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DNA

  • DNA helps with the following:

    • Forensic science

    • Paternal testing

    • DNA profiling

    • Study human migration patterns

    • Development of vaccines

    • Identification of biological remains

  • DNA stands for Deoxyribonucleic Acid and it is stored in the nucleus of the cell

  • It contains all the instructions for the function of the cell

  • DNA coils tightly to form a chromosome

    • This ensures no DNA is lost during cell division

  • Humans have 46 chromosomes in every cell

  • A gene is a segment of DNA that codes for a particular trait

    • E.g. ability to curl your tongue, hair colour, eye colour

  • Multiple genes can be found on a single chromosome (chromosome 1 has over 2000 genes on it!)

  • Nucleotide: Phosphate - Sugar - Base

  • The backbone of DNA = Phosphate and Sugar


DNA Replication

  • Helicase (Step 1)

    • The ‘unzipping’ enzyme

    • Comes and unwinds the DNA

    • Topoisomerase prevents DNA from supercoiling (stops overwinding of DNA)

  • Primase (Step 2)

    • Primase comes in and makes RNA primers on both strands

    • This is an important step so DNA polymerase knows where to start

  • Complementary Strands

    • Complementary strands are joined by hydrogen bonds and move in antiparallel directions

      • One strand goes 5’ to 3’ and the other goes 3’ to 5’ (based on the labelling of the sugar molecules Carbons)

  • DNA Polymerase (Step 3)

    • DNA polymerase builds the new strand in the 3’ to 5’ direction (using the old template strand as a base)

    • It also has a proofreading ability, so it rarely makes mistakes

    • The leading strand is complete, but the lagging strand is built in the wrong direction and has lots of gaps in it (known as Okazaki Fragments)

  • Ligase (Step 4)

    • Ligase comes along and fills in the gaps between each of the Okazaki Fragments

  • End Product

    • Two identical double-helix DNA molecules

    • This is semi-conservative because each of the two new copies contains one of the original strands of DNA

Cell Division

  • Cell division: the process by which one cell (parent cell) divides into two new identical daughter cells

    • This helps them grow and produce more cells

    • It also helps repair damaged tissues

  • Before cell division occurs, cell replicates all of its DNA so each daughter cell gets a complete copy of genetic information from the parent cell

  • Asexual Reproduction

    • Many organisms, especially unicellular organisms reproduce by cell division e.g. bacteria

  • Somatic (body) cells

    • All body cells in an organism have the same kind and number of chromosomes

    • We have 22 pairs of body chromosomes and 1 pair of sex chromosomes - a total of 23 chromosomes in the human body

  • Interphase

    • Period of cell growth and development

    • DNA replication occurs during this phase

    • Cells spend most of their time in this part of the cycle, where it is able to carry out normal cell activities

Mitosis

  • Prophase

    • Chromosomes coil up

    • Nuclear envelope disappears

    • Spindle fibers form

  • Metaphase

    • Chromosomes line up in the middle of the cell

    • Spindle fibers connect to chromosomes

  • Anaphase

    • Chromosome copies divide

    • Spindle fibers pull chromosomes to opposite poles

  • Telophase

    • Chromosomes uncoil

    • Nuclear envelopes form

    • 2 new nuclei are formed

    • Spindle fibers disappear


  • Cytokinesis

    • Division of the rest of the cellar after the nucleus divides

    • Cell then returns to interphase to continue to grow and perform cell activities

  • DNA controls all cell activities including cell division

    • If the DNA is damaged/mutated, then the cell loses their ability to control cell division

    • This causes super-dividing masses called tumors

      • Benign = Not cancerous

      • Malignant = Cancerous

Mutations

  • What is a mutation?

    • When DNA replicates itself, there are sometimes errors that aren’t detected and fixed (due to mutagens or chance)

    • Changes in the letters of the code also change the meaning of the code, which can have severe effects.

      • Too much or too little proteins could be produced.

