6 Flashcards
What is meiosis?
Meiosis occurs when sex cells (gametes) are formed in the ovaries and testes.
what happens during meiosis
4 gametes are produced from one daughter cell
Each gamete has half the number of chromosomes as the parent cell (in humans, 23 chromosomes, not 23 pairs)
its important to produce gametes with half the chromosome number because Human body cells need 46 chromosomes. If each gamete contained 46, after fertilisation, body cells would contain 92 chromosomes (not viable). By halving the number in meiosis, after fertilisation each cell has the correct number of chromosomes
interphase
DNA duplicates - this means that each chromosone now consists of 2 sister chromatids
2 chromosones
1 chromatids each
prophase 1
Nuclear membrane disappears
homologus chromosones (which code for the same thing) pair up - Crossing over occurs - this meaans that non-sister chromatids exchange some genetic material CREATING VARIATION
Spindle fibres form
2 chromosones
2 chromatids each
metaphase 1
Homologous chromosome pairs line up along the equator of the cell.
Spindle fibres attach to the chromosomes at the centromere
2 chromosones
2 chromatids each
anaphase 1
2Homologous chromosome pairs are pulled apart to opposite poles – the 2 sister chromatids stay together.
Any swapped genetic material is pulled apart too. This creates genetic differenciation
2 chromosones
2 chromatids each
telophase 1 & cytokinesis
snuclear membranes form around the now divided genetic material.
After cytokinesis, 2 daughter cells are formed WITH HALF THE CHROMOSOME NUMBER OF THE PARENT CELL.
Each chromosome still consists of 2 chromatids, but the chromosome number is half of the original cell
1 chromosones
2 chromatids each
prophase 2
Nuclear membrane disappears
Spindle fibres form
1 chromosones
2 chromatids each
metaphase 2
chromosomes (still consisting of chromatids) line up along the equator.
Spindle fibres attach to the chromosome at the centromere
1 chromosones
2 chromatids each
anaphase 2
chromatids are pulled apart towards opposite poles by spindle fibres
1 chromosones
2 chromatids each
telophase 2 & cytokinesis
nuclear membranes form around the now divided genetic material
Post-cytokinesis four daughter cells are formed. Each is genetically different to one another, and different to the parent cell.
Each daughter cell also has half the chromosomes number of the parent cell – haploid
1 chromosones
1 chromatids each
Sexual reproduction
Sexual reproduction is where genetic information from two organisms (a father and a mother) is combined to produce offspring which are genetically different to either parent.
This involves the fusion of male and female gametes. Because there are two parents, the offspring contain a mixture of their parents’ genes and are genetically different to their parents.
Gametes
In sexual reproduction the mother and father produce gametes by meiosis - e.g. egg and sperm cells in animals. In humans, each gamete contains 23 chromosomes half the number of chromosomes in a normal cell. (Instead of having two of each chromosome, a gamete has just one of each.)
This is why the offspring inherits features from both parents - it’s received a mixture of chromosomes from its mother and its father (and it’s the chromosomes that decide how you turn out). This mixture of genetic information produces variation in the offspring.
Asexual reproduction
In asexual reproduction there’s only one parent so the offspring are genetically identical to that parent. Asexual reproduction happens by mitosis - an ordinary cell makes a new cell by dividing in two. The new cell has exactly the same genetic information as the parent cell - it’s called a clone.
Advantages of sexual reproduction
Produces variation - If the environment changes, variation gives a survival advantage by natural selection
disadvantages of sexual reproduction
Can be time consuming and energy inefficient
Advantages of asexual reproduction
There only needs to be one parent. This means that asexual reproduction uses less energy than sexual reproduction, because organisms don’t have to find a mate. This also means that asexual reproduction is faster than sexual reproduction.
Another advantage is that many identical offspring can be produced in favourable conditions.
disadvantages of asexual reproduction
If all offspring are identical, they could all be at risk, e.g. if a new disease develops
rerproduction with both sexual and asexual reproduction
fungi, malarial parasites and some plants can do both sexual and asexual reproduction.
