IB Biology SL 2025 Exam - Unit 5 Study Guide

Covering topics from Unit 5 that could be on the 2025 test!

Parent Cell

The cell that divides

Daughter Cell

The cells that are produced from division

Cytokinesis in Plant Cells

Microtubules are built into a scaffold straddling the equator to assemble vesicles

Vesicles fuse together to form plate-shaped structures

Eventually, two complete layers of membrane are formed across the whole of the equator, connecting to the existing plasma membranes to complete cytokinesis

Cytokinesis in Animal Cells

Plasma membrane is pulled inwards towards the equator to form a cleavage furrow

Actin and myosin pull plasma membranes towards equator

When cleavage furrow reaches the center, the cell is pinched apart

Unequal Cytokinesis in Yeast

Yeast cells reproduce asexually through budding

Nucleus divides by mitosis, with small outgrowth from the mother cell receiving one nuclei but a small amount of cytoplasm

Dividing wall is constructed and separates the yeast cells

Unequal Cytokinesis in Oogenesis

Since females typically only produce one egg at a time, there is unequal division

In the first division, one large cell has nearly all cytoplasm and another smaller polar body

In the second division, the same thing happens, with the larger cell maturing into a oocyte ready for fertilization

Mitosis

Used to produce genetically identical cells

2n is the diploid number of chromosomes

Meiosis

Used to halve the chromosome number from diploid (2n) to haploid (n). Known as reduction division and occurs to keep the chromosome numbers down

Helps to generate genetic diversity

DNA replication as a prerequisite for both mitosis and meiosis

Cells that are preparing for nuclear division need to replicate their DNA in order for the daughter cells to have genetic information

Condensation of Chromosomes

Since DNA chromosomes are extremely long (50,000 micrometers) in relation to the nucleus size (less than 5 micrometers wide), they need to be condensed by shortening them in order to prevent tangles, breaks or knots

Movement of Chromosomes

Chromosomes are moved with microtubules (hollow cylinder of tubulin proteins that can be rapidly assembled or disassembled)

Microtubules can link up to kinetochores (which attach to the centromere). These kinetochores act as a motor by removing tubulin subunits, shortening the microtubules and bringing them to the poles

Interphase in Mitosis

Chromosomes are dispersed through nucleus

All DNA is replicated so each chromosome has 2 chromatids

Prophase in Mitosis

Chromosomes condense into thick, short structures

Microtubules grow from MTOCs at the poles to form a spindle-shaped array linking the poles of the cell

Nuclear membrane breaks down at the end of this phase

Metaphase in Mitosis

Microtubules growing attach to the centromere of each chromatid, meaning the sister chromatids are attached to opposite poles

Microtubules are put under tension to make sure attachment is correct. If it’s correct, the chromosomes cannot be pulled apart

Chromosomes are aligned at the equator at the end of this phase

Anaphase in Mitosis

Cohesin loops that held the chromatids are cut

Microtubules link each chromosome to one of the poles, with the kinetochore shortening the microtubule to bring the chromatids to the poles

All chromatids reach the poles at the end of this phase

Telophase in Mitosis

The chromosomes are pulled into a tight group near the MTOC and a nuclear membrane forms around them

Chromosomes decondense and spread out

Diploid Cells

Cells that have two sets of chromosomes and therefore homologous pairs of chromosomes

Haploid Cells

Cells that only have one set of chromosomes that are all non-homologous

Interphase I in Meiosis

All DNA is replicated in this process before reproduction occurs

Prophase I in Meiosis

Homologous chromosomes form parallel pairs

Crossing over occurs between non-sister chromatids in each bivalent

Chromatids shorten and thicken

Metaphase I in Meiosis

Nuclear membrane disperses and homologous chromosomes in each bivalent become attached to spindle microtubules