      • Proteins help control and regulate iron levels leading to too much iron in the blood which can damage organs.

  • Types of mutation

    • Substitution

      • Letters of code are changed from their original intended sequence

    • Insertion

      • Extra letters of code are inserted into the original intended sequence

    • Deletion

      • Letters of code are deleted from the original intended sequence

  • What is a mutagen?

    • A factor that triggers mutations

    • Over-exposure to X-Rays, UV rays, or chemicals in pesticides are all mutagens.

  • Are mutations inherited?

    • If the sperm and ovum carries a DNA abnormality then the child could be affected with a genetic disorder

    • Examples of genetic mutations include:

      • Red-green colour blindness

      • Haemophilia

      • Cystic Fibrosis

  • Mutations: Pros & Cons

    • Not all mutations are harmful, in fact, some species actually rely on mutations in order to survive

    • Insects that have slight variations or mutations to their genetic code may be resistant to pesticides, and these are the ones who survive while the non-resistant ones die out.

    • Sickle Cell mutation occurs when an Adenine is replaced by a Thymine in the haemoglobin code

      • If a child inherits the trait from one parent, it can be beneficial, as the child will have no/mild symptoms but is resistant to malaria

      • Two copies, one from each parent lead to misshapen red blood cells that cause pain and block vessels

Inheritance

  • Mendel

    • Our understanding of the patterns of inheritance of genetic disorders and other characteristics involving peas

    • Mendel carried out experiments on peas to determine how individual traits are inherited

  • Why did he use peas?

    • Peas grow quickly and therefore many generations can be grown in a relatively short time period

    • He used a large number of pea plants to complete his experiments

    • He found that each parent passed on half of its factors to each offspring and that certain factors were dominant over others

  • Alleles

    • Gene = proportion of DNA that determines a certain trait

    • Alleles: different versions of the same gene

    • Individuals inherit two alleles from their parents: either dominant or recessive alleles

  • Dominant or Recessive

    • Dominant allele: the allele that is expressed over the recessive allele

      • Represented by a capital letter (e.g. brown eyes are represented by ‘B’)

    • Recessive alleles: the allele that is only expressed if BOTH alleles inherited are recessive

      • Represented by a lowercase letter (e.g. blue eyes are represented by ‘b’)

    • In rare cases, both the recessive and dominant allele can be expressed and produce a blending effect (e.g. red and white carnations can produce pink carnations)

      • Represented with upper and lowercase letters (Rr)

  • Homozygous and Heterozygous

    • Let’s assume R = round pea shape (dominant trait) and r = oval pea shape (recessive trait)

    • Homozygous

      • Two copies of the same allele

      • EX: RR, rr

    • Heterozygous

      • Two different copies of an allele inherited from each parent

      • One dominant and one recessive

      • EX: Rr

  • Genotype and Phenotype

    • Genotype: Unique sequence of DNA

    • Phenotype: The expression of the genotype (i.e. how the organism looks)

Sex-Linked Traits

  • Requirements of Test Crosses

    • Large numbers of offspring are needed to produce reliable data

    • Now we have genetic screening and genome mapping, test crosses are less commonly used

    • The Human Genome Project (2003) identified all the genes in human DNA and has allowed researchers to develop genetic tests to diagnose diseases

  • Sex-Linkage

    • When a gene controlling the expression of a trait is located on a sex chromosome

      • e.g. X or Y

    • The Y chromosome is much shorter than the X and contains fewer genes

    • The X chromosome is longer and contains genes not present on the Y chromosome

    • Therefore, sex-linked conditions are usually X-linked

  • Sex-Linked Patterns

    • Sex-linked traits tend to affect males more than females because they only have one X chromosome and cannot mask the trait with a second X chromosome

    • Females have two X chromosomes and therefore can carry two different alleles on their X chromosome