Fungi
Fungi are made of masses of threads called hyphae (haploid)
In ideal conditions, fungi produce spores asexually. Spores disperse and germinate to produce clones of the parent
When conditions are not good, hyphae from 2 different fungi join to create a diploid hypha (sexual reproduction). The diploid hypha undergoes meiosis to produce haploid spores. These spores will be different to spores of either of the original parents
Plants e.g. strawberries
Sexually – gametes combine in pollination to form diploid seeds that germinate to produce a new plant. It will be genetically different to either parent
Asexually – can produce ‘runners’, that will create genetically identical plants. Can still produce new plants even is flowers are destroyed in frost, are eaten or are not pollinated.
Malarial parasite
Malaria is caused by a parasite that’s spread by mosquitoes. When a mosquito carrying the parasite bites a human, the parasite can be transferred to the human. The parasite reproduces sexually when it’s in the mosquito and asexually when it’s in the human host
Chromosomes
long Thread-like structures in the nucleus of a cell that contain DNA
DNA double helix
Shape of the DNA. 2 strands of nucleotides that wind up around each other like a twisted ladder to protect the bases.
DNA
DNA stands for deoxyribonucleic acid. It’s the chemical that all of the genetic material in a cell is made up from. It contains coded information - basically all the instructions to put an organism together and make it work. So it’s what’s in your DNA that determines what inherited characteristics you have and its sequence determines how our bodies are made.
Gene
Section of DNA found on a chromosone that contains the instructions for a particular characteristic.
DNA bases
Four chemicals found in DNA that make up the base sequence (A, T, C, G)
Genetic code
The sequence of bases within the DNA that ultimately codes for proteins
Structure of DNA - nucleotides
DNA is made up of a series of repeated units called nucleotides. Therefore, we call DNA a polymer
Structure of DNA
Each strand of DNA is made of a sugar-phosphate backbone (held together by phosphodiester bonds)
The two halves of the helix are held together by weak hydrogen bonds.
C-G = 3 H bonds
A-T = 2 H bonds
complementary base pairing
A and T always are complementary (they are always linked together on opposite strands)
C and G are complementary.
The bases consist of
A Adenine
T Thymine
G Guanine
C Cytosine
Genome
The Genome is the entire genetic material of an organism
benefits of nderstanding the human genome
Search for genes linked to different diseases
Better understand and possibly treat inherited disorders
Trace human migration patterns from prehistory - The human genome is mostly identical in all individuals, but as different populations of people migrated away from Africa, they gradually developed tiny differences in their genomes. By investigating these differences, scientists can work out when new populations split off in a different direction and what route they took.
When do Mutations occur?
Mutations occur continuously. They can occur spontaneously, e.g. when a chromosome isn’t quite replicated properly. However, the chance of mutation is increased by exposure to certain substances or some types of radiation.
Examples of mutations
Enzymes are proteins, if the shape of an enzyme’s active site is changed, its substrate may no longer be able to bind to it - see page 120.
Structural proteins like collagen could lose their strength if their shape is changed, making them pretty useless at providing structure and support.
silent mutations (in exons - coding pieces of DNA)
Not all mutations have a serious effect. We have two copies of every gene so if one is faulty, one may be okay. Sometimes if a base is changed in the DNA it does not code for a different amino acid/protein (silent mutation). If it is coding for an enzyme it may not lose its function.
mutations (in exons - coding pieces of DNA)
Some mutations change the DNA base sequence, and a different
amino acid is coded for, so we get a different protein. If this is an enzyme, it may mean it is not folded correctly (it would alter the intermolecular forces holding it in its 3D shape). This means the shape of the active site may be incorrect, so the enzyme doesn’t work.
mutations in body cells
If a mutation occurs in a body cell it may lead to cancer.
mutations in gametes
If a mutation occurs in the egg or sperm it can lead to the offspring having a genetic disorder
mutations (in introns - non-coding pieces of DNA)
mutations in introns could cause the gene to be switched on or off when it should not be. This would affect how genes are expressed.
E.g. we might get no protein synthesis when we need it, or we could get proteins made unnecessarily. Why may this be bad?
