what is sexual reproduction?
-where genetic information from two organisms is combined to produce genetically different offspring
What are gametes?
-reproductive cells produced by the mother and father
-only contain half the number of chromosomes - they are haploid
what is fertilisation?
-when the male gamete fuses with a female gamete to produce a fertilised egg (zygote)
-zygote is diploid and ends up with the full set of chromosomes
What is an embryo?
-the result of cell division from the zygote
-inherits characteristics from both parents
Asexual reproduction - advantages
Adv: -can produce lots of offspring very quickly because of the fast reproductive cycle.
-only one parent is needed, meaning organisms can reproduce whenever conditions are favourable
Asexual reproduction - disadvantages
-there’s no genetic variation, so if environment changes (unfavourable conditions), the whole population may be affected
Sexual reproduction - advantages
-creates genetic variation within the population, which means individuals with different characteristics
-species more adaptable to change in environment
-This can lead to natural selection and evolution
sexual reproduction - disadvantages
-takes more time and energy than asexual reproduction, so organisms produce fewer offspring in their lifetime
-organisms need to find and attract mates
-two parents are needed for sexual reproduction, can be a problem if individuals are isolated
Describe the process of Meiosis.
1. Each chromosome replicates & the 2 copies form an X shape.
2. The 2 sets of chromosomes ‘pair up’, forming 23 pairs, and the pairs then separate into 2 new cells.
3. Next, the 2 copies of of a chromosome in each X-shape split into 2 more new cells.
4. This leads to the production of; 4 Haploid genetically different gametes ( sex cells )
what are DNA strands?
- polymers made up of lots of repeating units called nucleotides
- each nucleotide consists of a sugar, phosphate group and one ‘base’
- 2 strands coiled to form a double helix
- The strands are linked by complementary bases joined by weak hydrogen bonds.
what does the sugar and phosphate groups do?
-they alternately form a ‘backbone’ to the DNA strands
What are the different bases?
One of four different bases joins to each sugar:
A (adenine, T (thymine), C (cytosine) and G (guanine)
A pairs with T
C pairs with G
what are chromosomes?
-long, coiled up molecules of DNA
-found in the nucleus of eukaryotic cells
What are genes?
A section of DNA on a chromosome that codes for a particular protein
What is a genome?
ALL of an organisms DNA codes for a genome.
SB3d Protien synthesis
Describe the process of Transcription in Protien synthesis
- An enzyme called RNA polymerase attaches to the DNA in front of a gene in a non-coding region. The enzyme separates the 2 DNA strands.
- The enzyme then moves along one DNA strand adding complementary RNA nucleotides. These contain the same bases as DNA except that Uracil ( U ) is used instead of Thymine ( T ).
- The nucleotides link to form a strand of messange RNA ( mRNA )
SB3d Protien synthesis
Describe the process Translation in Protien Synthesis
- The mRNA strands travel out of the nucleus through the pores in its membrane. In the Cytoplasm, mRNA strands attach to ribosomes.
- A ribosome moves along an mRNA strand 3 bases at a time. Each triplet of bases is called a codon. At each codon a tRNA molecule with complementary bases line up.
- Each tRNA molecule carries a specific amino acid. As the ribosomes moves along, it joins the amino acids from tRNA molecules together, forming a polypeptide chain.
- The polypeptide then folds up to form a protein with a specific shape.
SB3e Genetic Variations and phenotypes
Explain the effects of mutations in the;
A) Coding Region
B) Non-Coding Region
A) Mutations in the coding regions of DNA can lead to a change in phenotype as a different mRNA nucleotide will be binded, resulting a new different amino acid being brought by the tRNA.
B) RNA polymerase attaches to DNA bases infront of a gene. A mutation in this non-coding region may result in the RNA polymerase not binding well, reducing transcription.
- Such mutations can cause not enough B-polypeptide being made for haemoglobin, resulting in tiredness, weakness and shortness of breath.
SB3f Mendel
Describe the work of Gregor Mendel
- He bred pea plants together using a paintbrush to move pollen from one plant to the flower of another plant.
- A bag was then placed over the flower on the plant and sealed. Mendel planted the seeds that formed and observed the characteristics of the offspring.
Mendel summarised his work into 3 laws of inherintence;
- Each gamete recieves only one factor for a characteristic
- The version of a factor that a gamete recieves is random and does not depend on the other factors in the gamete.
- Some versions of a factor are more powerful than others and always have an effect in the offspring.
SB3h Inheritance
Explain how the sex of an offspring is determined at fertilisation.
- 2 of your chromosomes determine what sex you are. They are your sex chromosomes and there are 2 types, X and Y. Females have 2 X chromosomes while males have X and Y.
- A woman’s gametes all contain an X sex chromosomes but male sperm cells contain either an X or Y.
- Punnet squares can be used to show that the ratio of male to females ( XX’s to XY’s ) is 2:2 or 1:1, meaning there is an equal chance of the child being a male or female.
SB3i Multiple and missing alleles
Describe the inheritence of the ABO blood groups
- Using the ABO blood group system, everyone’s blood is either; A, B, AB, O. Your blood groups is determined by the ‘marker molecules’ on the surface of the red blood cells.
- There are 3 main types of these molecules; A, B or O. The gene that is responsible for these markers has 3 alleles; I^A, I^B, I^O. Everyone has 2 copies of the gene. ( I^O is recessive to both I^A & I^B )
- A person with the genotype I^AI^B shows the effect of both alleles and has the blood type AB. When both alleles affect the phenotype, we say they are codominant.
SB3i Multiple and missing alleles
Explain how sex-linked genetic disorders are inherited
- Chromosomes in diploid cells come in pairs. In most pairs, the chromosomes have the same gene. However, the human Y sex chromosome is missing some of the genes found on the X chromosome.
- This means a man (XY) will have only 1 allelel for some genes on the X chromosome ( since those genes are missing on the Y chromosome ).
- If the allelel for one of those X chromosomes genes causes a genetic disorder, then a man will develop that disorder.
SB3i Multiple and missing alleles
Explain why females are less likely to inherit genetic disorders
- If a woman (XX) inherits the ‘disorder allelel’, she may have a healthy allelel on her other X chromosome. If the disorder allelel is recessive, she will not get the disorder.
- If she were to inherit 2 recessive ‘disorder allelels’ she will develop the disorder, however the chance of a woman developing the disorder is much less than that of a man.
SB3j Gene mutation
Describe the outcomes of the Human Genome Project
- It produced a map of 3.3billion complementary base pairs in one set of 46 human chromosomes. Further work has found many sections of DNA that are the genes.
- Other human genomes have also been mapped, which has shown that there are variation between people but over 99% of DNA bases are the same.
- Mapping a person’s genome can indicate their risk of developing disease that are caused by different alleles of genes. It can also help identify which medicines might be best to treat a persons illness, because the alleles we have can affect how medicines work in the body.
