BIOLOGY DNA & GENETICS

DNA structure

Sugar Phosphate Backbone, Double helix, anti-parallel, phosphate, deoxyribose sugar, nitrogenous base ACTG (Complementary base pair rule)

DNA wrapped around histones

DNA Replication

Replication occurs at multiple sites along chromosomes to increase speed

Helicase breaks hydrogen bonds linking complementary base pairs.

Double helix unwinds and separates into two template strands, the point of separation is the replication fork

Primase attaches a primer to the template strands at a complementary sequence, creaking a 3’ position

Free DNA nucleotides pair up (A-T C-G) and form hydrogen bonds with template strands

DNA Polymerase joins nucleotides together in a 5’ to 3’ direction forming S-P backbone

Leading strand (5’ to 3’) forms towards replication fork and can synthesis continuously

Lagging strand is synthesised discontinuously away from replication fork, in 5’ to 3’ direction forming Okazaki fragments

Okazaki fragments are joined together by Ligase

Each new identical DNA molecule contains one original strand and one newly synthesised strand. (Semi-conservative Replication)

DNA Replication Diagram

Crossing Over

Occurs during Prophase-1

Matching regions on homologous chromosomes break and reconnect to the other chromosome. Results in genetic recombination.

Random Assortment

Occurs during metaphase-1

Paternal and maternal chromosomes are randomly arranged on each side of the equator, and are therefore randomly pulled to each pole.

Sexual Reproduction

Mother and Father both contribute half the offspring’s DNA, and therefore the offspring will have a random mix genes.

Haploid cell + haploid egg combine to produce diploid cell

Non-Disjunction

Any error during meiosis resulting in an abnormal number of chromosomes in the daughter cells.
ie spindle fibres taking both pairs of homologous chromosomes, instead of one.

Comparing Cell Division

Gene

A section of DNA that codes for a specific protein

Allele

An alternative form of a gene; codes for same trait, but different versions of the trait

Mutation

A permanent change in the DNA sequence, chromosome structure or number of chromosomes.

Induced Mutation

Caused by exposure to a mutagen

Spontaneous Mutations

Occur by chance during DNA replication or cell division.

Different causes of mutation

Mutagens - Chemical; Nitric Acid causes substitution. Physical; UV Radiation

Biological Agent - HPV

Errors in DNA Replication or Cell Division

Germline mutations

occur during meiosis, can be passed onto offspring if gametes are fertilised, does not affect health of original individual, but can result in infertile offspring

Somatic mutations

occur during mitosis, adult is affected, offspring is unaffected, can lead to cancer

Dominant

Only one copy of the allele is needed to be expressed in the phenotype

Recessive

Phenotype masked by the presence of a dominant gene - only expressed if two recessive alleles are present

Incomplete Dominance

Occurs when two different alleles are present, but neither allele is completely dominant

-Both alleles partially contribute to the phenotype and a third, intermediary phenotype is observed

Codominance

Occurs when two alleles are completely dominant

-Both alleles equally expressed in the phenotype

Polygenic Inheritance

For one characteristic, two or more genes and therefore two or more sets of alleles contribute to the phenotype

Coding DNA

DNA that is transcribed and translated

Noncoding DNA

DNA that does not get translated and therefore does not code for proteins.

Autosomal Recessive

Unaffected parents have affected offspring (skips a generation)

Autosomal Dominant

Affected offspring must have affected parent

Sex-linked Recessive

Affected males must express phenotype

Affected daughter must have an affected father

Affected mother must have affected sons

Sex-linked Dominant

Condition should appear in every generation

Affected males will not pass affected allele on to their sons - sons inherit father’s Y chromosome

Affected males will pass it onto their daughters because daughters always inherit father’s X chromosome

Sex-Linked Inheritance

Daughters get an X from mother and father

Sons get X from mother

Mitosis - Process

Interphase

Growth-1: Cell grows and carries out it’s normal tasks

Synthesis: DNA Replication - DNA Molecules in nucleus form exact copies

Growth-2: Cell prepares for division

Cell Division

Prophase: chromosomes condensed and visible, centrioles at opposite poles, phase ends with breakdown of nuclear membrane

Metaphase: Chromosomes align along equator, spindle fibres connect centrioles to chromosomes

Anaphase: Spindle fibres contract breaking the centromere, chromatids are pulled apart to opposite poles

Telophase: Spindle fibres disintegrate, nuclear membrane reforms

Cytokinesis

Animal Cell: Ring of filaments form around equator and tighten to split the cell

Plant Cell: New cell wall is created, splitting the two cells

Meiosis - Definition and Process

The process by which sex cells (gametes) are made in the gonads. It involves the division of a diploid germline cell into four genetically unique haploid daughter cells

