biology AQA igcse

What are the two main types of reproduction in plants?

Asexual reproduction and sexual reproduction.

Describe asexual reproduction in plants.

Asexual reproduction produces offspring that are genetically identical to the parent plant. Examples include runners, bulbs, tubers (with eye buds), and cuttings (which reproduce through mitosis).

What are the male gametes in plants?

Pollen grains.

What are the female gametes in plants?

Ova (or ovum).

What are the two main types of reproduction in plants?

Asexual reproduction and sexual reproduction.

Describe asexual reproduction in plants and give examples.

Asexual reproduction produces offspring that are genetically identical to the parent plant. Examples include runners, bulbs, tubers (with eye buds), and cuttings (which reproduce through mitosis).

What are the male gametes in plants?

Pollen grains.

What are the female gametes in plants?

Ova (or ovum).

What is the sequence of events in sexual reproduction in plants?

Pollination -> fertilisation -> germination. Following fertilisation, the egg cell develops into a zygote, then a seed, and finally forms part of a fruit.

Describe characteristics of insect-pollinated flowers.

• sticky stigma for pollen to stick on to insects
• large bright petals to attract insects
• anther + stigma located inside the flower so insect interacts with it
• pollen grains are large and sticky
• nectaries produce nectar to “reward” insects

Describe characteristics of wind-pollinated flowers.

• feathery stigma to catch pollen
• anther + stigma located outside the flower so wind catches the pollen
• small green petals

What happens during fertilisation in plants?

A pollen grain grows a tube through the style into the ovary. The ovule wall develops into a seed coat (testa). The testa contains an embryonic plant with a small root (radicle) and small shoots (plumule), one or two seed leaves (cotyledons), and a food store of starch for growth.

What is germination and what conditions are needed for it to start?

Germination is the process where a dormant seed, typically containing 10\% of water, begins to grow. Conditions needed are water, proper temperature, and oxygen.

Describe the structure of a male sperm cell.

• Tail: for movement
• Middle piece: contains mitochondria to produce energy for movement
• Acrosome: contains digestive enzymes
• Nucleus: located in the head, contains haploid DNA
• Cell membrane

Identify the main parts of the male reproductive system and their functions.

• Penis
• Sperm duct: a tube through which sperm travels
• Erectile tissue
• Testis: produces sperm
• Scrotum: skin that maintains the optimal temperature for sperm production
• Seminal vesicle: produces fluid to mix with sperm, helping sperm cells to survive longer

Describe the structure of a female egg cell.

• Nucleus: contains haploid DNA and controls cell division
• Cytoplasm: provides nutrients to the embryo
• Jelly coat: ensures that only 1 sperm cell can fertilise the egg; it hardens once a sperm cell enters
• Cell membrane

Identify the main parts of the female reproductive system and their functions.

• Fallopian tube/Oviduct: where the sperm cell meets the egg cell and fertilisation occurs
• Uterus/Womb: where the fertilised egg will attach to the wall
• Ovary: releases the egg cell
• Ligament: holds ovaries
• Cervix
• Vagina

What is the sequence of human development from sperm and egg to a baby?

Sperm cell + egg cell (ovum) = Zygote = Embryo = Foetus/Baby.

How long does human pregnancy typically last?

Pregnancy typically lasts 38-42 weeks.

What are the key structures within the womb that support a growing foetus?

• Amniotic fluid
• Amniotic sac
• Placenta
• Umbilical cord

What hormone is produced in the placenta and what is its role?

Progesterone is produced in the placenta to keep the uterus wall thick for the baby throughout pregnancy.

Describe the structure and function of the placenta.

The placenta has mother’s blood vessels and foetus’ blood vessels in close proximity, allowing for the exchange of nutrients and oxygen from the mother to the baby, and waste products from the baby to the mother.

What are the three stages of birth?

1. Dilation of cervix: The muscles of the uterus contract, and the amniotic fluid ruptures and escapes.
2. Delivery of baby: Uterine muscles contract to push out the baby.
3. Delivery of placenta: The placenta is delivered along with the membranes that had surrounded the baby.

What are the roles of progesterone and oestrogen during pregnancy?