      • This can be heterozygous or homozygous

      • But remember the dominant gene is always expressed

  • Rules for Sex Linkage

    • Only females can be carriers (heterozygous) for a recessive disease, males cannot be carriers because they are hemizygous and only have one X chromosome

    • Females cannot inherit an X-linked recessive condition from an unaffected father (there is only one X chromosome, so this is the dominant one)

  • Writing Conventions: Sex Linkage

    • We write the allele as a superscript to the sex chromosome (X)

      • E.g.: Haemophilia: X^H = unaffected ; X^h = affected

Pedigrees

  • A pedigree is a type of diagram used to represent the individuals in a family and track the inheritance of a particular trait in that family

  • Symbols:

    • Unshaded square = Normal male

    • Shaded square = Affected male

    • Half-shaded square = Carrier male

    • Line between a square and a circle = Parents/Married

    • Line joined ABOVE a square and a circle = Siblings

    • Unshaded circle = Normal female

    • Shaded circle = Affected female

    • Half-shaded circle = Carrier female

    • Oldest offspring on the left - offspring must be in birth order

    • A triangular line joining offspring = Identical twins

    • Line looking like ^ joining offspring = Non-identical twins

  • Autosomal Dominant

    • If both parents are affected and the offspring is unaffected, the trait must be dominant (parents must be heterozygous)

      • All affected individuals must have at least one affected parent

      • If both parents are unaffected, all offspring must be unaffected (homozygous recessive)

  • Autosomal Recessive

    • If both parents are unaffected and the offspring are affected, the trait must be recessive (both parents are heterozygous carriers)

    • If both parents show a trait, then all offspring will show the trait (homozygous recessive)

  • X-linked Dominant

    • If a male shows a trait, all daughters as well as his mother must show the trait.

    • Unaffected mothers cannot have affected sons

    • Tends to be more common in females

  • X-linked Recessive

    • If a female shows a trait, all the sons and their father must also have the trait

    • Unaffected mothers can have affected sons if she is a carrier (heterozygous)

    • X-linked recessive tend to be more common in males

SL

DNA

  • DNA helps with the following:

    • Forensic science

    • Paternal testing

    • DNA profiling

    • Study human migration patterns

    • Development of vaccines

    • Identification of biological remains

  • DNA stands for Deoxyribonucleic Acid and it is stored in the nucleus of the cell

  • It contains all the instructions for the function of the cell

  • DNA coils tightly to form a chromosome

    • This ensures no DNA is lost during cell division

  • Humans have 46 chromosomes in every cell

  • A gene is a segment of DNA that codes for a particular trait

    • E.g. ability to curl your tongue, hair colour, eye colour

  • Multiple genes can be found on a single chromosome (chromosome 1 has over 2000 genes on it!)

  • Nucleotide: Phosphate - Sugar - Base

  • The backbone of DNA = Phosphate and Sugar


DNA Replication

  • Helicase (Step 1)

    • The ‘unzipping’ enzyme

    • Comes and unwinds the DNA

    • Topoisomerase prevents DNA from supercoiling (stops overwinding of DNA)

  • Primase (Step 2)

    • Primase comes in and makes RNA primers on both strands

    • This is an important step so DNA polymerase knows where to start

  • Complementary Strands

    • Complementary strands are joined by hydrogen bonds and move in antiparallel directions

      • One strand goes 5’ to 3’ and the other goes 3’ to 5’ (based on the labelling of the sugar molecules Carbons)

  • DNA Polymerase (Step 3)

    • DNA polymerase builds the new strand in the 3’ to 5’ direction (using the old template strand as a base)

    • It also has a proofreading ability, so it rarely makes mistakes

    • The leading strand is complete, but the lagging strand is built in the wrong direction and has lots of gaps in it (known as Okazaki Fragments)

  • Ligase (Step 4)

    • Ligase comes along and fills in the gaps between each of the Okazaki Fragments

  • End Product

    • Two identical double-helix DNA molecules

    • This is semi-conservative because each of the two new copies contains one of the original strands of DNA

