Mitosis: Asexual reproductionCloning • Populations require variation • If all individuals in a population were exactly the same: • Life would be uninteresting • The population would be susceptible to a disease that had the potential to wipe the population out: all or nothing. No individual would have allele combinations to keep the population going so all would be affected. • Cloning is therefore a bad idea
A GAMETE IS ANOTHER NAME FOR A SEX CELL A MALE GAMETE IS USUALLY CALLED A SPERM CELL A FEMALE GAMETE IS USUALLY CALLED AN EGG OR OVUM
GENETICS - MEIOSIS MEIOSIS • Why do cells go through MEIOSIS? • To produce gametes. • To increase genetic variation. • To half the number of chromosomes. • Why?
GENETICS - MEIOSIS MEIOSIS • If gametes had 46 chromosomes and did not become haploid, how many would the offspring have? • 92 Chromosomes • How many would their offspring have? • 184 Chromosomes • How many would the next offspring have? • 368 Chromosomes
Meiosis – the mixing of genetics. Meiosis has two main jobs: • It takes one diploid cell and produces four haploid gametes ready for sexual reproduction. • Creates variation by producing new combinations of alleles
Variation Variation results from processes occurring in Meiosis in sexual reproduction Crossing-over and Independent assortmentduring meiosis are the two processes that produce variation by producing new combinations of alleles
Independent assortment in meiosis • Independent assortment this is where each pair of chromosomes line up at the equator and segregate into the gametes independently of the other pairs of chromosomes. • For each chromosome pair, the chromosome arrangement is random. As can be seen, there are two alignment possibilities which produces different combinations of alleles in the gametes. • Think to a scale of 46 chromosomes, or 23 pairs.
Independent assortment Independent assortment • In a cell with just two pairs of chromosomes there are four different types of outcomes for the way the chromosomes can separate.
Crossing over and recombination Crossing over and recombination of alleles When homologous pairs line up during the first division of meiosis the two homologous chromosomes exchange pieces of chromatid by a process called crossing over… This creates variation by producing new combinations of alleles (DNA) in the gametes and hence the offspring. Think to a scale of several thousand genes.
VARIATION AND FERTILISATION Fertilisation promotes variation as there is a random fusion of gametes at fertilisation bringing together chromosomes from two different parents creating new combinations of alleles in the offspring.
Importance of variation Variation is important for a population because: • If the environment changes then some individuals will have the allele combinations to allow for their survival and hence phenotype that makes them well adapted to the changing environment. Thus a population with genetic diversity is more stable in changing environmental conditions. • It provides variation amongst individuals for natural selection to act on.
Environmental changes • Environmental Changes include: • climatic changes • disease (resistance to disease is inherited) • changes in food resources • changes in predator pressures or grazing pressures
Gene Expression: How are genes expressed? • Genes are expressed by a process called protein synthesis. Every gene codes for a protein. • Every gene codes for several amino acids, which are protein monomers. • Examples of common proteins coded by genes: • Keratin (hair and nails) • Haemoglobin (carries oxygen to organs) • Collagen (skin) • Genes, made up of DNA code for mRNA codons. • 1 codon triplet (of 3 nucleotides) codes for 1 amino acid.
Mutations We think:
Mutations • What we don’t think of is:
The Truth About Mutations A mutation is defined as a permanent, random change to the DNA sequence of a cell or organisms’ DNA. The altered DNA sequence in turn may lead to the production of a different protein. This may be advantageous, silent, or disadvantageous.
Mutations 1. Any change in an organism’s genetic makeup 2. Ultimate source of genetic variation 3. Mutations create new alleles that provides the variation in a population.
Point Mutations • Substitutions: One nucleotide is substituted for another. May or may not change the amino acid sequence. • Insertions/Deletions (INDELS): One nucleotide is inserted or deleted. Frameshift of all amino acids after the insertion/deletion. New or non-functioning protein. • Stop Codon: A substitution or INDEL may result in a stop codon being introduced. No more amino acids. Incomplete protein.
MUTATE A SENTENCE THESUNWASHOTBUTTHEOLDMANDIDNOTGETHISHAT Separate into codons: THE SUN WAS HOT BUT THE OLD MAN DID NOT GET HIS HAT Substitution: THE RUN WAS HOT BUT THE OLD MAN DID NOT GET HIS HAT
MUTATE A SENTENCE THESUNWASHOTBUTTHEOLDMANDIDNOTGETHISHAT Separate into codons: THE SUN WAS HOT BUT THE OLD MAN DID NOT GET HIS HAT Insertion: THE SUN WAS SHO TBU TTH EOL DMA NDI DNO TGE THI SHA T
MUTATE A SENTENCE THESUNWASHOTBUTTHEOLDMANDIDNOTGETHISHAT Separate into codons: THE SUN WAS HOT BUT THE OLD MAN DID NOT GET HIS HAT Deletion: THE UNW ASH OTB UTT HEO LDM AND IDN OTG ETH ISH AT
Effects of Mutations on organism Positive / Beneficial people who have 1 Sickle Cell Anemia allele (they are heterozygous) have immunity to Malaria, Immunity to HIV Neutral Eye color, Birth marks, no effect under current environmental conditions Harmful Sickle Cell Anemia (both alleles affected), cystic fibrosis – reduces survival chances or reproductive success Lethal haemophilia, muscular dystrophy
Mutagens • Mutations are spontaneous or induced by mutagens. They can occur if there is a mistake in the base pairing during replication • mutagens • Ionising radiation - eg: UV rays, gamma rays, x- rays • Chemicals - eg: dioxin, formaldehyde • Viruses - eg: hepatitis, HPV
Somatic vs Gametic Mutations SOMATIC MUTATIONS Some people may have mutations in the cells of their body. They are not passed on to the next generation / not inherited but may cause cancer (so are not retained in the population) GAMETIC MUTATIONS occur only in the germ cells (cells that give rise to gametes) or gametes. Gametic mutations can be passed to offspring / inherited so can be retained in the population.
Mutations, Sexual Reproduction and Variation • Mutations are the only novel form of variation in a population or species. • Sexual reproduction, whilst assisting in the continuation of variation in a population, merely reshuffles the alleles of genes that already exist in a population. The variety in allele combinations means that only perhaps some individuals will suffer or be susceptible to an environmental change. The population as a whole is more likely to survive.
How do meiosis and sexual reproduction contribute to variation • Independent assortment • Crossing over • Sexual reproduction - zygote formation from two genetically different individual’s gametes. • Gametes contain half the alleles of an individual. The allele combinations are in random assortment. • Sexual reproduction involves the random combination of two individuals randomly combined alleles to create an individual genetically different from any other in the species.
How does mutation contribute to variation • Mutations occur spontaneously in organisms in nature. They are a product of millions of DNA replication errors over millions of years. • Mutations have the potential to new genes, or novel alleles in a gene. • If a mutation affects a somatic cell during an organism’s lifetime, there is no chance of the alleles being passed to future generations. • If a mutation affects a gametic cell (egg or sperm) or the germ cells (pre-gametes) of an organism, there is potential for the novel allele created to be passed down to future generations.
Natural Selection • Natural selection occurs when the environment favours certain phenotypes over others. It is organisms with favourable phenotypes, whose genes are passed on. • Alleles that have positive or no impact on an organism’s survival are more likely to be inherited. • Alleles that have a negative effect on an organism’s survival are less likely to be inherited. Negative alleles may still be inherited. • They may be recessive alleles (therefore not be expressed in the phenotype) – sickle cell anemia • They may have their effect/express their phenotypes later in life (post-reproduction) – Huntington’s disease