Reproduction (AQA GCSE Biology)

During meiosis the chromosome number is halved from diploid (46) to haploid (23). For this reason meiosis is known as reduction division.

The process of meiosis involves the following events:

• During the first division homologous chromosomes pair up and recombine, resulting in genetic variation

• The second division separates sister chromatids

Meiosis increases genetic variation by creating new combinations of maternal and paternal chromosomes in each gamete, leading to unique offspring.

The daughter cells produced by meiosis can be described as follows:

• Four daughter cells

• Cells are genetically different

• Cells are haploid

• The cells produced are gametes

Meiosis is crucial because it ensures that gametes (sperm and egg cells) have half the chromosome number, allowing for the correct chromosome number after fertilisation.

Meiosis produces four daughter cells.

Some key differences between meiosis and mitosis might include:

• Mitosis involves one division WHILE meiosis involves two

• Mitosis produces diploid daughter cells WHILE meiosis produces haploid daughter cells

• Mitosis produces two daughter cells WHILE meiosis produces four

• Daughter cells in mitosis are genetically identical to each other and the parent cell WHILE in meiosis they are genetically different

• Mitosis is used for growth and repair WHILE meiosis produces sex cells for sexual reproduction

Advantages of sexual reproduction are:

• Produces variation in the offspring

• If the environment changes, variation gives a survival advantage by natural selection

• Natural selection can be speeded up by humans in selective breeding to increase food production

Sexual reproduction requires the production of specialised gametes and the search for a mate, which can be energetically costly and time-consuming.

Advantages of asexual reproduction:

• Only one parent needed

• More time and energy efficient as do not need to find a mate

• Faster than sexual reproduction

• Many identical offspring can be produced when conditions are favourable

Asexual reproduction lacks genetic variation, making offspring vulnerable to changes in the environment. It may also lead to the accumulation of harmful mutations and limit adaptability to new conditions.

Malarial parasites reproduce asexually in the human host, but sexually in the mosquito.

Many fungi reproduce asexually by spores but also reproduce sexually to give variation.

False.

Many plants produce seeds sexually, but also reproduce asexually by runners such as strawberry plants, or bulb division such as daffodils.

The genome is the entire set of genetic material of an organism.

DNA (deoxyribonucleic acid) is the molecule that contains the instructions for growth and development of all organisms.

False.

DNA is a polymer made up of two strands forming a double helix.

Chromosomes are located in the nucleus of cells.

A gene is a short length of DNA found on a chromosome that codes for a particular sequence of amino acids, to make a specific protein.

Genes code for different types of proteins.

The Human Genome Project was an international, collaborative research effort to determine the DNA sequence of the entire human genome and record every gene in human beings.

The human genome can help study human migration patterns because different populations of humans living in different parts of the world have developed very small differences in their genomes.

It is important to understand the human genome because it allows us to:

• Search for genes linked to different types of disease

• Understand and treat inherited disorders

• Trace human migration patterns from the past

The four nucleotide bases in DNA are:

• T (Thymine)

• A (Adenine)

• G (Guanine)

• C (Cytosine)

Complementary base pairing refers to the specific pairings of nucleotide bases in DNA.

• Adenine pairs with thymine (A with T)

• Guanine pairs with cytosine (G with C)

False.

A pairs with T

DNA is a polymer made up of two strands coiled around each other to form a double helix. The strands consist of a sugar-phosphate backbone with nitrogenous bases (A, T, C and G) attached to the backbone via complementary base pairing.

The backbone of a DNA strand is formed from the deoxyribose sugar of one nucleotide base and the phosphate molecule of the adjacent nucleotide base.

A polymer is a large molecule made up of many repeating subunits, in this case, nucleotides.

Three bases code for one amino acid during protein synthesis.

Protein synthesis is the process by which proteins are made in a cell using the DNA code to code for the correct sequence of amino acids to create the protein

The DNA template (mRNA) is read by the ribosome and carrier molecules (tRNA) bring new amino acids to the ribosome in the correct order to form the protein chain according to the DNA triplet code.

Genetic variants in coding DNA can alter the activity of a protein, potentially changing its function or effectiveness.

Genetic variants in non-coding DNA can alter how genes are expressed, potentially changing when, where, or how much a protein is produced.

When the protein chain is complete it folds up to form a unique shape. This unique shape enables the proteins to do their job as enzymes, hormones or forming structures in the body such as collagen.

A mutation is a change in DNA structure that may result in a change in the protein synthesized by a gene.

False.

Most mutations do not alter the protein, or only alter it slightly so that its appearance or function is not changed.

A mutation may code for an altered protein with a different shape, causing an substrate to no longer fit the active site of the enzyme.

Inherited disorders are caused by the inheritance of certain alleles from parents to offspring.

Cystic fibrosis is a genetic disorder of cell membranes that results in the body producing large amounts of thick, sticky mucus in the air passages.

The cystic fibrosis allele is recessive

Polydactyly is a genetic disorder that causes someone to be born with extra fingers or toes.

The polydactyly allele is dominant.

Embryo screening is the process of analysing the genes of an embryo before implantation or while in the womb to detect genetic disorders.

False.

Embryo screening raises economic, social, and ethical concerns, such as the potential destruction of embryos with genetic disorders.

Gene therapy is the process of inserting normal alleles into the chromosomes of an individual who carries defective alleles.

One argument for gene therapy is that it can help alleviate suffering in people with genetic disorders.

One argument against gene therapy is that...

• Some people are concerned it may lead to mutations

• It may trigger an immune response

• Some people think it is 'unnatural'

There are 46 chromosomes (23 pairs of chromosomes) found in a normal human body cell.

Sex chromosomes are the chromosomes that determine the biological sex of an individual; in humans they are the X and Y chromosomes.

In biological females the sex chromosomes are XX.

In biological males the sex chromosomes are XY.

False.

Biological sex is determined by an entire chromosome pair (XX for females, XY for males), not by a single gene.

True.

The father determines the biological sex of the child because he can pass on either an X or a Y chromosome.

The probability of a child inheriting two X chromosomes is 1/2 or 50 %.