Bivalents spread out in equator

Pairing of homologous chromosome ends but chiasmata prevents separation

Anaphase I in Meiosis

Kinetochores shorten spindle microtubules, pulling chromosomes towards poles

Chiasmata slide to end of chromosomes

Pairs of homologous chromosomes separate and move to opposite poles

Telophase I & Cytokinesis in Meiosis

Nuclear membrane is assembled around the chromosomes at each pole

Chromosomes decondense

Cytokinesis divides cytoplasm, resulting in two haploid cells

Prophase II in Meiosis

Because the chromosomes already have two chromatids, there is no need for DNA replication

Chromatids are condensed

Metaphase II in Meiosis

Nuclear membranes disperse and chromatids of chromosomes became attached to spindle microtubules from opposite poles

Chromosomes spread out on equator

Cohesin loops are cut allowing them to separate

Anaphase II in Meiosis

Kinetochores on centromere shorten spindle microtubules

These chromosomes have reached the poles at the end of the phase

Telophase II & Cytokinesis in Meiosis

Nuclear membranes are assembled around chromosomes at each pole

Decondensation spreads chromosomes out throughout the 4 nuclei

Cytokinesis occurs, dividing both cells to produce 4 haploid cells

Down syndrome and non-disjunction

Non-disjunction: pair of homologous chromosomes might move to the same pole in Anaphase I or both chromatids move to the same pole in Anaphase II

Down syndrome: non-lethal non-disjunction event that occurs when a person has 3 copies of chromosome 21

Meiosis as a source of variation

Crossing over - due to the homologous chromosomes pairing up early on, the two strands of the DNA helices are cut at the same point and replaced with an equivalent strand in the other chromatid, resulting in a mutual exchange of DNA

Random orientation - Since the chromatids have random orientations when they are brought to the equator, there is a 50% chance of one going to a certain side, increasing genetic variation

Sexual Reproduction

Two parents: one male and one female

Meiosis is used once per generation

Offspring are genetically different from their parents

New gene combinations are produced each generation

Genetic variation is generated

Offspring may be better adapted to their parents if the environment is changing

Asexual Reproduction

One parent

Mitosis is used throughout the life cycle

Offspring are genetically identical to each other and the parent

Existing gene combinations are maintained down the generations

No genetic variation

Role of meiosis and fusion of gametes in the sexual life cycle

Fusion of gametes allows for offspring to be produced (fertilization)

However, without meiosis to halve the chromosome number and maintain genetic diversity, it would be impossible to fuse gametes and the sexual life of eukaryotes could not occur

Differences between male and female sexes in sexual reproduction

Male gametes travel to the female gametes

Male gametes are smaller than the female gametes

Male gametes have less food reserves than the female gametes

More male gametes in quantity than female gametes

Oestradiol

Produced by the corpus luteum

Rises to a peak at the end of the follicular phase. Stimulates repair and thickening of endometrium after menstruation and an increase in FSH receptors that make the follicles more receptive to FSH

Progesterone

Another hormone produced by the corpus luteum

Rise at the start of the luteal phase, reach a peak, and then drop back to a low level by the end of this phase

Promotes the thickening and maintenance of the endometrium

Inhibits FSH and LH secretion by the pituitary gland

Luteinizing Hormone (LH)

Protein hormone produced in the pituitary gland

Increases sharply on Day 14 as it stimulates the completion of meiosis in the oocyte and partial digestion of the follicle wall to open it during ovulation

Promotes the post-ovulation development of the wall of the follicle into the corpus luteum

Follicle-Stimulating Hormone (FSH)

Another protein hormone produced in the pituitary gland

Rises to a peak at the end of the menstrual cycle to stimulate the development of follicles, each containing an oocyte and follicular fluid

Stimulates secretion of oestradiol by the follicle wall3

Positive Feedback

Amplify an initial stimulus, leading to an intensified response

Negative Feedback

Output of a system influences its own input, causing the process to slow down or halt, thus maintaining a stable internal environment

Fertilization in Humans

First sperm that manages to penetrate the zone pellucida on the egg binds and the membranes of the sperm and egg fuse together

Layer of glycoprotein around the egg hardens to prevent entry of more sperm

Nuclei from sperm and egg remain separate until the first mitosis, in which both 23 chromosomes are released and participate jointly in mitosis using the same spindle of microtubules. This produces the diploid number of 46