• Progesterone: Maintains high levels to keep the walls of the uterus thick for the baby.
• Oestrogen: Maintains high levels to produce milk for the baby after delivery.

Where are FSH (Follicle-Stimulating Hormone) and LH (Luteinising Hormone) produced?

Both FSH and LH are produced in the pituitary gland.

What are the roles of hormones in male reproduction?

• FSH: stimulates sperm production.
• LH: instructs the testes to secrete testosterone.
• Testosterone: controls the development of male secondary sexual characteristics (e.g., pubic hair, voice cracking, muscle growth).

What are the roles of hormones in female reproduction?

• FSH & LH: control the release of oestrogen from the ovaries.
• Oestrogen: controls the development of secondary sexual characteristics (e.g., hip and breast development, voice deepening, pubic hair).

What is the purpose of the menstrual cycle in females?

The menstrual cycle controls the development of the uterus lining so that if the egg (ovum) is fertilised, the lining will be ready to receive it. Otherwise, the wall breaks down and then starts building itself up again.

Outline the main phases of the menstrual cycle.

1. Day 1-5: Uterine lining breaks down, menstruation occurs.
2. Day 6-10: Uterine lining thickens again.
3. Day 11-18: Uterine lining thickens more; ovulation happens somewhere in between these days, usually on day 14.
4. Day 19-28: Uterine lining continues to thicken until day 28 and then breaks down if no fertilisation occurs.

How long do egg cells and sperm cells typically live?

• Egg cell lives up to one day (24h).
• Sperm cell lives up to three days (72h) for more chances of fertilisation.

What are the roles of FSH, LH, Oestrogen, and Progesterone during the menstrual cycle?

• FSH: stimulates a follicle (ball of cells) to develop.
• LH: breaks the follicle to release the egg cell (on ovulation day); the follicle then changes to a corpus luteum, which produces progesterone.
• Oestrogen: develops the uterine wall up to ovulation.
• Progesterone: takes over from oestrogen, building up the wall (in case of fertilisation) until the 28^{th} day. If fertilisation does not occur, its levels drop, leading to the shedding of the uterine walls.

What is DNA?

DNA (deoxyribose nucleic acid) is a double-stranded polymer in the shape of a helix. It contains the instructions/information for making proteins within a cell.

What is the structural hierarchy from nucleotides to DNA?

Nucleotides -> Gene -> Chromosome -> DNA -> Nucleus.

Describe the structure of a DNA nucleotide.

A DNA nucleotide consists of:

• Deoxyribose pentose sugar (pentagon shape)
• Phosphate group: connects to the next pentose sugar to form a chain of nucleotides
• Nitrogenous base: connects to another base to form a connection between two strands of nucleotides

What bonds are found in the DNA structure?

• Phosphodiester bonds: between the phosphate group and deoxyribose sugar, forming the sugar-phosphate backbone of each strand.
• Hydrogen bonds: between complementary nitrogenous bases, linking the two DNA strands together.
• A=T (Adenine and Thymine) have two hydrogen bonds.
• G≡C (Guanine and Cytosine) have three hydrogen bonds.

What are the four complementary nitrogenous bases in DNA and how do they pair?

The four nitrogenous bases are adenine (A), thymine (T), cytosine (C), and guanine (G).

• Adenine (A) always pairs with Thymine (T).
• Cytosine (C) always pairs with Guanine (G).

What is the 'universal code' of DNA?

Every 3 nucleotide bases (known as a triplet or codon) code for one amino acid.

Outline the process of Transcription in protein synthesis.

1. DNA helicase (enzyme) unzips the DNA strand by breaking hydrogen bonds.
2. RNA polymerase makes the RNA strand by choosing one template strand from the two in DNA. This enzyme builds RNA nucleotides (found in the cytoplasm), which bind to their complementary bases on the DNA template strand.
3. The DNA segment is transcribed into mRNA (messenger RNA), which then leaves the nucleus through nuclear pores to the ribosomes (the site of protein synthesis).
4. A phosphodiester bond is formed between the ribose sugar and phosphate group, creating the sugar-phosphate backbone of the mRNA strand.
5. DNA ligase (enzyme) zips the DNA strands back together.

