MODULE 5: HEREDITY

1: Reproduction - How does reproduction ensure the continuity of a species?

Mechanisms of reproduction that ensure the continuity of a species

o Animals (advantages of external and internal fertilisation)

Internal ExternalPerformed by Terrestrial plants, birds,

mammals, reptilesMarine organisms (fish, sponges, amphibians)

Gametes produced Many male, few female MillionsFertilisation chance High LowFrequency More frequent Less frequentEnvironment Water body, vulnerable

to external conditionsFemale body/eggs, safe until birth

Advantages Higher fertilisation chance, offspring protected

Many offspring produced, rapid colonisation, simpler and faster

o Plants (asexual and sexual reproduction)

Asexual SexualHow it is performed Runners (outgrowth

stem -> new plant), budding, spores, etc.

Pollen lands on stigma, goes through pollen tube to ovary, fertilises ova. Can be self-pollinated or cross-pollinated.

Cell division Mitosis MeiosisOffspring Genetically identical to

each other and parentCombination of info from parents

Parents needed One TwoAdvantages Quickly produces many

offspringMore variation

Disadvantages Less variation—more risk under environmental change

Complex, takes more time and energy

o Fungi (budding, spores): Fungi can release spores, single cells produced by mitosis that can grow into a new fungus. They can also produce a replica of itself (via mitosis) that grows as a new organism, attached to the parent.

o Bacteria (binary fission): This is when bacteria undergoes mitosis and splits into two new organisms.

o Protists (binary fission, budding): See above. Protists are usually algae/fungi.

Features of fertilisation, implantation and hormonal control of pregnancy and birth in mammals

o Fertilisation: Meiosis is undergone in the gonads, producing gametes (female: ovum, male: sperm). The male deposits sperm into the female reproductive system, where it travels into the oviduct, where fertilisation occurs. When that happens, the two gametes fuse to have 46 chromosomes. It becomes a zygote, which grows by mitosis as it travels to the uterus.

o Implantation: The zygote travels into the uterus and implants into its wall. The blastocyst (zygote’s outer layers) grow into the endometrium (uterus lining), which handles nutrition and waste for the first 2-4 weeks until the placenta forms.

o Hormonal control of pregnancy and birth: Luteinising hormone: Triggers ovulation and

development of corpus luteum. Oestrogen : Main female sex hormone, triggers ovulation

(release of egg from ovary). Maintains and stimulates production of other hormones, e.g. oxytocin.

Human chorionic gonadotrophin: Released during implantation -> keep the corpus luteum active -> ensure adequate progesterone -> maintain endometrium.

Progesterone : Released by corpus luteum during implantation/first trimester to maintain endometrium, then by the placenta to prevent contractions and miscarriage.

Oxytocin : Released towards end of third trimester to stimulate contractions, then milk production.

Prolactin : Released during third trimester to stimulate lactation.

o Other pregnancy terms: Gestation (development of offspring in uterus), oocyte (ovary cell, becomes ovum through meiosis), trimester (3 months).

Impact of scientific knowledge on the manipulation of plant and animal reproduction in agriculture

Reproductive technology

Definition/example Advantage Disadvantage

Artificial insemination

Inserting semen from a selected male into a selected female of same species.E.g. used in cattle, sheep, race horses.

Can inseminate many females from one male, transport is easier and more cost effective, can be used in conservation.

Reduces genetic variation -> susceptibility to environmental change

Artificial pollination

Dusting of pollen onto female stigmas.E.g. Mendel’s pea plants, modern agriculture.

Simple and inexpensive, consistency in growth rates/food quality, can create new varieties and increase genetic diversity (short-term)

Decreases genetic variation, monocultures increase risk of disease/pests

Whole-organism cloning

Producing a genetically identical organism without sexual reproduction.E.g. plants: cuttings, graftingsE.g. animals: Dolly the sheep, somatic cell nuclear transfer (put body cell nucleus in enuculeated egg cell, implant into surrogate mother).

Organisms with desired characteristics produced, predictable growth/yield, conservation.

Expensive with limited advantages, raises ethical issues, poses health issues to animals, reduces genetic diversity (identical plants are all susceptible to the same things).

2: Cell Replication - How important is it for genetic material to be replicated exactly?