- We may lack a protein we need
- We may make a protein when we don’t need it. If this is an enzyme it
may catalyze a reaction we don’t want to happen, or simply waste energy making proteins we do not need.
introns
Introns are non-coding pieces of DNA – but they are used in gene expression. They ‘switch on’ genes, meaning they control whether their respective exon needs to undergo protein synthesis.
Cystic fibrosis - what is it?
Cystic fibrosis is an inherited disorder of the cell membranes. It results in the body producing a lot of thick sticky mucus in the air passages (which makes breathing difficult). It also results in movement of substances in and out of a cell becoming more difficult
Cystic fibrosis - type of allele
Its caused by a recessive allele.
Therefore, the presence of two alleles will be needed
to cause the characteristic to be seen in the
phenotype . because its recessive, people with only 1 copy of the allele won’t have the disorder - they’re known as carriers - they carry the faulty allele, but don’t have any symptoms.
Polydactyly - what is it?
Polydactyly is an inherited disorder where a baby’s born with extra fingers or toes. It doesn’t usually cause any other problems so isn’t life-threatening.
Polydactyly - type of allele
Its caused by a dominant allele and so can be inherited if just one parent carries the defective allele. The parent that has the defective allele will have the condition too since the allele is dominant.
Types of embryonic screening - Pre-implantation genetic diagnosis (PGD)
During IVF, it’s possible to remove a cell from each embryo and analyse its genes. Embryos with ‘healthy’ alleles would be implanted into the mother - the ones with ‘faulty’ alleles destroyed.
Types of embryonic screening - Chorionic villus sampling (CVS)
CVS involves taking a sample of cells from part of the placenta (where the baby’s umbillical cord joins with the mothers uterus) and analysing their genes. The part of the placenta that’s taken and the embryo develop from the same original cell - so they have the same genes. If the embryo is found to have an inherited disorder, the parents can decide whether or not to terminate (end) the pregnancy.
Embryonic screening
Embryonic screening is a way of detecting inherited disorders in embryos
protein synthesis - transcription
DNA unzips (this is done using RNA polymerase enzymes & only the gene that needs transcribing is unzipped)
RNA polymerase binds to and starts ‘reading’ the DNA template strand
Complementary RNA nucleotides line up alongside the DNA template strand - In RNA, the base T is replaced with U
e.g. mRNA: AGCUCA
DNA template strand TCGAGT
A single stranded copy of the DNA is made – this copy is called mRNA. We only make mRNA of the genes that we need – not the whole DNA strand. (The double stranded DNA is too large to leave the nucleus so a smaller single stranded mRNA is needed to fit through the nuclear membrane)
Once the mRNA is made, it is released from the DNA, and the DNA reforms the double helix
The introns (non-coding regions) are cut out
mRNA leaves the nucleus and moves to the ribosomes in the cytoplasm
protein synthesis - transalation
mRNA binds to the ribosomes.
Here, the ribosome reads the mRNA in groups of 3 bases – a triplet (codon). 3 bases codes for 1 amino acid
Carrier molecules (tRNA) with the complementary base sequence to the mRNA triplet bind to the mRNA at the ribosome. Attached to the other end of the carrier molecule is an amino acid.
The next three bases are read and another carrier molecule (with a complementary base sequence to the mRNA) brings in another amino acid
A peptide bond forms between the amino acid. The first carrier can now leave.
A third amino acid is brought in by another carrier molecule, which forms a peptide bond to the growing chain.
Once all the mRNA has been read, we are left with a sequence of amino acids. We call this a polypeptide chain (primary structure).
It can then fold into a 3D shape
Larmarck’s theory
Within a species, organisms all start off looking very similar
Some individuals would change as a result of use or disuse of a part of the body
e.g. repeated use of an arm would increase its size. This gives the individual an acquired characteristic
Offspring would inherit their parent’s acquired characteristics (the trait is inherited by the offspring) and develop it further
Eventually, over time, the environment would have directly affected individuals so changing the nature of the species
similar
change- use/disuse
inherit acuired charicteristic from parent
If a donkey and a horse mate then a mule is produced.
A mule is infertile. A donkey and horse are separate species.
Why may mules be infertile?
