Heredity - bio

DNA, protein synthesis, cell division, mutation, and inheritance

What is DNA

A helical double stranded molecule that occurs bound to proteins in chromosomes in the nucleus

Contains genetic instructions for building and maintaining an organism

Deoxyribonucleic acid.

The two strands run in the opposite directions meaning it is anti-parallel

What is DNA made up of:

A nucleotide consisting of a phosphate group, a sugar, and a nitrogenous base (one of four)

Name the nitrogenous bases and how they align in the DNA strand

(complimentary base pairing)

Adenine, guanine, thymine, and cytosine
Adenine pairs with thymine, and guanine with cytosine

Held together by weak hydrogen bonds, A and T have 2, C and G have 3

Sugar-phosphate backbone of each strand is held together by covalent bonds (that align)

What are pyrimidines and purines?

They are nitrogenous bases that are crucial components of nucleic acids

Purines have two fused rings and pyrimidines have a single six-membered ring

Adenine and guanine are purines

Thymine and cytosine are Pyrimidines

Labelled diagram of DNA

Compare DNA in prokaryotes and eukaryotes

E: combination of DNA and histone proteins found in nucleus (chromatin) - linear and organised
P: DNA found in cytosol as unbound, circular DNA

What is the structure of chloroplast and mitochondrial DNA?

They are small and circular, not wrapped around histones - (like bacterium DNA) - there are usually many copies of DNA in a single mitochondrion/chloroplast

Similarities between DNA in mitochondria and chloroplasts are important in the endosymbiont theory, suggesting that they originated as free-living prokaryotic cells

Function of DNA

To store and contain the genetic information that codes for traits (passed on to future generations) - life needs this to continue

Distinguishing features between prokaryotic and eukaryotic cells

Location and form of DNA in prokaryotes and eukaryotes

E: linear chromosome in membrane bound nucleus with genetic information

P: circular chromosome in nucleoid region with genetic information

What is the process of DNA replication?

The process where DNA undergoes to make a complete and identical copy of itself (preparing for cell division and ready to divide for growth) - an exact copy is needed

Consisting of one parental strand and one new strand

Why is DNA replication semi-conservative?

One of the two strands is conserved or retained from one generation to the next, while the other strand is new

Name enzymes involved in DNA replication:

• DNA helicase

• DNA polymerase

• RNA primase

• DNA ligase

What does DNA helicase do?

Unwinds and separates the double strand, breaking the hydrogen bonds

What does DNA polymerase do?

Adds free nucleotides to the template strand in a 5’ to 3’ direction

What does RNA primase do?

Attaches short segments to DNA to show polymerase where to go/start adding nucleotides

What does DNA ligase do?

Removes the primers

Explain the central dogma of protein synthesis

Describes the flow of genetic information from DNA (transcribed) to RNA (translated) to protein - only flows in one direction

What is epigenetics?

Refers to factors that effect the expression of genes that may be inherited

Epigenetic processes are important in the development of cancer and differentiation of embryonic tissues

What does acetylation mean?

It means that the genes are ‘switched on’ by acetyl tags on the histones

Form of histone modification that enhances gene expression. It loosens the interactions between DNA and histones, allowing easier access to DNA

What does methylation mean?

The turning off of genes - silencing

These are added to a cytosine in the DNA that are next to guanine

Methyl tags block transcription factors from binding to DNA

Causes DNA to bind more tightly to histones

Interactions leading to gene expression?

• Cell signaling

• Paracrines and Autocrines: chemical signals nearby and within a cell

• Hormones

• Environmental factors

What is RNA?

Ribonucleic acid

A single-stranded molecule with ribose sugar (instead of deoxyribose sugar)

Contains uracil instead of thymine but still contains a nucleotide

Compare DNA with RNA

Function of Antibodies:

Specialised proteins that defend the body from antigens - travel through blood stream to identify/defend bacteria/viruses

E.g. IgG

Function of contractile proteins

Responsible for movement and involved in muscle contraction + movement.

E.g. Actin in the cytoskeleton

Function of enzymes

Facilitate biochemical reactions, speeding up the chemical reactions.

E.g. Lactase - breaks down the sugar lactose in milk

Function of hormonal proteins

Messenger proteins which help coordinate certain bodily activities

E.g. Insulin - regulates metabolism by controlling the blood-sugar concentration

Function of structural proteins

Fibrous, stringy and provide support.