Use of hormones in in vitro fertilization (IVF) treatment

Every day for about 2 weeks, the woman has an injection containing a drug to suspend the menstrual cycle by stopping FSH and LH

Intramuscular injections of FSH are given daily for 7-12 days, stimulating more follicles to develop

When follicles are 18mm in diameter, they are stimulated to mature by an injection of human chorionic gonadotropin, in which the eggs will be collected 34-35 hours after through a minor surgical procedure

Each egg is mixed with 50,000-100,000 sperm in sterile conditions in a shallow dish. If fertilization is successful, one or more embryos are placed in the uterus

Sexual reproduction in flowering plants

Flowers reproduce by pollination, in which pollen is created from the anthers

Fertilization happens inside an ovule, in the ovary. The pollen tube carrying the male gametes grows towards one ovule and into the end. When the pollen tube reaches the center, the male gametes are released

Product of fertilization is a zygote, which develops into an embryo with an embryo root, an embryo shoot, and either one or two embryo leaves

Features of an insect-pollinated flower

Large, brightly colored petals to advertise the flower, act as a landing stage and guide the insect’s movements to the anther or stigma

Scent is secreted to advertise the flower

Pollen grains are large and spiky to stick to insects

Stigma is large and sticky to collect pollen from visiting insects

Nectaries secrete nectar that is attractive to insects, and are positioned deep so that insects must brush past anthers and stigma to reach it

Methods of promoting cross-pollination

Wind pollination - adapting structure so wind can take pollen more efficiently

Water pollination - less common but some plants rely on water to carry pollen

Animal interactions

Self-incompatibility mechanisms to increase genetic variation within a species

Adaptations to facilitate transfer of pollen from one plant to another by an outside agent

Separation of anthers and stigmas in separate male and female flowers both on the same and different plants

Maturity of anthers and stigmas at different times (protandry - anthers first; protogyny - stigma first)

Dispersal and germination of seeds

Dry & explosive

Fleshy and attractive for animals to eat

Feathery or winged to catch the wind

Covered in hooks that catch onto the coats of animals

Production of haploid gametes in parents and their fusion to form a diploid zygote as the means of inheritance

During meiosis, the diploid cells halve into haploid cells. This allows the parents to be able to individually contribute to the offspring when mitosis happens

Methods for conducting genetic crosses in flowering plants

This can be done with a paint brush to perform the cross by transfer of pollen or an anther with pollen dabbed directly onto the stigma

Gregor Mendel’s method revealed more about what the crosses could do to produce new plants

P Generation

The parents in reproduction

F₁ Generation

The first filial generation of reproduction from the parents

Homozygous

The gametes that are produced have the same allele of this gene (GG or gg)

Heterozygous

The gametes that are produced have different alleles of this gene (Gg)

Genotype

Combination of alleles, such as GG, Gg, or gg

Phenotype

The observable traits or characteristics of an organism

Mainly due to the interaction between the genotype of an organism and the environment it exists

Effects of dominant and recessive alleles on phenotype

The dominant allele will always overpower the recessive allele. If there is either one or two, the dominant one will be the phenotype

If there are two recessive alleles, they will be a different phenotype

Phenotypic Plasticity

A form of adaptation that’s reversible because genes have only been switched on or off

Pale skin turning to darker skin when exposed to more sunlight due to increased black melanin production in the skin

Phenylketonuria

Example of a genetic disease due to a recessive allele

Produced by mutation of the gene coding for the enzyme phenylalanine hydroxylase, which is used to convert phenylalanine to tyrosine, which can be used to make proteins

This is located on chromosome 12

Single-nucleotide polymorphisms and multiple alleles in gene pools

SNPs - where different bases can be present in a gene. Because of this variety, there will be many different alleles of a gene in the gene pool

ABO blood groups as an example of multiple alleles

There are 6 different alleles, with 4 possible combinations

I_AI_A = Type A Blood

I_Ai = Type A Blood

I_BI_B = Type B Blood

I_Bi = Type B Blood

I_AI_B = Type AB Blood

ii = Type O Blood

Incomplete Dominance

An intermediate phenotype that combines both genes (pink flowers coming from red and white flowers reproducing)