Evidence from the fossil record
As each layer of rock is added the one on top must be younger than the one below so we can tell the relative ages of rocks.
It also shows us how organisms that lived in the past differ from those living today
It shows us how long a species existed for – when does it disappear from the fossil record?
5 kingdom system
Linnaeus created the 5 kingdom system. Below kingdom there are smaller subdivisions…
As you descend the classification system, organisms grouped within it get more similar.
Traditionally, organisms have been classified according to a system first proposed in the 1700s by Carl Linnaeus, which groups living things according to their characteristics and the structures that make them up.
In this system (known as the Linnaean system), living things are first divided into the 5 kingdoms (e.g. Animals, plants, Fungi, Protists, Prokaryotes). The kingdoms are then subdivided into smaller and smaller groups - phylum, class, order, family, genus, species.
three-domain system
n 1990, Carl Woese proposed the three-domain system. Using evidence gathered from new chemical analysis techniques such as RNA sequence analysis, he found that in some cases, species thought to be closely related in traditional classification systems are in fact not as closely related as first thought - Archaea & bacteria
In the three-domain system, organisms are first of all split into three large groups called domains:
- Archaea
Organisms in this domain were once thought to be primitive bacteria, but they’re actually a different type of prokaryotic cell. They can be found in extreme places such as hot springs and salt lakes.
- Bacteria
This domain contains true bacteria like E. coli and Staphylococcus. Although they often look similar to Archaea, there are lots of biochemical differences between them.
- Eukaryota
This domain includes a broad range of organisms including fungi, plants, animals and protists (page 136).
These are then subdivided into smaller groups kingdom, phylum, class, order, family, genus, species.
Evolutionary trees
Evolutionary trees show how scientists think different species are related to each other.
They show common ancestors and relationships between species. The more recent the common ancestor, the more closely related the two species and the more characteristics they’re likely to share.
Scientists analyse lots of different types of data to work out evolutionary relationships. For living organisms, they use the current classification data (e.g. DNA analysis and structural similarities). For extinct species, they use information from the fossil record
cloning animals - embryo transplants
Farmers can use embryo transplants to produce cloned offspring from their best bull and cow:
- Sperm cells are taken from a prize bull and egg cells are taken from a prize cow. The sperm are then used to artificially fertilise an egg cell.
- The embryo that develops is then split many times (to form clones) before any cells become specialised.
- These cloned embryos can then be implanted into lots of other cows…
- where they grow into calves (which will all be genetically identical to each other).
cloning animals - Adult cell cloning
Adult cell cloning can also be used to make animal clones. Adult cell cloning involves taking an unfertilised egg cell and removing its nucleus. The nucleus is then removed from an adult body cell (e.g. skin cell) and is inserted into the ‘empty’ egg cell. The egg cell is then stimulated by an electric shock - this makes it divide, just like a normal embryo.
When the embryo is a ball of cells, it’s implanted into the uterus (womb) of an adult female (the surrogate mother). Here the embryo grows into a clone of the original adult body cell as it has the same genetic information.
Restriction enzymes
Restriction enzymes cut open the plasmid leaving ‘sticky ends’ – one strand of the dsDNA has an overhang
genetic engineering - process
- A plasmid (vector) is obtained from within a bacterium
- It is cut open using restriction enzymes
- At the same time, the required gene (e.g. coding for the desired characteristic) is cut from the donor organism using the same restriction enzymes used to cut open the vector.
- The two sticky ends can overlap and be joined together using DNA ligase.
- This is now called recombinant DNA
- The recombinant DNA (plasmid + required gene) is inserted into a new bacterium through heat shock, or electroporation - the bacterium is now transgenic
- The transgenic organism is checked to check it is expressing the required gene. This is done by inserting a genetic marker gene which could be a fluorescence gene or an antibiotic resistance gene (i.e. if the flourescence gene is inserted, it’ll glow OR it’ll survive exposure to antibiotics). The assumption is, that if the marker gene has been inserted and expressed, so have the desired gene)
- The bacterium is allowed to divide and begins producing the desired protein (insulin)
- Insulin can be extracted and purified for use in humans