E.g. collagens - support for connective tissues, such as tendons and ligaments

Function of storage proteins

Store amino acids

E.g. Ovalbumin - found in egg whites

Function of transport proteins

Carrier proteins that move molecules from one place to another around the body.

E.g. Hemoglobin transports oxygen through the blood

What are proteins?

large biomacromolecules and macromolecules that contain one or more long chains of amino acid residues

What is protein synthesis

Process where cells produce proteins from instructions encoded in genes found in the coding section of the cell’s DNA (two steps transcription + translation)

Involves transcription of a gene into mRNA in the nucleus

• mRNA is translated into amino acids

Steps of transcription

1. RNA polymerase binds to the promoter site of the DNA

2. RNA polymerase separates the DNA strands

3. New nucleotides are inserted to the template strand, thymine is replaced with Uracil

4. This continues until the terminator is reached

5. As RNA polymerase moves along the DNA, the hydrogen bonds are reformed

Steps of translation

1. mRNA attaches to ribosome

2. As mRNA moves along ribosome, tRNA brings in an amino acid

3. The ribosome hitches together the amino acids to form polypeptide chains

4. When the end is reached, the polypeptide is sent to ER to be modified into functioning protein

How to read an amino acid table?

What does the redundancy/degeneracy of the genetic code mean?

It means that multiple codons can specify the same amino acid

This is because there are 64 possible codons but only 20 amino acids

What is the end product of mRNA?

A polypeptide - synthesised through process of translation

What is an exon?

The coding region of RNA/DNA

What is an intron?

Non-coding region of RNA/DNA

Role of mRNA

Messenger RNA: transports the code from the nucleus into the cytoplasm and the ribosome

*carries information

Role of tRNA

Transfer RNA: found in the cytoplasm, a tRNA carries a specific amino acid to the ribosome for incorporation into the growing peptide.

tRNA anticodon matches opposite codon in the mRNA

Role of the ribosome

Acts as the site where mRNA is (read) translated into a specific sequence for amino acids, forming a polypeptide chain of protein

What are anticodons?

A set of three consecutive nucleotides (part of tRNA molecule) and is complementary to a codon

Sequence is read three bases at a time

Difference between triplet code, anti-codons, and codons

Triplet code: sequence of three nucleotides in mRNA/DNA that determines a specific amino acid or stop signal

Anti-codon: sequences of three nucleotides on tRNA that is complementary to a codon on mRNA

Codon: a specific sequence of three nucleotides that codes for a particular amino acid or stop signal (during protein synthesis)

mRNA codon table

• just know that it lists the codons for each of the 20 amino acids

What is gene expression

The gene being transcribed into mRNA and translated into a protein

Regulated by proteins that bind to a specific base sequence in DNA

Impacts on gene expression:

Environment of a cell and organism: can influence the way genes are expressed (affects chromatin structure and gene expression)

E.g. diet, stress, toxins, hormones, temperature

What helps regulate transcription - eukaryotes

Nucleosomes: act as physical barriers and a substrate for epigenetic modifications that can affect the transcription process of DNA

• Provide the “architectural framework” for transcription

What is binary fission ? (process)

Asexual reproduction where a prokaryotic cell divides into two identical daughter cells

DNA SEGREGATES AND CELL DIVIDES ALL IN ONE STEP

1. single chromosome is tightly coiled (prior)

2. Genetic material in chromosome uncoils and replicates

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

4. New cell wall starts to grow cleavage furrow develops in cell membrane

5. New cell wall fully develops

6. 2 cells separate (cytokinesis), forming two identical daughter cells

The steps of mitosis:

Type of nuclear division in somatic cells that maintains the parental diploid number of chromosomes in the daughter cells

Prophase:

1. Chromatin threads condense to form chromosomes (two sister chromatids held together by centromere)

2. Nuclear membrane disintegrates and nucleolus disappears

3. Spindle begins to form, fibres attach to each chromsome at its centromere

Metaphase:

1. The chromosomes move to the centre of the cell and line up along the equator

2. Centromeres of chromosomes are aligned on the equator

3. Spindle fibres attach to the centromere

Anaphase:

1. Sister chromatids separate and are pulled to opposite part of the cell

2. Each pole has complete identical set of maternal and paternal chromosomes

3. Sister chromatids are now chromosomes

Telophase

1. Chromosomes decondense (forms chromatin)

2. Two new nuclear envelopes form (one for each new daughter cell)

3. Spindle disappears

4. Cell elongates and is ready for cytokinesis to separate.

Compare mitosis and meiosis (with drawings)

