Reproduction & Inheritance (Edexcel International A Level (IAL) Biology): Flashcards

Females have two X chromosomes, while males have one X and one Y chromosome.

A carrier is a female with one dominant and one recessive allele for a sex-linked gene, who does not show the disease herself but can pass the recessive allele to her offspring.

Linked genes on the same autosome are inherited together, not independently, during meiosis.

Crossing over is the process by which non-sister chromatids exchange alleles during prophase I of meiosis I, resulting in new combinations of alleles.

A chiasma (plural: chiasmata) is the point where non-sister chromatids become entangled and crossing over occurs during prophase I of meiosis I.

Crossing over occurs during prophase I of meiosis I.

The paired homologous chromosomes during prophase I are known as bivalents.

Crossing over increases genetic variation by creating new combinations of alleles on chromosomes.

The process that produces gametes which are genetically different from each other is called meiosis.

Independent assortment is the production of different combinations of alleles in daughter cells due to the random alignment of homologous chromosome pairs during metaphase I of meiosis I.

False.

Homologous chromosomes are separated during meiosis I, not meiosis II. Meiosis II separates sister chromatids.

The formula 2^n is used, where n is the number of chromosomes in a haploid cell.

During metaphase I, the alignment of homologous chromosome pairs is random, leading to genetic variation.

A haploid nucleus has a single set of chromosomes (n), as found in gametes after meiosis.

The two processes are crossing over and independent assortment. Both increase genetic variation among gametes.

A gamete is a sex cell of an organism, such as a sperm or egg (ovum).

Fertilisation is the fusion of the nuclei from a male gamete (sperm cell) and a female gamete (egg cell) to form a zygote.

Gametes are formed during meiosis and only have one copy of each chromosome, so they are haploid cells.

A human sperm cell contains 23 single chromosomes in its nucleus.

A haploid cell has only one set of chromosomes; in humans, this means 23 chromosomes.

The acrosome contains digestive enzymes that break down the protective glycoprotein layer (zona pellucida) around the egg cell.

Sperm cells have a flagellum that allows them to swim towards the egg and many mitochondria to provide energy for this movement.

False.

After fertilisation, the zona pellucida forms an impenetrable barrier to prevent other sperm nuclei from entering the egg.

The egg cell contains a large amount of cytoplasm with stored food to nourish the developing embryo.

The corona radiata is the layer of follicle cells that forms a protective coating around the secondary oocyte.

Fertilisation is the fusion of the nuclei from a male gamete (sperm cell) and a female gamete (egg cell).

Fertilisation usually occurs in the oviduct, where the sperm meets the egg cell.

During sexual intercourse, semen is ejaculated high up into the vagina of the female, near the cervix.

True.

During the acrosome reaction, the sperm cell releases enzymes that digest a path through the zona pellucida.

The cortical reaction prevents more than one sperm from entering the egg cell by causing the zona pellucida to thicken and harden.

A zygote is a fertilised egg cell formed when the male and female gamete nuclei fuse. It contains the full 46 chromosomes, half from each parent.

The acrosome reaction is when the head of the sperm cell releases enzymes that digest a path through the zona pellucida, allowing the sperm to pass through the egg cell membrane.

The zygote divides by mitosis to form two cells, which continue to divide to form an embryo. The new cells later become specialised to form the body tissues of the offspring.

Sexual reproduction in flowering plants requires the transfer of pollen between the male and female parts of flowers.

The transfer of pollen from the anther to the stigma is known as pollination.

Pollination is the transfer of pollen from the anther (male part) to the stigma (female part) of a flower.

False.

Flowers usually contain both male and female reproductive parts.

The anther is the part of the flower where pollen is produced.

The stigma is the part of the female reproductive organ of a flower which receives the pollen.

After pollination, the pollen grain germinates and a pollen tube grows from the pollen grain down the style to the ovary.

The generative nucleus divides by mitosis to form two haploid male nuclei.

Double fertilisation is the process where one haploid male nucleus fuses with the egg cell nucleus to form a diploid zygote, and the other fuses with two polar nuclei to form a triploid endosperm nucleus.

The endosperm nucleus formed during double fertilisation is triploid.

Mitosis is the process of nuclear division that produces two genetically identical daughter nuclei with the same number of chromosomes as the parent cell nucleus.

Mitosis produces daughter nuclei that are genetically identical to the parent cell nucleus.

The four main stages of mitosis are prophase, metaphase, anaphase, and telophase.