Cell Division

  • Cell division: the process by which one cell (parent cell) divides into two new identical daughter cells

    • This helps them grow and produce more cells

    • It also helps repair damaged tissues

  • Before cell division occurs, cell replicates all of its DNA so each daughter cell gets a complete copy of genetic information from the parent cell

  • Asexual Reproduction

    • Many organisms, especially unicellular organisms reproduce by cell division e.g. bacteria

  • Somatic (body) cells

    • All body cells in an organism have the same kind and number of chromosomes

    • We have 22 pairs of body chromosomes and 1 pair of sex chromosomes - a total of 23 chromosomes in the human body

  • Interphase

    • Period of cell growth and development

    • DNA replication occurs during this phase

    • Cells spend most of their time in this part of the cycle, where it is able to carry out normal cell activities

Mitosis

  • Prophase

    • Chromosomes coil up

    • Nuclear envelope disappears

    • Spindle fibers form

  • Metaphase

    • Chromosomes line up in the middle of the cell

    • Spindle fibers connect to chromosomes

  • Anaphase

    • Chromosome copies divide

    • Spindle fibers pull chromosomes to opposite poles

  • Telophase

    • Chromosomes uncoil

    • Nuclear envelopes form

    • 2 new nuclei are formed

    • Spindle fibers disappear


  • Cytokinesis

    • Division of the rest of the cellar after the nucleus divides

    • Cell then returns to interphase to continue to grow and perform cell activities

  • DNA controls all cell activities including cell division

    • If the DNA is damaged/mutated, then the cell loses their ability to control cell division

    • This causes super-dividing masses called tumors

      • Benign = Not cancerous

      • Malignant = Cancerous

Mutations

  • What is a mutation?

    • When DNA replicates itself, there are sometimes errors that aren’t detected and fixed (due to mutagens or chance)

    • Changes in the letters of the code also change the meaning of the code, which can have severe effects.

      • Too much or too little proteins could be produced.

      • Proteins help control and regulate iron levels leading to too much iron in the blood which can damage organs.

  • Types of mutation

    • Substitution

      • Letters of code are changed from their original intended sequence

    • Insertion

      • Extra letters of code are inserted into the original intended sequence

    • Deletion

      • Letters of code are deleted from the original intended sequence

  • What is a mutagen?

    • A factor that triggers mutations

    • Over-exposure to X-Rays, UV rays, or chemicals in pesticides are all mutagens.

  • Are mutations inherited?

    • If the sperm and ovum carries a DNA abnormality then the child could be affected with a genetic disorder

    • Examples of genetic mutations include:

      • Red-green colour blindness

      • Haemophilia

      • Cystic Fibrosis

  • Mutations: Pros & Cons

    • Not all mutations are harmful, in fact, some species actually rely on mutations in order to survive

    • Insects that have slight variations or mutations to their genetic code may be resistant to pesticides, and these are the ones who survive while the non-resistant ones die out.

    • Sickle Cell mutation occurs when an Adenine is replaced by a Thymine in the haemoglobin code

      • If a child inherits the trait from one parent, it can be beneficial, as the child will have no/mild symptoms but is resistant to malaria

      • Two copies, one from each parent lead to misshapen red blood cells that cause pain and block vessels

Inheritance

  • Mendel

    • Our understanding of the patterns of inheritance of genetic disorders and other characteristics involving peas

    • Mendel carried out experiments on peas to determine how individual traits are inherited

  • Why did he use peas?

