B13 - Genetics And Evolution Flashcards
Sexual reproduction
• two parents
• fusion of male and female gametes to form a zygote
• characteristics combination of both parents
• leads to genetic variation
gametes formed by meiosis, zygote divides by mitosis
• 23 pairs of chromosomes
• involves fertilisation
Asexual reproduction
• one parent
• no fusion of gametes so no mixing of genetic information
• produce clones - genetic material identical to parent
• no genetic variation in offspring
• mitosis is the only type of cell division involved
• genes are passed on from parent to offspring
• no fertilisation
What types of organisms can asexually reproduce?
• mainly bacteria - very fast! x10 faster than quickest animals
• starfish
• strawberries - grow stems called runners
• potatoes - tubers
• spider plants - grow new plants on their stems, if cut off and put in soil they grow
Meiosis
• parent cell
• chromosomes make identical copies of themselves
• similar chromosomes pair up
• sections of DNA get swapped
• first cell division - chromosome pairs separate
• second cell division
Cells in the reproductive organs divide by meiosis to form the gametes (sex cells).
• Body cells have two sets of chromosomes, gametes have only one set.
• In meiosis, the genetic material is copied and then the cell divides twice to form four gametes, each with a single set of chromosomes.
• All gametes are genetically different from each other.
• Gametes join at fertilisation to restore the normal number of chromosomes. The new cell divides by mitosis. The number of cells increases and as the embryo develops, the cells differentiate.
How is meiosis different to mitosis?
• genetically varied
• 23 chromosomes in a gamete
• divides twice
• four daughter cells
• the types of cells that are made are gametes
• sexual reproduction
• used in sexual reproduction
Similarities between meiosis and mitosis
• both types of cell division
• chromosomes duplicate in both
• all other subcellular structure have to replicate
How is mitosis different to meiosis?
• genetically identical daughter cells
• each daughter cell has 46 chromosomes
• cell only divides once
• two identical daughter cells are made
• normal body cells are made
• daughter cells are for growth and repair
• used in asexual
Number of chromosomes in one cell at the start/end of meiosis
Start = 16 (8 pairs) - doubles to 32 then splits 4 ways
End - 8
Explain why the change in the number of chromosomes is important (meiosis)
Meiosis forms gametes, and two gametes need to fertilise each other to keep chromosome number the same generation after generation
Describe how meiosis produces cells that are genetically different
Meiosis produces cells that are genetically different when DNA is exchanged between gametes, and when homologous chromosome pairs randomly orient themselves.
Who discovered DNA?
• Watson and Crick won Nobel prize for DNA discovery
• Rosalind Franklin’s work
• DNA shape is a ‘double helix’
• genes are short sections of DNA that code for particular proteins
What is the human genome?
All the DNA of an organism
Human Genome Project
• started in 1986 - officially in 1990
• completed in 2003 - under budget, two years early
• composite derived from several individuals - sequences
• all information
Aims of the human genome project
• identify all 20-25000 genes in human DNA
• find where each gene is located
• determine sequences of 3 billion chemical base pairs that make up human DNA
• took 15 years
• cost US 13 billion dollars
• helps prevent problems by manipulating DNA
(The project opens up controversial/ethical issues)
What is an allele?
• different forms of the same gene
What does homozygous mean?
An individual with two identical alleles for a characteristic
What does heterozygous mean?
• an individual with different alleles for a characteristic
What is a genotype?
Describes alleles present, genetic make up of an individual regarding a particular characteristic, eg Bb
What does phenotype mean?
• physical appearance of an individual regarding a particular characteristic, eg hair or eye colour
What does recessive mean?
Alleles that are only expressed if both are present, eg rr
What does dominant mean?
A dominant allele is always expressed in the phenotype, even if only one copy is present
Cystic fibrosis
• single gene disorder
• when our lungs make too much mucus
• no cure
• affects digestion - don’t produce enzymes
• genetic degenerative disease
• 9000 suffers in the UK
• life expectancy = 61
• nebulisers and other medication, eg vitamins and antibiotics
• physiotherapy - aims to remove secretions from the lungs to improve airway clearance and lung function
• no digestive enzymes - food passes through the intestines without nutrients being absorbed
• can lead to: weight loss, diabetes and osteoporosis
• linked to a gene on chromosome 7 - codes for a protein called CFTR
• defective alleles
Polydactyl
Having extra fingers or toes
Most inherited disorders are caused by recessive alleles but this is caused by a dominate allele
Embryonic Screening
• ivf
• eggs are extracted and mixed with sperm
• take DNA from embryos and look at it - if carrying a defective allele, they are destroyed
• healthy embryos implanted back into the uterus, hopefully will grow into a baby
• a way of detecting inherited disorders
• methods such as pre-implantation genetic diagnosis (PGD), and chronic villus sampling (CVS)
Arguments for embryonic screening
• helps to stop people suffering from certain inherited disorders
• treating disorders costs gov lots of money (and tax payers) so could reduce healthcare costs
• during IVF, most embryos are destroyed - PGD just ensures selected one is healthy
• if an inherited disorder is diagnosed through CVS, parents have a choice
Arguments against embryonic screening
• may come to a point where people want to pick the most desirable embryo
• increases prejudice - implies having a genetic disorder makes you ‘undesirable’
• after PGD, rejected are destroyed
• risk CVS causes miscarriage
• could lead to an abortion (CVS)
• expensive to screen embryos
What is the male gamete formed in flowering plants?
Pollen (grain)
What is the mean mass of DNA in arbitrary units in a sperm cell?
2
What is the mean mass of DNA in arbitrary units in each cell of an embryo?
4
DNA is a….
Polymer
made up of two strands forming a double helix
What is a gene?
A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein.
What are most genetic characteristics a result of?
Most characteristics are the result of multiple genes interacting, rather than a single gene.
(However can be a single gene)
Sex chromosomes
Ordinary human body cells contain
23 pairs of chromosomes; 22 control general body characteristics only, but the sex chromosomes carry the genes that determine sex.
• In human females the sex chromosomes are the same
(XX) whilst in males the sex chromosomes are different (XY).