During metaphase, chromosomes line up at the equator of the cell.

Spindle fibres are protein microtubules that attach to chromosomes and pull the sister chromatids apart to opposite poles of the cell during mitosis.

False.

Cytokinesis is not a stage of mitosis. It is a separate part of the cell cycle that occurs after nuclear division.

Sister chromatids are two identical copies of a chromosome joined together at the centromere, formed during DNA replication.

In prophase, chromosomes condense and become visible, and the nuclear envelope breaks down.

During anaphase, sister chromatids separate at the centromere and are pulled to opposite poles of the cell by spindle fibres.

Asexual reproduction is the production of new individuals from a single parent where the offspring are genetically identical to the parent.

The three phases of the cell cycle are interphase, nuclear division (mitosis), and cell division (cytokinesis).

During interphase, the cell grows, replicates its DNA, and prepares for mitosis.

False.

DNA replication occurs during the S phase (synthesis phase) of interphase, not G1.

Chemical signals called cyclins trigger movement from one phase of the cell cycle to the next.

Cytokinesis is the process where the whole cell divides after nuclear division, resulting in two genetically identical daughter cells.

A meristem is a region in plants where active cell division occurs, leading to growth. In roots, meristems are found just behind the protective root cap.

The meristem in a plant root is found just behind the protective root cap.

In the root tip meristem, there is a zone of cell division containing cells undergoing mitosis.

The squash technique is used to spread out root tip cells so individual cells undergoing mitosis can be observed.

Root tips are commonly stained with acetic orcein to make chromosomes visible under the microscope.

False.

Cells undergoing mitosis are found in a specific zone of cell division within the root tip meristem, not randomly distributed.

The squash technique involves staining and gently pressing root tip cells into a thin layer so individual cells and their chromosomes can be observed clearly under a microscope.

A water bath at 60°C is used to warm hydrochloric acid during root tip preparation.

Stains such as acetic orcein bind to chromosomes, making them more visible and allowing the different stages of mitosis to be observed under the microscope.

After preparing and staining a root tip slide, it is observed under an optical microscope.

The mitotic index is the proportion of cells in a sample that are undergoing mitosis.

The formula for calculating the mitotic index is: number of cells with visible chromosomes ÷ total number of cells.

The mitotic index can be given as a percentage by multiplying the answer by 100.

False.

You divide the number of cells with visible chromosomes by the total number of cells.

In a sample where 32 out of 42 cells have visible chromosomes, the mitotic index is 0.76.

The mitotic index is 0.44. This is calculated as (14 + 5 + 3 + 6) ÷ (36 + 14 + 5 + 3 + 6) = 28 ÷ 64 = 0.44.

To find the mitotic index, count the number of cells with visible chromosomes and divide by the total number of cells.

The mitotic index is 0.27, calculated as 20 ÷ 75 = 0.27.

A stem cell is a cell that can divide an unlimited number of times and has the potential to remain a stem cell or differentiate into a specialised cell.

Potency is the ability of a stem cell to differentiate into more specialised cell types.

A totipotent cell can differentiate into any cell type found in an embryo as well as extra-embryonic cells such as the placenta and umbilical cord.

A pluripotent cell can differentiate into any cell type found in an embryo but cannot form the placenta or umbilical cord.

A multipotent cell is an adult stem cell that can only specialise into certain types of cells and is no longer pluripotent.

The zygote formed when a sperm fertilises an egg cell is totipotent.

False.

Pluripotent cells can differentiate into any cell type found in the embryo but cannot form the placenta or umbilical cord.

Totipotent cells are present up to the 16-cell stage of embryo development (around the fourth day after fertilisation).

In plants, stem cells are found in areas of growth such as shoots and roots.

Embryonic stem cells can differentiate into almost any cell type, making them highly valuable for the therapeutic treatment of many diseases.

The use of adult stem cells is less controversial because the donor can give permission and no viable embryo is destroyed.

Stem cell therapy for leukaemia often involves a bone marrow stem cell transplant to replace damaged bone marrow.

True.

Adult stem cells can only differentiate into a limited range of cell types, which limits their use for treating some diseases.

A morula is a solid ball of totipotent cells formed by mitotic division of the zygote in the early stages of mammalian embryo development.

A blastocyst is a hollow ball of cells that develops from the morula, consisting of an outer cell layer and an inner cell mass of pluripotent stem cells.

Differential gene expression is the process by which only certain genes are activated and expressed in a stem cell, leading to specialisation.