    • Peas grow quickly and therefore many generations can be grown in a relatively short time period

    • He used a large number of pea plants to complete his experiments

    • He found that each parent passed on half of its factors to each offspring and that certain factors were dominant over others

  • Alleles

    • Gene = proportion of DNA that determines a certain trait

    • Alleles: different versions of the same gene

    • Individuals inherit two alleles from their parents: either dominant or recessive alleles

  • Dominant or Recessive

    • Dominant allele: the allele that is expressed over the recessive allele

      • Represented by a capital letter (e.g. brown eyes are represented by ‘B’)

    • Recessive alleles: the allele that is only expressed if BOTH alleles inherited are recessive

      • Represented by a lowercase letter (e.g. blue eyes are represented by ‘b’)

    • In rare cases, both the recessive and dominant allele can be expressed and produce a blending effect (e.g. red and white carnations can produce pink carnations)

      • Represented with upper and lowercase letters (Rr)

  • Homozygous and Heterozygous

    • Let’s assume R = round pea shape (dominant trait) and r = oval pea shape (recessive trait)

    • Homozygous

      • Two copies of the same allele

      • EX: RR, rr

    • Heterozygous

      • Two different copies of an allele inherited from each parent

      • One dominant and one recessive

      • EX: Rr

  • Genotype and Phenotype

    • Genotype: Unique sequence of DNA

    • Phenotype: The expression of the genotype (i.e. how the organism looks)

Sex-Linked Traits

  • Requirements of Test Crosses

    • Large numbers of offspring are needed to produce reliable data

    • Now we have genetic screening and genome mapping, test crosses are less commonly used

    • The Human Genome Project (2003) identified all the genes in human DNA and has allowed researchers to develop genetic tests to diagnose diseases

  • Sex-Linkage

    • When a gene controlling the expression of a trait is located on a sex chromosome

      • e.g. X or Y

    • The Y chromosome is much shorter than the X and contains fewer genes

    • The X chromosome is longer and contains genes not present on the Y chromosome

    • Therefore, sex-linked conditions are usually X-linked

  • Sex-Linked Patterns

    • Sex-linked traits tend to affect males more than females because they only have one X chromosome and cannot mask the trait with a second X chromosome

    • Females have two X chromosomes and therefore can carry two different alleles on their X chromosome

      • This can be heterozygous or homozygous

      • But remember the dominant gene is always expressed

  • Rules for Sex Linkage

    • Only females can be carriers (heterozygous) for a recessive disease, males cannot be carriers because they are hemizygous and only have one X chromosome

    • Females cannot inherit an X-linked recessive condition from an unaffected father (there is only one X chromosome, so this is the dominant one)

  • Writing Conventions: Sex Linkage

    • We write the allele as a superscript to the sex chromosome (X)

      • E.g.: Haemophilia: X^H = unaffected ; X^h = affected

Pedigrees

  • A pedigree is a type of diagram used to represent the individuals in a family and track the inheritance of a particular trait in that family

  • Symbols:

    • Unshaded square = Normal male

    • Shaded square = Affected male

    • Half-shaded square = Carrier male

    • Line between a square and a circle = Parents/Married

    • Line joined ABOVE a square and a circle = Siblings

    • Unshaded circle = Normal female

    • Shaded circle = Affected female

    • Half-shaded circle = Carrier female

    • Oldest offspring on the left - offspring must be in birth order

    • A triangular line joining offspring = Identical twins

    • Line looking like ^ joining offspring = Non-identical twins

  • Autosomal Dominant

    • If both parents are affected and the offspring is unaffected, the trait must be dominant (parents must be heterozygous)

      • All affected individuals must have at least one affected parent

      • If both parents are unaffected, all offspring must be unaffected (homozygous recessive)

  • Autosomal Recessive

    • If both parents are unaffected and the offspring are affected, the trait must be recessive (both parents are heterozygous carriers)

    • If both parents show a trait, then all offspring will show the trait (homozygous recessive)

  • X-linked Dominant

    • If a male shows a trait, all daughters as well as his mother must show the trait.

    • Unaffected mothers cannot have affected sons

    • Tends to be more common in females

  • X-linked Recessive

    • If a female shows a trait, all the sons and their father must also have the trait

    • Unaffected mothers can have affected sons if she is a carrier (heterozygous)

    • X-linked recessive tend to be more common in males

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