Processes involved in cell replicationo Mitosis and Meiosis

Mitosis Meiosis1 division, produces two daughter cells

2 divisions, produces 4 gametes (sex cells)

Daughter cells are identical to parent and each other

Gametes are all different to each other

Number of chromosomes maintained Number of chromosomes halvedOccurs in asexual reproduction and in growth and repair

Occurs to make gametes for sexual reproduction

Meiosis 1: Interphase (diploid, chromatids are joined at centrometre) -> P1 (chromosomes condense, homologous chromosomes form bivalents, crossing over occurs and genetic material is exchanged) -> M1 (spindle fibres align bivalents at centre) -> A1 (bivalents are split, chromosomes move to opposite poles) -> T1 (cell divides into two haploid daughter cells, nuclear membrane reforms)

Meiosis 2: P2 (chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles) -> M2 (spindle fibres align chromosomes) -> A2 (sister chromatids separate and move to opposite poles) -> T2 (chromosomes decondense, nuclear membrane reforms, cells divide into four haploid daughter cells)

Mitosis: Interphase (chromatids joined at centrometre to form chromosome) -> Prophase (chromosomes condense, nuclear membrane dissolves) -> Metaphase (bivalents lined up at centre by spindle fibres) -> Anaphase (chromosomes pulled apart and become chromatids) -> Telophase (nuclear membrane reforms) -> Cytokinesis (cytoplasm divides, two diploid daughter cells are made)

o DNA replication using the Watson and Crick DNA model (incl. nucleotide composition, pairing and bonding)

Nucleotide composition: DNA stands for deoxyribonucleic acid. It is a double helix, a twisted ladder. DNA is made up of nucleotides, each containing a sugar, a phosphate and a base. The sugar and phosphate make up the backbone and the base pairs the rungs.

Pairing: DNA is made up of four nitrogen bases, held together by hydrogen bonds. Adenine and Cytosine always pair, and Guanine and Thymine always pair.

DNA replication: Both strands of a DNA molecule are mirror images of each other. The molecule is untwisted and ‘unzipped’ by enzymes, breaking the hydrogen bonds and forming a leading and a lagging strand. Enzymes connect nucleotides with complementary bases on the original strands to form two identical DNA strands.The leading strand is synthesised continuously (no breaks) in the direction of the replication fork (where the strands separate). The lagging strand is synthesised discontinuously (in fragments) in the opposite direction by Okazaki fragments.DNA replication occurs during interphase of meiosis and mitosis.

Effect of the cell replication process on the continuity of a species

o DNA replication allows cells to be replicated through meiosis and mitosis, which are essential processes in reproduction, as mitosis allows simpler organisms such as bacteria to multiply through asexual reproduction, while meiosis creates gametes which allow sexual reproduction. So cell replication is essential to species continuity by allowing asexual and sexual reproduction.

3: DNA and Polypeptide Synthesis - Why is polypeptide synthesis important?

Compare the forms in which DNA exists in eukaryotes and prokaryotesProkaryotic EukaryoticFound freely in cytoplasm Found in nucleusNaked (doesn’t bond w/ protein) Bound to histone proteins for

strength/stabilityCompact genomes (little repetitive DNA)

Genomes have lots of non-coding + repetitive DNA

Contains plasmids (DNA molecule separate of chromosomal DNA)

No plasmids

Circular Linear

Model the process of polypeptide synthesis

o Transcription and translation Transcription: After DNA unzips, a gene’s DNA

sequence is copied (transcribed) to make an RNA molecule. The enzyme RNA polymerase builds an mRNA molecule by pairing nucleotides with complementary bases on the non-coding strand (template strand)The info is transcribed in codons/triplets (groups of three bases). The mRNA strand is the same as the coding strand except uracil replaces thymine. The molecule begins to move away and transcription ends when the enzyme reaches a stop codon. The introns (segment of a gene that doesn’t code for proteins) are removed, leaving the exons (codes for proteins/polypeptides). The mRNA then moves out of the nucleus and onto a ribosome.