Prophase-1: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form pairs (bivalents), crossing over occurs, centrioles move to opposite poles

Metaphase-1: Bivalents line up along equator of cell, spindle fibres from opposing centrioles connect to centromeres

Anaphase-1: Spindle fibres contract and homologous chromosomes move to opposing poles

Telophase-1: Chromosomes decondense, nuclear membrane reforms, cell divides (cytokinesis) to form two haploid daughter cells

Prophase-2: Chromosomes condense, nuclear membrane dissolves, centrioles move to opposite poles

Metaphase-2: Sister chromatids line up along equator, spindle fibres connect to centromeres

Anaphase-2: Spindle fibres contract, the sister chromatids are pulled apart, and the centromere breaks

Telophase-2: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) into four unique haploid daughter cells

Binary Fission - Definition and Process

A process of asexual reproduction whereby a prokaryotic cell divides into two identical daughter cells

1. Prior to binary fission, single chromosome is tightly coiled

2. Genetic material in chromosome and plasmid replicates and separates

3. Original and replicated chromosomes attach to cell membrane; are pulled to separate poles as cell elongates

4. New cell wall (septum) starts to grow; cleavage furrow develops in cell membrane

5. New septum fully develops

6. Two cells are separate, forming two identical daughter cells, chromosomes become tightly coiled again

Gene Mutations

Substitution

Addition

Deletion

Substitution

A nucleotide swap, which can result in:

• Synonymous - Mutation codes for same amino acid so effect is neutral

• Missense - Codes for a different amino acid

• Nonsense - Prematurely codes for a stop codon, halting translation. Results in a short useless polypeptide - harmful

Addition

Addition of one nucleotide - creates a frameshift resulting in a completely new sequence of amino acids, additionally, either synonymous, missense or nonsense mutations

A frameshift mutation near the start of an amino acid sequence will have much more effect than one at the end

Deletion

Deletion of one nucleotide - creates a frameshift resulting in a completely new sequence of amino acids, additionally, either synonymous, missense or nonsense mutations

A frameshift mutation near the start of an amino acid sequence will have much more effect than one at the end

Chromosomal/Block Mutations

Deletion

Inversion

Translocation

Duplication

Deletion

Middle piece of the chromosome falls out and the two ends rejoin

• Results in a loss of genetic material

Inversion

Middle piece of the chromosome falls out, rotates 180° and rejoins

• No loss of genetic material

Translocation

A piece of a chromosome breaks off and joins onto another non-homologous chromosome - when the chromosomes are passed to gametes some will receive extra genes, while some will be gene deficient

Duplication

A piece is lost from one chromosome and added to its homologue. One will be deficient while the other receives double the genes.

Euploid, Aneuploid, Monosomy, Trisomy

Euploid: A cell containing chromosomes correctly organised into complete sets

Aneuploid: A cell missing one or more chromosomes.

• Monosomy: One copy of a chromosome that should be present in two copies

• Trisomy: Third copy of a chromosome that should be present in two copies

Transcription

DNA code is copied into mRNA

1. DNA unwinds and unzips due to helicase, which breaks the hydrogen bonds

2. This exposes the nitrogenous bases of the template strand (3' to 5’)

3. RNA Polymerase binds to the promoter region

4. mRNA is synthesised from the start codon

5. RNA Polymerase makes mRNA by adding nucleotides under the complementary base pair rule, in a 5’ to 3’ direction

6. Instead of thymine, uracil pairs with adenine

7. mRNA synthesis finishes when the RNA Polymerase reaches the terminator region after the stop codon at the end of the gene

8. The mRNA released is single stranded and contains introns and exons

9. mRNA undergoes splicing, where introns are removed

10. The DNA zips up and the mRNA leaves the nucleus via a nuclear pore

Translation

mRNA at the ribosome is used to build a polypeptide

1. mRNA attaches to the ribosome at the start codon (AUG)

2. tRNA molecules each carry a specific amino acid

3. Each tRNA molecule has an anticodon that corresponds with a specific mRNA codon

4. As the ribosome reads the mRNA codons, tRNA molecules bring the appropriate amino acid in the correct sequence

5. tRNA detaches and collects another amino acid from the cytoplasm

6. Adjacent amino acids are joined together by peptide bonds

7. When the stop codon is reached, the mRNA is released from the ribosome

8. The polypeptide chain detaches and folds into a protein

DNA to mRNA to tRNA to Amino Acid

Punnett Square Marking Key

Polygenic Inheritance vs Monogenic Inheritance

Comparing DNA and RNA

Comparing nDNA and mtDNA