Reproduction

Sexual and Asexual Reproduction

Human cells contain 23 pairs of chromosomes. In mitosis one cell is copied into two identical cells.

  • In humans, the gametes(sex cells) are sperm cells and egg cells.
  • In gametes chromosomes are single and not paired therefore human sperm and egg cells contain 23 single chromosomes.
    Gametes are made by the process of meiosis.
    Meiosis produces non-identical cells.

Meiosis and Fertilisation

Advantages/Disadvantages of Sexual/Asexual Reproduction

Features of Sexual Reproduction

  • Involves the fusion of male+female gametes, this is known as fertilisation.
  • There is a mixing of genetic information, since every gamete is different there will be a variation of offspring.

Features of Asexual Reproduction

  • Only one parent, therefore does not involve the fusion of gametes
  • No mixing of genetic information, there will be no variation of offspring. Offspring produced in this way are called clones.
    -Asexual reproduction involves mitosis.

In a normal human cell there are 23 pairs of chromosomes. A human gamete contains 23 single chromosomes. Meiosis takes place in he reproductive organs.

Stages of Meiosis

  • All chromosomes are copied and the cells divide into two.
  • Cells divide again forming gametes.
  • In meiosis , four gametes are produced from one original cell- each of them being genetically different from eachother, each contain different alleles.
    In sexual reproduction, gametes join together(fuse) this is called fertilisation.
  • After fertilisation the cell now has the normal number of chromosomes(23 pairs).
  • After fertilisation, the new cell divides by mitosis, producing a clump of identical cells, this is called an embryo.
  • Cell differentiation occurs as the embryo develops.

Sexual Reproduction

There is a variation of offspring, this is a survival advantage by natural selection. This means there is a greater chance of survival if conditions become challenging.

Asexual Reproduction

There is no need to find a mate as there is only one parent involved. This is more efficient in regards to time and energy. This means asexual reproduction is faster than sexual reproduction.

However asexual reproduction can be risky, because all offspring are genetically identical, there is a risk they could all die if conditions become unfavourable.

DNA and the Genome

Chromosomes are found in the nucleus of cells. They contain the molecule DNA, our DNA determines all of our inherited features.

  • DNA consists of two strands, each strand is a polymer.
  • Each strand is made by joining up lots of smaller molecules.
  • In DNA, the two strands wrap up around eachother to form a double helix.
  • A gene is a small section of DNA.
  • Proteins are made by joining amino acids, each gene encodes for a specific sequence of amino acids to make a specific protein.

The Genome is the entire genetic material of an organism, the human genome is the entire genetic material that makes a human.
Its benefits are:

  • Search for genes linked to a disease
  • Help and treat inherited disorders
  • Trace human migration patterns from the past

Mutations

The base sequence of a gene determines the amino acid sequence of a protein. Once a protein is formed, it will fold up into a unique shape, this unique shape will determine its function.

  • A change to a base is called a mutation.
  • Mutations are common and often still lead to the same protein being made.
  • Chromosomes contain non-coding parts of DNA, these regions switch on/off.
  • Mutations in these non-coding regions can affect how genes are switched on/off.
  • A gene may be turned on when it should not be, this could lead to the cell producing a protein when it should not have at the time.
  • This could have a significant impact on the cell, e.g. uncontrollable mitosis leading to cancer.

Alleles

Alleles are versions of a gene.

  • The genotype tells us the alleles present
  • The phenotype is the characteristics caused by a persons alleles.
  • Homozygous- Two copies of the same allele.
  • Heterozygous- Two different alleles
    Alleles can be dominant or recessive, the dominant allele will always be expressed in the phenotype even if there is only one copy present, a recessive allele will only be expressed when two copies are present.

Inheritance of Sex

Humans contain 23 pairs of chromosomes in normal body cells. 22 of those chromosome pairs contain the genes which determine inherited characteristics ONLY.

  • The 23rd pair are called the sex chromosomes, these determine the sex of an offspring.
  • Males have the sex chromosome 'XY'
  • Females have the sex chromosome 'XX'
    By completing a Punnett square we can see that the probability of male and female offspring are both 50% or a 1:1 ratio. This is just a probability and in actual fact, all offspring of parents could be male or all could be female.

Arguments for/against embryonic screening

For: Reduces number of people suffering, treating diseases can be expensive
Against: Parents may decide to pick desired characteristics for offspring (designer babies), may promote prejudice as people with certain disorders may feel unwanted.