Translation: The ribosomes move along the mRNA, attaching tRNA molecules by temporarily pairing anticodons (correspond to codons) with corresponding codons, while another enzyme makes peptide bonds between the amino acids (1 codon = 1 amino acid). The tRNA breaks off, leaving the chain of amino acids—a polypeptide. The polypeptide may be joined by others, then it is folded into its shape to form a protein.

o Importance of mRNA and tRNA in transcription and translationmRNA is important to convey genetic information from DNA, serving as a messenger and specifying the amino acid sequence of the DNA.tRNA is important as it decodes an mRNA sequence into a polypeptide chain and then a protein.

o Function and importance of polypeptide synthesisPolypeptide synthesis forms polypeptides that fold to form proteins. Proteins have many essential roles within our cells. See ‘structure and function of proteins in living things).

o Assess how genes and environment affect phenotypic expressionCharacteristics are determined by genes (by directly coding for certain characteristics and features, e.g. eye colour) and the environment, depending on its various features that affect the organism, such as food and water availability. E.g. two plants growing in environments with different quantities of sunlight, moisture, and nutrients will result in different growth rates and yield.Examples: The Himalayan Rabbit (low temperatures->black fur, high temperatures->brown fur), the water buttercup (leaves above the water are broad and lobed, leaves under the water are thin and finely divides)

Structure and function of proteins in living thingso Primary structure: Chain of amino acidso Secondary structure: Folded polypeptide -> forms structural

proteins (bone, muscle, etc.)o Tertiary structure: Complex, 3D shape -> forms hormones

(chemical messengers) and enzymes (catalyse chemical reactions)

o Quaternary structure: Made of 2+ polypeptides -> form haemoglobin (carries oxygen)

4: Genetic Variation - How can the genetic similarities and differences within and between species compared?

Conduct practical investigations to predict variations in the genotype by modelling meiosis, incl. the crossing over of homologous chromosomes, fertilisation and mutations

o Homologous chromosomes/alleles: Chromosomes come in pairs (one from father, one from mother) called homologous pairs, containing equivalent sets of genes, allowing different alleles (alternate forms of a characteristic) to exist. One allele is often recessive while one is dominant, and the dominant one is usually expressed over the recessive one. (e.g. in Tt, T will be expressed)Mendel’s pea plant experiments produced this model of inheritance.

o Fertilisation: When sex cells (haploid number: 23 chromosomes each) fuse, they create a zygote (diploid number: 46 chromosomes). Homologous chromosomes line up and separate at random during meiosis, meaning that there are many possible gametes. Each gamete gets one of the four chromatids shown below. Each gamete also represents an allele.

o Crossing over: Occurs during prophase 1 of meiosis when homologous chromosomes pair up. Maternal and paternal chromosomes of each pair may tangle together and exchange segments of genes, making new gene combinations.

Model the formation of new genotypes produced during meiosis

o Interpreting autosomal, sex-linkage, co-dominance, incomplete dominance, and multiple alleles

Autosomal: Relating to chromosomes that aren’t sex chromosomes, e.g. in Mendel’s pea plants. Each plant carries 2 genes for a characteristic, each an alternate form of the characteristic (like tall and short). These genes are called alleles. One allele is dominant (T) and expressed over the other (t), which is recessive. If the two alleles for a characteristic are the same, it is homozygous (TT, tt), if different, it is heterozygous (Tt). Each parent passes one gene, so their offspring has two genes for the characteristic.The rhesus system only has two alleles, and is an example of dominant and recessive alleles (Rh+ is dominant to Rh-).

Sex-linkage: Relating to sex chromosomes (like female XX and male XY). Thomas Morgan, through fruit flies, found that sex chromosomes often carry only 1 gene

instead of 2 because the Y chromosome is smaller and has less genetic material, and doesn’t carry genes. Many recessive conditions are mainly expressed in males because a male only inherits one gene for a characteristic and it is always expressed, even if it is recessive, because he cannot inherit a dominant gene on his Y to mask its effect. (see punnett squares)

Co-dominance: Two alleles are both dominant and both apparent in the phenotype. An example of polygenic inheritance, where the inheritance of a characteristic is controlled by 2 or more genes.

Incomplete dominance: Two alleles blend together and show a blended effect in the phenotype (e.g. white + red flower = pink flower)

Multiple alleles: When there are over two alleles for a characteristic. E.g. ABO blood groups (A and B are dominant over O, A and B are co-dominant and = AB)

Blood group Genotype Antigens (are recognised

by antibodies)

Antibodies (recognise

antigens for attack)

A IAi or IAIA A BB IBi or IBIB B A

AB IAIB A + B NoneO ii None A + B

o Constructing/interpreting info from pedigrees and Punnett squares

Punnett squares: Allows you to predict the phenotypes of the offspring of two parents, using the genotypes of the parents.