Process of meiosis

Type of cell division that produces gametes with half the number of chromosomes as the original cell

Prophase I:

• Chromosomes condense + become visible

• Homologous chromosomes (matching pairs) pair up

• Crossing over: sections of DNA are exchanged between homologous chromosomes (increasing genetic diversity)

Metaphase I:

• Homologous chromosome pairs line up in middle of cell

• Independent assortment: chromosomes are randomly arranged (variation)

Anaphase I:

• Homologous chromosomes are pulled apart to opposite ends of cells

Telophase I & Cytokinesis:

• 2 new cells form, each with half the original chromosome number (haploid)

• Nuclear membranes may form temporarily

MEIOSIS II (similar to mitosis)

Prophase II:

• Chromosomes condense again in born cells

• Spindle fibres form

Metaphase II:

• Chromosomes line up at the centre of the cell

Anaphase II:

• Sister chromatids are pulled apart to opposite sides

Telophase II & Cytokinesis:

• 4 haploid cells form, each genetically unique

Where does meiosis occur?

Males: testes (sperm cell)

Females: ovaries (egg cell)

Process of fertilisation

(leads to variation)

Any sperm can fertilise any egg, leading to even more variation

• Creates a new organism allowing life to continue

Is the DNA sequence unchangeable?

No

May arise spontaneously during DNA replication, or cell division, or may be induced by physical or environmental factors (mutagens), or through biological agents.

What is a mutagen?

An agent capable of inducing mutations

What is a mutation?

A source of new alleles - a permanent change in the DNA sequence of an individual

What are the types of physical mutagens?

• UV LIGHT: structural distortion by cross-linking neighboring nucleotides

• X-RAYS: gene and chromosome aberrations

• NUCLEAR RADIATION: breaks in DNA strands

  • High energy radiation causes damage to DNA

What are the types of chemical mutagens?

• MUSTARD GAS: affects guanine, causing substitution mutation

• 2-AMINOPURINE, 5-BROMURACIL: nucleotide substitution

• COLCHICINE: prevents spindle formation in mitosis; doubles chromosome number

• NITRIC ACID: adenine in DNA is deaminated (behaves like guanine)

Difference between somatic and gametic mutations (and their effects)

Somatic - MITOSIS: occurs in body cells and are not inherited

E.g. mutations that lead to cancer (in a tissue)

Gametic - MEIOSIS: occur in reproductive cells (gametes) and can be passed on to offspring

E.g. Embryo becomes diploid (2 haploid gametes fusing)

What are cell division errors?

Occurs during mitosis or meiosis and is when the chromosomes aren’t sorted properly and end up with less or more chromosomes

Can result in unequal crossing over when chromosomes are misaligned

What are DNA replication errors

Base-pair substitution results in either a mistake being fixed or a mutation

But this is rare as DNA mechanisms are very effective

What are point mutations (and examples)

A genetic alteration where a single nucleotide base in RNA or DNA is changed, can be neutral, harmful or beneficial to the organism

Includes substitution, insertion and deletion

What is insertion/deletion?

Mutation where one or more nucleotide pairs have been added/deleted to a segment of DNA

FRAMESHIFT MUTATION - READING FRAME CHANGES

Codons are read wrong and the wrong amino acid is added to a protein

What is substitution?

A mutation where a single nucleotide is swapped for another in the original gene sequence

Missense: base substituted codes for a different amino acid, a different amino acid is made

Silent: base substituted still codes for the same amino acid, no effect

Nonsense: base substituted produces a stop codon, protein can no longer be made

What are chromosomal mutations?

Changes in the number or structure of chromosomes (may alter gene locations or copy numbers leading to genetic disorder)

What is monoploidy and one effect

Number change:

An individual that contains one half the normal number of chromosomes

• Effect: unmasks harmful alleles

*very rare

What is polyploidy and one effect?

Number change

When an organism gains a full set of extra chromosomes

Lethal in humans, but common in plants

Chromosomal mutation during meiosis where cells contain more than 2 haploid (n sets) of chromosomes

What is aneuploidy and one effect?

Number change:

NON-DISJUNCTION:

When an organism gains or loses an extra chromosome

• E.g. trisomy: down syndrome (extra chromosome 21)

Results in the formation of two types of gametes in equal proportions (one type has two copies of a particular chromosome and the other has none).

What is deletion and its effect?