Every nucleus within the stem cells of a multicellular organism contains the same genes, meaning all stem cells have an identical genome.

Controlling gene expression is the key to development, as it causes stem cells to differentiate into specialised cell types.

During differentiation, only certain genes in the DNA are activated and expressed.

A transcription factor is a protein that controls gene expression by binding to specific regions of DNA.

mRNA is transcribed only from active genes, and is then translated to proteins that help determine cell specialisation.

A specialised cell is a cell whose structure and proteins are adapted to carry out a particular function, produced by expressing only certain genes.

Repressors prevent RNA polymerase from binding to the promoter of a gene, thereby inhibiting transcription.

An operon is a section of DNA containing a cluster of structural genes transcribed together and control elements like promoters and operators.

An activator is a transcription factor that increases the rate of transcription of a gene by helping RNA polymerase bind to the promoter.

False.

The gene is still present in the DNA — it is simply not expressed (not transcribed) in that cell type.

Because of differential gene expression: each cell type transcribes and translates only a specific subset of its genes, making different proteins suited to its role.

Transcription factors bind to a region of DNA called the promoter, controlling whether transcription of a gene can begin.

Specific genes are switched on → mRNA is transcribed from them → the mRNA is translated into proteins → these proteins give the cell its specialised structure and function.

Irreversible specialisation is when a cell, after differentiation, remains in its specialised form and cannot revert to a stem cell.

A post-transcriptional modification is a change made to an mRNA transcript after it is produced, such as splicing, which can regulate gene expression.

In eukaryotes, transcription happens in the nucleus, while translation occurs in the cytoplasm.

During transcription, both introns and exons are copied to produce pre-mRNA.

Splicing is a post-transcriptional modification in which introns are removed and exons are joined together in pre-mRNA to produce mature mRNA.

Introns are non-coding regions of DNA within a gene, while exons are coding regions that are expressed in the final mRNA.

After splicing, the resulting mRNA contains only coding sequences of the gene.

Alternative splicing is the process where exons of pre-mRNA are joined in different combinations to produce multiple mature mRNAs from a single gene, allowing one gene to code for more than one protein.

True.

Alternative splicing of pre-mRNA allows a single gene to produce multiple different proteins by varying which exons are included in the mature mRNA.

A phenotype is the observable characteristics of an organism, determined by its genotype and environmental factors.

Phenotypic variation is the difference in phenotypes between organisms of the same species.

The four different human blood groups are due to different individuals having two of three possible alleles for the single ABO gene.

Genetically identical plants can grow to different heights when grown in different environmental conditions, demonstrating that environmental factors can cause phenotypic variation.

Phenotype is determined by the expression of the genotype and the interaction of the environment with it.

Genetic variation is the small differences in DNA base sequences between individuals of the same species, and is inherited from generation to generation.

Monogenic inheritance involves characteristics controlled by a single gene, usually showing discontinuous variation, while polygenic inheritance involves several genes and usually results in continuous variation.

Variation in phenotype caused solely by environmental pressures cannot be inherited by offspring.

Epigenetics is the control of gene expression by factors other than DNA sequence, typically involving the switching on and off of genes without changing the genetic code.

False.

DNA methylation suppresses gene expression by making chromatin more tightly packed and inhibiting transcription factor binding.

Addition of acetyl groups to lysine residues on histone proteins causes DNA to be less tightly wrapped and gene expression to be activated.

Yes, epigenetic changes such as DNA methylation or histone modification can be passed on to daughter cells after cell division, allowing the same pattern of gene expression to be maintained.

Continuous variation is when phenotypes show a range of values between two extremes and do not fall into discrete categories.

The mass or height of a human is an example of continuous variation.

Continuous variation can be identified by the lack of discrete categories and the presence of a range of values between two extremes.

Continuous variation differs from discontinuous variation because phenotypes do not fall into discrete categories.

Polygenic inheritance is the control of a phenotype by several genes, often resulting in continuous variation.

Monogenic inheritance usually results in discontinuous variation.

Characteristics controlled by a single gene are called monogenic characteristics.

Monogenic inheritance involves a single gene controlling a characteristic, while polygenic inheritance involves several genes influencing a phenotype.

True.

Polygenic inheritance involves several genes, so the phenotype shows continuous variation.

At the genetic level, different alleles at a single locus have a small effect on the phenotype.

Phenotype is affected by both genotype and environment, so environmental factors can influence the expression of traits and contribute to continuous variation.