Codominance

No intermediate between two genes, resulting in an intermediate phenotype (AB blood)

Sex determination in humans and inheritance of genes on sex chromosomes

Since females are XX, they will always give one X chromosome to the offspring

Since males are XY, they may give the X or Y chromosome, which determines the gender of the offspring

If a person has XXY, this is a boy with Klinefelter’s disease, showing how the presence of a Y chromosome determines the male

If a person is X, this is a girl with Turner’s disease, showing how the absence of a Y chromosome determines the female

Haemophilia as an example of a sex-linked genetic disorder

People who have haemophilia lack or have a defective form of Factor VIII, which is responsible for clotting

This is located on the X chromosome, and since it’s recessive, males are much more common to have this disease than females since males are XY but females are XX

Pedigree Chart

Continuous variation

Continuous range of types is possible

Trait is influenced by multiple genes if there is a genetic cause

Environmental factors may influence the trait

Tree height, body mass of animals, skin color

Discrete Variation

Separate categories of the variant, with no intermediates in between

Trait is influenced by just one or at most a few genes if there is a genetic cause

Environmental factors do not usually influence the trait

ABO blood groups, number of eggs laid, left/right handed snail shells

Box-and-whisker plots to represent data for a continuous variable such as student height

Chromosomes

DNA within the nucleus that exist as long single molecules

Chromatids

The two strands that can be seen in a chromosome as the process of mitosis/meiosis continues

Microtubule Organizing Centers (MTOCs)

Areas that are found at the poles of the cell that can reassemble microtubules

Testis

Produces sperm and testosterone

Scrotum

Holds testes at a lower than core body temperature

Epididymis

Stores sperm until ejaculation

Sperm Duct

Transfers sperm during ejaculation

Seminal Vesicle & Prostate Gland

Secrete fluid containing alkali, proteins and fructose that is added to sperm to make semen

Urethra

Transfers semen during ejaculation and urine during urination

Penis

Penetrates vagina for ejaculation of semen near the cervix

Male Reproductive System

Ovary

Produces eggs, oestradiol and progesterone

Oviduct

Collects eggs at ovulation, provides a site for fertilization then moves the embryo to the uterus

Uterus

Provides for the needs of the embryo and then fetus during pregnancy

Cervix

Protects fetus during pregnancy and then dilates to provide a birth canal

Vagina

Stimulates penis to cause ejaculation and provides a birth canal

Vulva

Protects internal parts of the female reproductive system

Female Reproductive System

Ovarian Cycle

Consists of the follicular phase and luteal phase

Follicular Phase

First half of ovarian cycle

In each follicle, eggs are stimulated to grow, with the most developed follicle breaking open and releasing the egg into the oviduct (ovulation, usually on Day 14)

Luteal Phase

Second half of ovarian cycle

The wall of the follicle that released an egg develops into the corpus luteum. However, if fertilization does not occur, the corpus luteum breaks down and the ovary returns to the follicular phase

Uterine Cycle

Happens concurrently with the ovarian cycle

Endometrium becomes thickened and more richly supplied with blood during luteal phase

If there is no embryo, thickening starts to break down towards the end of the luteal phase and sheds in material form

Menstruation Cycle Chart

Stamens

Male parts of the flower. Consist of:

Anther - supported by a stalk called the filament. Diploid cells inside the anther divide by meiosis to produce 4 haploid cells, each developing into a pollen grain. Pollen grain divides by mitosis to produce 3 haploid nuclei, with two used as male gametes

Carpels

Female parts of the flower. Consist of:

Ovary - contains one or more ovules. One cell in the center of the ovule grows large and divides by meiosis to produce 4 haploid nuclei. One of these haploid nuclei divide 3 times by mitosis to produce 8 haploid nuclei, one which is a female gamete

Stigma - where the flower receives and traps pollen grains

Style - connects the ovary to the stigma

Reproductive System of a Flower