Pedigree charts: Show inheritance patterns and allows inheritance of genetic disorders to be followed.

Problems usually whether the condition is dominant or recessive and for the genotypes of certain members.Rules: If two affected have an unaffected child, the trait is dominant. If two unaffected have an affected child, the trait is dominant. If every affected person has an affected parent, the trait is dominant.

Use data to represent the frequencies of characteristics in a population to identity trends, relationships, limitations, etc.

o Examining frequency data Gene frequency: How often a certain allele for a gene

occurs in a population. Gene pool: Every allele of every gene in a population of

a certain species. Hardy-Weinberg principle: In a sexually-reproducing

population with random matings, gene frequency will not change unless certain changes/events are occurring.

Changes: mutation, emigration (organisms leave, shrinking gene pool), immigration (organisms enter population, increasing gene pool), natural selection (see next point).

Natural selection: Certain alleles are selected for or against, depending on whether they increase or decrease an organism’s survival chance. Advantageous alleles are passed on to offspring who survive to reproduce and pass on the characteristic -> frequency of this allele increases.

o Analysing single nucleotide polymorphism (SNP)

Definition : Variation in a single nucleotide among a species/population’s DNA. E.g. at a specific position on the DNA strand, most people have an A, while some others have a C. This variation means that there is a SNP in this position. Other than SNPs, all humans have the same DNA.

Effects : Most SNPs occur in non-coding DNA, usually having no effect, but some can e.g. change the structure of tRNA molecules. Other, occur in genes and can cause changes to a nucleotide/protein (maybe stopping the protein from working), while some don’t any effect at all.

Applications : Can be used as markers to find disease-causing genes, and to track the inheritance of disease in families. The progression of a disease can also be dependent on certain SNPs, the analysis of which can open up personalised treatment options.

5: Inheritance Patterns in a Population - Can population genetic patterns be predicted with any accuracy?

Technologies used to determine inheritance patterns in a population

o DNA sequencing Used to determine the sequence of bases in DNA Sanger method: Genes/DNA is isolated and replicated

using PCR (polymerase chain reaction), the sequence is graphed by a computer.

Maxim Gilbert method: Chemicals used to identify a specific base, electrophoresis used to compare base patterns.

o DNA profiling

Used to identity and compare individuals based on their DNA sequence (and satellite DNA made up of STRs—short tandem repeats—sections of DNA unique to every individual)

A DNA sample is collected, DNA is isolated, PCR amplifies STRs, gel/capillary electrophoresis detects difference in size.

Applications: Paternity testing (comparing offspring’s DNA with potential fathers), forensic investigations (identifying suspects/victims with crime scene DNA)

Investigate data analysis from a large-scale collaborative project to identify trends, patterns and relationships

o Human Genome Project (HGP) A publicly funded international scientific project, aiming

to determine the DNA sequence of humans. The development of technology accelerated progress greatly.

Gave insight into human evolution/ancestry, allows for improved/personalised disease treatments, allows determination of disease inheritance

o Conservation population genetics Used to maintain biodiversity and genetic variation, as

genetic variation = organisms adapt to change = conservation of species. Harmful alleles can be detected and bred out, advantageous ones can be introduced.

Methods used: field observation, sampling, statistical analysis, DNA analysis (SNPs, haplotypes—groups of genes inherited together, GWAS—genome wide association study).

o Population genetics to determine disease/disorder inheritance

Enables scientists to better diagnose disease, study disease inheritance and improve treatment options.

Black urine disease—causes many issues. DNA sequencing/other technology found this disease was caused by a mutation in a specific gene.

o Population genetics in human evolution Anthropological genetics aims to explain causes of

human diversity (mutation, natural selection, genetic drift) and pathways of evolution.

Genetic evidence: Comparing human and chimp genomes, accumulated differences can be used to determine how long ago the species separated.

Modern humans: By comparing SNPs of indigenous people from different locations, we get clues about the patterns of migration and interbreeding that occurred.

Mitochondrial and Y-chromosomal DNA: Modern humans are all descended from a wave of migration out of Africa 60,000 years ago. We know this from mt-DNA, which is passed only from mother to daughter. Studies of SNPs in mt-DNA shows that all people are descended directly from a woman living in Africa 150,000 years ago. The same has been determined for males from Y-chromosomal DNA.