Outline the process of Translation in protein synthesis.

1. tRNA (transfer RNA) carries the first amino acid from the cytoplasm to the start codon (AUG) of the mRNA strand at the ribosome.
2. A second tRNA arrives, and its anticodon binds to the next complementary codon on the mRNA strand.
3. More tRNA molecules arrive at the ribosome, binding their anticodons to the codons on the mRNA strand. Peptide bonds are formed between the amino acids, creating a polypeptide chain.
4. Translation stops when a “stop codon” (UAA/UAG/UGA) is reached at the end of the mRNA chain.

How many different combinations of codons are there, and how many different types of amino acids are in the human body?

There are 64 different combinations of codons (4^3), and 20 different types of amino acids in our body.

Describe DNA replication and why it is called semi-conservative.

DNA replication uses the original DNA strand as a template to make new DNA molecules. Each new DNA molecule consists of one strand from the original (old) DNA and one newly synthesized strand. This is called semi-conservative replication because half of the original DNA molecule is conserved in each new molecule.

What is a genetic mutation?

A genetic mutation occurs when the sequence of bases in the DNA is altered or changed.

Describe the effect of different types of genetic mutations on the DNA sequence.

• Duplication of a base: shifts the entire sequence forward and changes all subsequent codons (amino acids).
• Deletion of a base: shifts the entire sequence backwards and changes all subsequent codons (amino acids).
• Substitution of a base: does not shift the entire sequence and only changes one codon (one amino acid).
• Inversion of a base: does not shift the entire sequence and only changes one or two codons (amino acids).

What are mutagens? Give examples.

Mutagens are substances that can cause mutations. Examples include ionising radiation and chemicals such as mustard gas, nitrous oxide, and certain chemicals found in cigarettes and food.

Describe the main parts of a chromosome.

• Centromere: the middle part which connects the two chromatids into a pair.
• Chromatid: one part of a duplicated chromosome, with one chromatid typically inherited from each parent to form a pair.

What are the types of chromosomes found in humans?

• Autosomes: 22 pairs of chromosomes that define the characteristics of an organism. They are homologous pairs, containing the same genes from both parents (one chromosome from each parent forms one pair).
• Sex chromosomes (Gamete chromosomes): define the gender of an organism (XX for female; XY for male).

What is chiasmata?

Chiasmata refers to the crossing over of gene parts between homologous chromosomes (from mom and dad) to make one final recombinant chromosome for the organism, increasing genetic variation.

Give an example of a chromosomal mutation.

Down’s syndrome is a chromosomal mutation where an individual has an extra chromosome on chromosome number 21.

What is mitosis? What is its purpose and what does it produce?

Mitosis is a type of cell division that occurs in body cells for growth and repair. It happens asexually and produces 2 genetically identical diploid daughter cells.

Outline the stages of mitosis and key events in each.

1. Interphase:
   • Nuclear membrane is present.
   • Chromosomes are not visible.
   • DNA replication takes place.
2. Prophase:
   • Chromosomes condense/supercoil and become visible.
   • Nuclear membrane breaks down.
   • Centrioles migrate to the opposite poles of the cell.
3. Metaphase:
   • Chromosomes align at the equator (middle part) of the cell.
   • Centrioles form spindle fibres to attach to the centromere (middle part) of each chromosome.
4. Anaphase:
   • Spindle fibres contract and shorten.
   • Chromatids are pulled to the opposite poles, splitting each chromosome in half.
5. Telophase (Cytokinesis):
   • Chromosomes disappear as they decondense.
   • 2 Nuclear membranes reform.
   • Centrioles are not visible.
   • Cytokinesis begins, where the cell membrane and cytoplasm start to divide, forming two identical daughter cells.

What is meiosis? What is its purpose and what does it produce?

Meiosis is a type of cell division in body cells to form gametes for reproduction. It happens sexually and forms 4 genetically different haploid cells.

List the general stages of meiosis.

1. Interphase
2. Prophase 1
3. Metaphase 1
4. Anaphase 1
5. Prophase 2
6. Metaphase 2
7. Anaphase 2
8. Telophase 2 and cytokinesis