Structure change:

The loss of nucleotides from a site within the original gene

• Effect: frame shift mutation - reading frame for the corresponding amino acids has been nudged away from the original (all codons downstream of mutation are affected)

• Non-coding section: no effect

• Coding section: lost, proteins not made

What is inversion and its effect?

Structure change:

Chromosome segment breaks off, flips around backwards, and reattaches.

• Effect: less dramatic, may disrupt a gene where it causes two different genes to fuse together. If chromosomes do not align properly affected individual may have reduced fertility

What are translocations and its effect?

Structure change:

When a section of one chromosome breaks off and reattaches to another

• Effect: several negative impacts but commonly links to cancer and infertility

What are duplications and its effects?

Structure change:

When an extra copy is made of a section of a chromosome and inserted either into the same chromosome or another.

• Effect: harmful and advantageous:

  gain of new function of organism or commonly results in protein function loss

Why is Mendel’s work important?

He was the founder of genetics and through his work with pea plants he discovered the basics from which all genetics stems

Mendelian inheritance refers to how certain traits are inherited in a predictable way, for a monohybrid cross, and that dominant alleles will mask recessive alleles

What are Mendel’s three laws?

• Law of dominance

  • Hybrid offspring’s will only inherit the dominant trait in the phenotype, alleles suppressed are the recessive traits (masked)

• Law of independent assortment:

  • States that a pair of trait segregates independently of another pair during gamete formation. As the individual heredity factors assort independently, different traits get equal opportunities to occur together.

• Law of segregation

  • States that during production of gametes, two copies of each hereditary factor segregate so offspring get one factor from each parent

State Mendel's laws to predict inheritance when there are 2 different genes

• Law of Segregation and law of independent assortment:

  Segregation states that alleles for a trait separate during gamete formation, while independent assortment states that alleles for different traits assort independently during gamete formation

What is incomplete dominance?

When neither allele ‘dominates’ the other. Both alleles are present in the heterozygous genotype and both contribute to a phenotype (3 phenotypes may occur)

E.g. Snapdragon with RR (Red) WW (white) RW( (pink)

What is codominance?

When both alleles are equally dominant. Both present in the heterozygous genotype and combine.

E.g. Speckled chicken, BB (black), WW (white), BW (white and black spots)

How are genes with multiple alleles inherited?

Occurs with genes that have more than 2 different alleles, though an individual will only have two of the alleles in its genotype

E.g. Inheritance of human blood groups show multiple alleles, complete dominance, and co-dominance.

What is polygenetic inheritance and why do some genes show in continuous variation?

Polygenetic inheritance: Transmission between generations of characteristics that are controlled by polygenes

Some genes show continuous variation because traits such as height and eye colour fall among a range of values rather than being categorised into a group

How are genes on the sex chromosome inherited?

Genes on the sex chromosomes are sex-linked.

The X comes from the mother, and the Y comes from the father

• Females: receive one X from the mother and one X from the father

• Males: receive one X from the mother and one Y from the father

Identify the symbols on a pedigree chart:

How are genes with multiple alleles inherited?

Each gene can have several different versions (alleles) but an individual can only carry two - one from each parent

Rules for a sex-linked inheritance pattern?

1. Affected female has to have an affected father

2. Affected males cannot have affected sons

Therefore would be autosomal

Rules for autosomal inheritance pattern? (Dominant vs Recessive)

1. If it is recessive, affected offspring can’t have affected parents

2. If two affected parents have an affected child, it must be dominant

Life cycle of a cell?

The sequence of events from one cell division to another

Begins when the cell is formed from its parent cell and completed with its own division.

Stage between cell divisions is called interphase, incorporates a period of metabolic activity and growth (G1), duplication of chromosomes and centrosomes (Synthesis phase), and further growth and reproduction of organelles as the cell prepares to divide (G2)

Cells in G0 have withdrawn from the active cell cycle and can only re-enter the cell under certain circumstances

What does a genome consist of?

All the nucleotides, the complete set, in the DNA of the cell

Contains all the information needed for a cell's growth, survival and replication

Describe the movement of the genetic material in a cell during the cell cycle

During interphase, cells that undergo division go through phases where genetic material replicates within the nucleus.

In mitosis, chromatin threads condense to form chromosomes. As the nuclear membrane disintegrates, the chromosomes move to the center of the cell. Chromatids are separated by shortening of spindles, pulling each chromatid towards the opposite pole of the cell. Newly formed nuclear membranes form around decondensed chromatin.

The cell undergoes cytokinesis, forming two identical daughter cells and marking one full cycle