it is not in the presence of the dominant allele, that is if the individual is homozygous for the recessive allele. ● These are now known as the Mendelian ratios. This lead to the idea of dominant and recessive traits which Mendel termed as factors.

Law of Segregation After many repetitions, Mendel determined that traits must be passed on by hereditary units which he called factors. Even if they are not expressed, they can be passed on. ● Organisms inherit two copies of gene, one from each parent ● Organisms donate only one copy in each gene to their gametes because the genes separate during gamete formation.

Genes and Alleles ● It is now known that Mendel’s factors were variation of genes we call alleles. ● If an allele is dominant, it will be always expressed. If the allele is recessive, it will only be expressed when paired with another recessive allele. ● By convention, dominant alleles were denoted with capital letters and recessive alleles are donated with lowercase letters.

Genotypes and Phenotypes ● As diploid individuals, we have two copies of every gene meaning that we have two alleles per gene. ● Based on the relationship between dominant and recessive alleles, individuals can both show the same trait without having the same alleles. ● We use the term phenotype to indicate physical appearance of a specific trait and we use the term genotype to indicate the genetic combination of alleles. ● If an individual has the same allele for both copies, they would be homozygous for that allele. If the individual has two different alleles, they would be heterozygous for that allele.

Punnett Squares ● Punnett squares are a diagram that summarizes every possible combination from each allele from each parent; a tool for determining the probability of a single offspring having a particular trait. ● Probability is the likelihood that an outcome will

occur if it is a matter of chance.

Dihybrid Cross:● Matings between purebred plants that differed in two traits. Example:- A mating between a Heterozygous Yellow round and Heterozygous Yellow Round. Parent 1:- RrYy (RY, Ry, rY,

ry)

Parent 2:- RrYy (RY, Ry, rY,

ry)

Law of Independent Assortment This second law of inheritance states that inheritance of alleles for one trait does not affect the inheritance of alleles for another trait (on a separate chromosome). This is means that offspring may have new combinations of alleles that are not present in either parent. Product Law:- The probability of two independent random events both occurring is the product of the individual probabilities of the events.

Discontinuous Variation When the expression of the products of one gene has no bearing on the expression of the products of a second gene. Example:- Pea plants were Tall or Short and Yellow or Green. There is no in-between value.

Continuous Variation Continuous variation results in unclear results due to phenotypic variation that is along a spectrum. One gene may be affected by the expression of another gene. An additive allele can have a partial influence on phenotype, leading to continuous variation. Example:- in the general population there are many variations of skin colour, from pale, white to dark black. This is because skin colour is controlled by more than two genes and not by one specific one. An additive allele:- an allele that has a partial influence on a phenotype.

Continuous Variation explains the substantial variation of phenotypes found in nature.

Variations in Hereditary Complete Dominance:- A situation where an allele will determine the phenotype, regardless of the presence of another. Incomplete Dominance:- A situation where neither allele dominates the other and both have an influence on the individual; results in partial expression of both traits. Codominance:- A situation where both alleles ...